urllib -- Python library reference

urllib -- Python library reference
Prev: cgi (cgi.html)
Top: Top (top.html)
# 11.2. Standard Module `urllib`
This module provides a high-level interface for fetching data across
the World-Wide Web.  In particular, the `urlopen` function is
similar to the built-in function `open`, but accepts URLs
(Universal Resource Locators) instead of filenames.  Some restrictions
apply --- it can only open URLs for reading, and no seek operations
are available.
it defines the following public functions:
Restrictions:
- • Currently, only the following protocols are supported: HTTP, (versions
0.9 and 1.0), Gopher (but not Gopher-+), FTP, and local files.
• The caching feature of `urlretrieve()` has been disabled until I
find the time to hack proper processing of Expiration time headers.
• There should be a function to query whether a particular URL is in
the cache.
• For backward compatibility, if a URL appears to point to a local file
but the file can't be opened, the URL is re-interpreted using the FTP
protocol. This can sometimes cause confusing error messages.
• The `urlopen()` and `urlretrieve()` functions can cause
arbitrarily long delays while waiting for a network connection to be
set up. This means that it is difficult to build an interactive
web client using these functions without using threads.
• The data returned by `urlopen()` or `urlretrieve()` is the
raw data returned by the server. This may be binary data (e.g. an
image), plain text or (for example) HTML. The HTTP protocol provides
type information in the reply header, which can be inspected by
looking at the `Content-type` header. For the Gopher protocol,
type information is encoded in the URL; there is currently no easy way
to extract it. If the returned data is HTML, you can use the module
`htmllib` to parse it.
• Although the `urllib` module contains (undocumented) routines to
parse and unparse URL strings, the recommended interface for URL
manipulation is in module `urlparse`.


urlparse -- Python library reference

urlparse -- Python library reference
Prev: nntplib (nntplib.html)
Top: Top (top.html)
# 11.7. Standard Module `urlparse`
This module defines a standard interface to break URL strings up in
components (addessing scheme, network location, path etc.), to combine
the components back into a URL string, and to convert a ``relative
URL'' to an absolute URL given a ``base URL''.
The module has been designed to match the current Internet draft on
Relative Uniform Resource Locators (and discovered a bug in an earlier
draft!).
It defines the following functions:


Variable Index -- Python library reference
Prev: Function Index (function_index.html)
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# Variable Index


voice objects -- Python library reference
Prev: macspeech (macspeech.html)
Top: Top (top.html)
## 14.8.1. voice objects
Voice objects contain the description of a voice. It is currently not
yet possible to access the parameters of a voice.


whrandom -- Python library reference

whrandom -- Python library reference
Prev: rand (rand.html)
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# 5.3. Standard Module `whrandom`
This module implements a Wichmann-Hill pseudo-random number generator.
It defines the following functions:


Window Objects -- Python library reference
Prev: STDWIN Functions (stdwin_functions.html)
Top: Top (top.html)
## 15.1.2. Window Objects
Window objects are created by `stdwin.open()`.  They are closed
by their `close()` method or when they are garbage-collected.
Window objects have the following methods:


Footnotes

guido@cwi.nl


Contents

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## Contents
- Introduction (node2.html#SECTION00200000000000000000)
- Notation (node3.html#SECTION00210000000000000000)
Lexical analysis (node4.html#SECTION00300000000000000000)
- Line structure (node5.html#SECTION00310000000000000000)
- Comments (node6.html#SECTION00311000000000000000)
Explicit line joining (node7.html#SECTION00312000000000000000)
Implicit line joining (node8.html#SECTION00313000000000000000)
Blank lines (node9.html#SECTION00314000000000000000)
Indentation (node10.html#SECTION00315000000000000000)
Other tokens (node11.html#SECTION00320000000000000000)
Identifiers (node12.html#SECTION00330000000000000000)
- Keywords (node13.html#SECTION00331000000000000000)
Literals (node14.html#SECTION00340000000000000000)
- String literals (node15.html#SECTION00341000000000000000)
Numeric literals (node16.html#SECTION00342000000000000000)
Operators (node17.html#SECTION00350000000000000000)
Delimiters (node18.html#SECTION00360000000000000000)
Data model (node19.html#SECTION00400000000000000000)
- Objects, values and types (node20.html#SECTION00410000000000000000)
The standard type hierarchy (node21.html#SECTION00420000000000000000)
Special method names (node22.html#SECTION00430000000000000000)
- Special methods for any type (node23.html#SECTION00431000000000000000)
Special methods for attribute access (node24.html#SECTION00432000000000000000)
Special methods for sequence and mapping types (node25.html#SECTION00433000000000000000)
Special methods for sequence types (node26.html#SECTION00434000000000000000)
Special methods for numeric types (node27.html#SECTION00435000000000000000)
Execution model (node28.html#SECTION00500000000000000000)
- Code blocks, execution frames, and name spaces (node29.html#SECTION00510000000000000000)
Exceptions (node30.html#SECTION00520000000000000000)
Expressions and conditions (node31.html#SECTION00600000000000000000)
- Arithmetic conversions (node32.html#SECTION00610000000000000000)
Atoms (node33.html#SECTION00620000000000000000)
- Identifiers (Names) (node34.html#SECTION00621000000000000000)
Literals (node35.html#SECTION00622000000000000000)
Parenthesized forms (node36.html#SECTION00623000000000000000)
List displays (node37.html#SECTION00624000000000000000)
Dictionary displays (node38.html#SECTION00625000000000000000)
String conversions (node39.html#SECTION00626000000000000000)
Primaries (node40.html#SECTION00630000000000000000)
- Attribute references (node41.html#SECTION00631000000000000000)
Subscriptions (node42.html#SECTION00632000000000000000)
Slicings (node43.html#SECTION00633000000000000000)
Calls (node44.html#SECTION00634000000000000000)
Unary arithmetic operations (node45.html#SECTION00640000000000000000)
Binary arithmetic operations (node46.html#SECTION00650000000000000000)
Shifting operations (node47.html#SECTION00660000000000000000)
Binary bit-wise operations (node48.html#SECTION00670000000000000000)
Comparisons (node49.html#SECTION00680000000000000000)
Boolean operations (node50.html#SECTION00690000000000000000)
Expression lists and condition lists (node51.html#SECTION006100000000000000000)
Summary (node52.html#SECTION006110000000000000000)
Simple statements (node53.html#SECTION00700000000000000000)
- Expression statements (node54.html#SECTION00710000000000000000)
Assignment statements (node55.html#SECTION00720000000000000000)
The pass statement (node56.html#SECTION00730000000000000000)
The del statement (node57.html#SECTION00740000000000000000)
The print statement (node58.html#SECTION00750000000000000000)
The return statement (node59.html#SECTION00760000000000000000)
The raise statement (node60.html#SECTION00770000000000000000)
The break statement (node61.html#SECTION00780000000000000000)
The continue statement (node62.html#SECTION00790000000000000000)
The import statement (node63.html#SECTION007100000000000000000)
The global statement (node64.html#SECTION007110000000000000000)
The access statement (node65.html#SECTION007120000000000000000)
The exec statement (node66.html#SECTION007130000000000000000)
Compound statements (node67.html#SECTION00800000000000000000)
- The if statement (node68.html#SECTION00810000000000000000)
The while statement (node69.html#SECTION00820000000000000000)
The for statement (node70.html#SECTION00830000000000000000)
The try statement (node71.html#SECTION00840000000000000000)
Function definitions (node72.html#SECTION00850000000000000000)
Class definitions (node73.html#SECTION00860000000000000000)
Top-level components (node74.html#SECTION00900000000000000000)
- Complete Python programs (node75.html#SECTION00910000000000000000)
File input (node76.html#SECTION00920000000000000000)
Interactive input (node77.html#SECTION00930000000000000000)
Expression input (node78.html#SECTION00940000000000000000)
Index (node79.html#SECTION001000000000000000000)
About this document ... (node80.html#SECTION001100000000000000000)
---
guido@cwi.nl


Indentation

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## Indentation
Leading whitespace (spaces and tabs) at the beginning of a logical
line is used to compute the indentation level of the line, which in
turn is used to determine the grouping of statements.
First, tabs are replaced (from left to right) by one to eight spaces
such that the total number of characters up to there is a multiple of
eight (this is intended to be the same rule as used by Unix). The
total number of spaces preceding the first non-blank character then
determines the line's indentation. Indentation cannot be split over
multiple physical lines using backslashes.
The indentation levels of consecutive lines are used to generate
INDENT and DEDENT tokens, using a stack, as follows.
Before the first line of the file is read, a single zero is pushed on
the stack; this will never be popped off again. The numbers pushed on
the stack will always be strictly increasing from bottom to top. At
the beginning of each logical line, the line's indentation level is
compared to the top of the stack. If it is equal, nothing happens.
If it is larger, it is pushed on the stack, and one INDENT token is
generated. If it is smaller, it must be one of the numbers
occurring on the stack; all numbers on the stack that are larger are
popped off, and for each number popped off a DEDENT token is
generated. At the end of the file, a DEDENT token is generated for
each number remaining on the stack that is larger than zero.
Here is an example of a correctly (though confusingly) indented piece
of Python code:
```text
def perm(l):
# Compute the list of all permutations of l

if len(l) <= 1:
return [l]
r = []
for i in range(len(l)):
s = l[:i] + l[i+1:]
p = perm(s)
for x in p:
r.append(l[i:i+1] + x)
return r
```
The following example shows various indentation errors:
```text
def perm(l): # error: first line indented
for i in range(len(l)): # error: not indented
s = l[:i] + l[i+1:]
p = perm(l[:i] + l[i+1:]) # error: unexpected indent
for x in p:
r.append(l[i:i+1] + x)
return r # error: inconsistent dedent
```
(Actually, the first three errors are detected by the parser; only the
last error is found by the lexical analyzer --- the indentation of
`return r` does not match a level popped off the stack.)
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---
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Other tokens

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# Other tokens
Besides NEWLINE, INDENT and DEDENT, the following categories of tokens
exist: identifiers, keywords, literals, operators, and delimiters.
Spaces and tabs are not tokens, but serve to delimit tokens. Where
ambiguity exists, a token comprises the longest possible string that
forms a legal token, when read from left to right.
---
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Identifiers

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# Identifiers
Identifiers (also referred to as names) are described by the following
lexical definitions:
```text
identifier: (letter|"_") (letter|digit|"_")*
letter: lowercase | uppercase
lowercase: "a"..."z"
uppercase: "A"..."Z"
digit: "0"..."9"
```
Identifiers are unlimited in length. Case is significant.
---
- Keywords (node13.html#SECTION00331000000000000000)
---
guido@cwi.nl


Keywords

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## Keywords
The following identifiers are used as reserved words, or
keywords of the language, and cannot be used as ordinary
identifiers. They must be spelled exactly as written here:
```text
access del from lambda return
and elif global not try
break else if or while
class except import pass
continue finally in print
def for is raise
```
---
guido@cwi.nl


Literals

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# Literals
Literals are notations for constant values of some built-in types.
---
- String literals (node15.html#SECTION00341000000000000000)
Numeric literals (node16.html#SECTION00342000000000000000)
---
guido@cwi.nl


String literals

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## String literals
String literals are described by the following lexical definitions:
```text
stringliteral: shortstring | longstring
shortstring: "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring: "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
shortstringitem: shortstringchar | escapeseq
shortstringchar: <any ASCII character except "\" or newline or the quote>
longstringchar: <any ASCII character except "\">
escapeseq: "\" <any ASCII character>
```
In ``long strings'' (strings surrounded by sets of three quotes),
unescaped newlines and quotes are allowed (and are retained), except
that three unescaped quotes in a row terminate the string. (A
``quote'' is the character used to open the string, i.e. either
`'` or `"`.)
Escape sequences in strings are interpreted according to rules similar
to those used by Standard C. The recognized escape sequences are:
In strict compatibility with Standard C, up to three octal digits are
accepted, but an unlimited number of hex digits is taken to be part of
the hex escape (and then the lower 8 bits of the resulting hex number
are used in all current implementations...).
All unrecognized escape sequences are left in the string unchanged,
i.e., the backslash is left in the string. (This behavior is
useful when debugging: if an escape sequence is mistyped, the
resulting output is more easily recognized as broken. It also helps a
great deal for string literals used as regular expressions or
otherwise passed to other modules that do their own escape handling.)
---
guido@cwi.nl


Numeric literals

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## Numeric literals
There are three types of numeric literals: plain integers, long
integers, and floating point numbers.
Integer and long integer literals are described by the following
lexical definitions:
```text
longinteger: integer ("l"|"L")
integer: decimalinteger | octinteger | hexinteger
decimalinteger: nonzerodigit digit* | "0"
octinteger: "0" octdigit+
hexinteger: "0" ("x"|"X") hexdigit+

nonzerodigit: "1"..."9"
octdigit: "0"..."7"
hexdigit: digit|"a"..."f"|"A"..."F"
```
Although both lower case `l' and upper case `L' are allowed as suffix
for long integers, it is strongly recommended to always use `L', since
the letter `l' looks too much like the digit `1'.
Plain integer decimal literals must be at most 2147483647 (i.e., the
largest positive integer, using 32-bit arithmetic). Plain octal and
hexadecimal literals may be as large as 4294967295, but values larger
than 2147483647 are converted to a negative value by subtracting
4294967296. There is no limit for long integer literals apart from
what can be stored in available memory.
Some examples of plain and long integer literals:
```text
7 2147483647 0177 0x80000000
3L 79228162514264337593543950336L 0377L 0x100000000L
```
Floating point literals are described by the following lexical
definitions:
```text
floatnumber: pointfloat | exponentfloat
pointfloat: [intpart] fraction | intpart "."
exponentfloat: (intpart | pointfloat) exponent
intpart: digit+
fraction: "." digit+
exponent: ("e"|"E") ["+"|"-"] digit+
```
The allowed range of floating point literals is
implementation-dependent.
Some examples of floating point literals:
```text
3.14 10. .001 1e100 3.14e-10
```
Note that numeric literals do not include a sign; a phrase like
`-1` is actually an expression composed of the operator
`-` and the literal `1`.
---
guido@cwi.nl


Operators

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# Operators
The following tokens are operators:
```text
+ - * / %
<< >> & | ^ ~
< == > <= <> != >=
```
The comparison operators `<>` and `!=` are alternate
spellings of the same operator.
---
guido@cwi.nl


Delimiters

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# Delimiters
The following tokens serve as delimiters or otherwise have a special
meaning:
```text
( ) [ ] { }
, : . " ` '
= ;
```
The following printing ASCII characters are not used in Python. Their
occurrence outside string literals and comments is an unconditional
error:
```text
@ $ ?
```
They may be used by future versions of the language though!
---
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Data model

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# Data model
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- Objects, values and types (node20.html#SECTION00410000000000000000)
The standard type hierarchy (node21.html#SECTION00420000000000000000)
Special method names (node22.html#SECTION00430000000000000000)
- Special methods for any type (node23.html#SECTION00431000000000000000)
Special methods for attribute access (node24.html#SECTION00432000000000000000)
Special methods for sequence and mapping types (node25.html#SECTION00433000000000000000)
Special methods for sequence types (node26.html#SECTION00434000000000000000)
Special methods for numeric types (node27.html#SECTION00435000000000000000)
---
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Introduction

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# Introduction
This reference manual describes the Python programming language.
It is not intended as a tutorial.
While I am trying to be as precise as possible, I chose to use English
rather than formal specifications for everything except syntax and
lexical analysis. This should make the document more understandable
to the average reader, but will leave room for ambiguities.
Consequently, if you were coming from Mars and tried to re-implement
Python from this document alone, you might have to guess things and in
fact you would probably end up implementing quite a different language.
On the other hand, if you are using
Python and wonder what the precise rules about a particular area of
the language are, you should definitely be able to find them here.
It is dangerous to add too many implementation details to a language
reference document --- the implementation may change, and other
implementations of the same language may work differently. On the
other hand, there is currently only one Python implementation, and
its particular quirks are sometimes worth being mentioned, especially
where the implementation imposes additional limitations. Therefore,
you'll find short ``implementation notes'' sprinkled throughout the
text.
Every Python implementation comes with a number of built-in and
standard modules. These are not documented here, but in the separate
Python Library Reference document. A few built-in modules are
mentioned when they interact in a significant way with the language
definition.
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Objects, values and types

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# Objects, values and types
Objects are Python's abstraction for data. All data in a Python
program is represented by objects or by relations between objects.
(In a sense, and in conformance to Von Neumann's model of a
``stored program computer'', code is also represented by objects.)
Every object has an identity, a type and a value. An object's
identity never changes once it has been created; you may think of it
as the object's address in memory. An object's type is also
unchangeable. It determines the operations that an object supports
(e.g. ``does it have a length?'') and also defines the possible
values for objects of that type. The value of some objects can
change. Objects whose value can change are said to be mutable;
objects whose value is unchangeable once they are created are called
immutable. The type determines an object's (im)mutability.
Objects are never explicitly destroyed; however, when they become
unreachable they may be garbage-collected. An implementation is
allowed to delay garbage collection or omit it altogether --- it is a
matter of implementation quality how garbage collection is
implemented, as long as no objects are collected that are still
reachable. (Implementation note: the current implementation uses a
reference-counting scheme which collects most objects as soon as they
become unreachable, but never collects garbage containing circular
references.)
Note that the use of the implementation's tracing or debugging
facilities may keep objects alive that would normally be collectable.
Some objects contain references to ``external'' resources such as open
files or windows. It is understood that these resources are freed
when the object is garbage-collected, but since garbage collection is
not guaranteed to happen, such objects also provide an explicit way to
release the external resource, usually a `close` method.
Programs are strongly recommended to always explicitly close such
objects.
Some objects contain references to other objects; these are called
containers. Examples of containers are tuples, lists and
dictionaries. The references are part of a container's value. In
most cases, when we talk about the value of a container, we imply the
values, not the identities of the contained objects; however, when we
talk about the (im)mutability of a container, only the identities of
the immediately contained objects are implied. (So, if an immutable
container contains a reference to a mutable object, its value changes
if that mutable object is changed.)
Types affect almost all aspects of objects' lives. Even the meaning
of object identity is affected in some sense: for immutable types,
operations that compute new values may actually return a reference to
any existing object with the same type and value, while for mutable
objects this is not allowed. E.g. after
```text
a = 1; b = 1; c = []; d = []
```
`a` and `b` may or may not refer to the same object with the
value one, depending on the implementation, but `c` and `d`
are guaranteed to refer to two different, unique, newly created empty
lists.
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The standard type hierarchy

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# The standard type hierarchy
Below is a list of the types that are built into Python. Extension
modules written in C can define additional types. Future versions of
Python may add types to the type hierarchy (e.g. rational or complex
numbers, efficiently stored arrays of integers, etc.).
Some of the type descriptions below contain a paragraph listing
`special attributes'. These are attributes that provide access to the
implementation and are not intended for general use. Their definition
may change in the future. There are also some `generic' special
attributes, not listed with the individual objects: `__methods__`
is a list of the method names of a built-in object, if it has any;
`__members__` is a list of the data attribute names of a built-in
object, if it has any.
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Special method names

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# Special method names
A class can implement certain operations that are invoked by special
syntax (such as subscription or arithmetic operations) by defining
methods with special names. For instance, if a class defines a
method named `__getitem__`, and `x` is an instance of this
class, then `x[i]` is equivalent to `x.__getitem__(i)`.
(The reverse is not true --- if `x` is a list object,
`x.__getitem__(i)` is not equivalent to `x[i]`.)
Except for `__repr__`, `__str__` and `__cmp__`,
attempts to execute an
operation raise an exception when no appropriate method is defined.
For `__repr__`, the default is to return a string describing the
object's class and address.
For `__cmp__`, the default is to compare instances based on their
address.
For `__str__`, the default is to use `__repr__`.
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Special methods for attribute access (node24.html#SECTION00432000000000000000)
Special methods for sequence and mapping types (node25.html#SECTION00433000000000000000)
Special methods for sequence types (node26.html#SECTION00434000000000000000)
Special methods for numeric types (node27.html#SECTION00435000000000000000)
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Special methods for any type

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## Special methods for any type
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Special methods for attribute access

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## Special methods for attribute access
The following methods can be used to change the meaning of attribute
access for class instances.
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## Special methods for sequence and mapping types
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Special methods for sequence types

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## Special methods for sequence types
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Special methods for numeric types

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## Special methods for numeric types
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Execution model

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# Execution model
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Exceptions (node30.html#SECTION00520000000000000000)
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# Code blocks, execution frames, and name spaces
A code block is a piece of Python program text that can be
executed as a unit, such as a module, a class definition or a function
body. Some code blocks (like modules) are executed only once, others
(like function bodies) may be executed many times. Code blocks may
textually contain other code blocks. Code blocks may invoke other
code blocks (that may or may not be textually contained in them) as
part of their execution, e.g. by invoking (calling) a function.
The following are code blocks: A module is a code block. A function
body is a code block. A class definition is a code block. Each
command typed interactively is a separate code block; a script file is
a code block. The string argument passed to the built-in function
`eval` and to the `exec` statement are code blocks.
And finally, the
expression read and evaluated by the built-in function `input` is
a code block.
A code block is executed in an execution frame. An execution
frame contains some administrative information (used for debugging),
determines where and how execution continues after the code block's
execution has completed, and (perhaps most importantly) defines two
name spaces, the local and the global name space, that affect
execution of the code block.
A name space is a mapping from names (identifiers) to objects.
A particular name space may be referenced by more than one execution
frame, and from other places as well. Adding a name to a name space
is called binding a name (to an object); changing the mapping of
a name is called rebinding; removing a name is unbinding.
Name spaces are functionally equivalent to dictionaries.
The local name space of an execution frame determines the default
place where names are defined and searched. The global name
space determines the place where names listed in `global`
statements are defined and searched, and where names that are not
explicitly bound in the current code block are searched.
Whether a name is local or global in a code block is determined by
static inspection of the source text for the code block: in the
absence of `global` statements, a name that is bound anywhere in
the code block is local in the entire code block; all other names are
considered global. The `global` statement forces global
interpretation of selected names throughout the code block. The
following constructs bind names: formal parameters, `import`
statements, class and function definitions (these bind the class or
function name), and targets that are identifiers if occurring in an
assignment, `for` loop header, or `except` clause header.
A target occurring in a `del` statement is also considered bound
for this purpose (though the actual semantics are to ``unbind'' the
name).
When a global name is not found in the global name space, it is
searched in the list of ``built-in'' names (which is actually the
global name space of the module `__builtin__`). When a name is not
found at all, the `NameError` exception is raised.
footnode.html#585
The following table lists the meaning of the local and global name
space for various types of code blocks. The name space for a
particular module is automatically created when the module is first
referenced. Note that in almost all cases, the global name space is
the name space of the containing module --- scopes in Python do not
nest!
Notes:
The built-in function `vars()` returns a dictionary representing
the current local name space. The effect of modifications to this
dictionary on the name space are undefined.
footnode.html#586
---
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---
---
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Notation

---
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---
# Notation
The descriptions of lexical analysis and syntax use a modified BNF
grammar notation. This uses the following style of definition:
```text
name: lc_letter (lc_letter | "_")*
lc_letter: "a"..."z"
```
The first line says that a `name` is an `lc_letter` followed by
a sequence of zero or more `lc_letter`s and underscores. An
`lc_letter` in turn is any of the single characters `a' through `z'.
(This rule is actually adhered to for the names defined in lexical and
grammar rules in this document.)
Each rule begins with a name (which is the name defined by the rule)
and a colon. A vertical bar (`|`) is used to separate
alternatives; it is the least binding operator in this notation. A
star (`*`) means zero or more repetitions of the preceding item;
likewise, a plus (`+`) means one or more repetitions, and a
phrase enclosed in square brackets (`[ ]`) means zero or one
occurrences (in other words, the enclosed phrase is optional). The
`*` and `+` operators bind as tightly as possible;
parentheses are used for grouping. Literal strings are enclosed in
quotes. White space is only meaningful to separate tokens.
Rules are normally contained on a single line; rules with many
alternatives may be formatted alternatively with each line after the
first beginning with a vertical bar.
In lexical definitions (as the example above), two more conventions
are used: Two literal characters separated by three dots mean a choice
of any single character in the given (inclusive) range of ASCII
characters. A phrase between angular brackets (`<...>`) gives an
informal description of the symbol defined; e.g. this could be used
to describe the notion of `control character' if needed.
Even though the notation used is almost the same, there is a big
difference between the meaning of lexical and syntactic definitions:
a lexical definition operates on the individual characters of the
input source, while a syntax definition operates on the stream of
tokens generated by the lexical analysis. All uses of BNF in the next
chapter (``Lexical Analysis'') are lexical definitions; uses in
subsequent chapters are syntactic definitions.
---
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---
---
guido@cwi.nl


Exceptions

---
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---
# Exceptions
Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors or other exceptional
conditions. An exception is raised at the point where the error
is detected; it may be handled by the surrounding code block or
by any code block that directly or indirectly invoked the code block
where the error occurred.
The Python interpreter raises an exception when it detects an run-time
error (such as division by zero). A Python program can also
explicitly raise an exception with the `raise` statement.
Exception handlers are specified with the `try...except`
statement.
Python uses the ``termination'' model of error handling: an exception
handler can find out what happened and continue execution at an outer
level, but it cannot repair the cause of the error and retry the
failing operation (except by re-entering the the offending piece of
code from the top).
When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.
Exceptions are identified by string objects. Two different string
objects with the same value identify different exceptions.
When an exception is raised, an object (maybe `None`) is passed
as the exception's ``parameter''; this object does not affect the
selection of an exception handler, but is passed to the selected
exception handler as additional information.
See also the description of the `try` and `raise`
statements.
---
guido@cwi.nl


Expressions and conditions

---
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---
# Expressions and conditions
Note: In this and the following chapters, extended BNF notation
will be used to describe syntax, not lexical analysis.
This chapter explains the meaning of the elements of expressions and
conditions. Conditions are a superset of expressions, and a condition
may be used wherever an expression is required by enclosing it in
parentheses. The only places where expressions are used in the syntax
instead of conditions is in expression statements and on the
right-hand side of assignment statements; this catches some nasty bugs
like accidentally writing `x == 1` instead of `x = 1`.
The comma plays several roles in Python's syntax. It is usually an
operator with a lower precedence than all others, but occasionally
serves other purposes as well; e.g. it separates function arguments,
is used in list and dictionary constructors, and has special semantics
in `print` statements.
When (one alternative of) a syntax rule has the form
```text
name: othername
```
and no semantics are given, the semantics of this form of `name`
are the same as for `othername`.
---
- Arithmetic conversions (node32.html#SECTION00610000000000000000)
Atoms (node33.html#SECTION00620000000000000000)
- Identifiers (Names) (node34.html#SECTION00621000000000000000)
Literals (node35.html#SECTION00622000000000000000)
Parenthesized forms (node36.html#SECTION00623000000000000000)
List displays (node37.html#SECTION00624000000000000000)
Dictionary displays (node38.html#SECTION00625000000000000000)
String conversions (node39.html#SECTION00626000000000000000)
Primaries (node40.html#SECTION00630000000000000000)
- Attribute references (node41.html#SECTION00631000000000000000)
Subscriptions (node42.html#SECTION00632000000000000000)
Slicings (node43.html#SECTION00633000000000000000)
Calls (node44.html#SECTION00634000000000000000)
Unary arithmetic operations (node45.html#SECTION00640000000000000000)
Binary arithmetic operations (node46.html#SECTION00650000000000000000)
Shifting operations (node47.html#SECTION00660000000000000000)
Binary bit-wise operations (node48.html#SECTION00670000000000000000)
Comparisons (node49.html#SECTION00680000000000000000)
Boolean operations (node50.html#SECTION00690000000000000000)
Expression lists and condition lists (node51.html#SECTION006100000000000000000)
Summary (node52.html#SECTION006110000000000000000)
---
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Arithmetic conversions

---
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---
# Arithmetic conversions
When a description of an arithmetic operator below uses the phrase
``the numeric arguments are converted to a common type'',
this both means that if either argument is not a number, a
`TypeError` exception is raised, and that otherwise
the following conversions are applied:
- first, if either argument is a floating point number,
the other is converted to floating point;
else, if either argument is a long integer,
the other is converted to long integer;
otherwise, both must be plain integers and no conversion
is necessary.
---
guido@cwi.nl


Atoms

---
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---
# Atoms
Atoms are the most basic elements of expressions. Forms enclosed in
reverse quotes or in parentheses, brackets or braces are also
categorized syntactically as atoms. The syntax for atoms is:
```text
atom: identifier | literal | enclosure
enclosure: parenth_form | list_display | dict_display | string_conversion
```
---
- Identifiers (Names) (node34.html#SECTION00621000000000000000)
Literals (node35.html#SECTION00622000000000000000)
Parenthesized forms (node36.html#SECTION00623000000000000000)
List displays (node37.html#SECTION00624000000000000000)
Dictionary displays (node38.html#SECTION00625000000000000000)
String conversions (node39.html#SECTION00626000000000000000)
---
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Identifiers (Names)

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---
## Identifiers (Names)
An identifier occurring as an atom is a reference to a local, global
or built-in name binding. If a name is assigned to anywhere in a code
block (even in unreachable code), and is not mentioned in a
`global` statement in that code block, then it refers to a local
name throughout that code block. When it is not assigned to anywhere
in the block, or when it is assigned to but also explicitly listed in
a `global` statement, it refers to a global name if one exists,
else to a built-in name (and this binding may dynamically change).
When the name is bound to an object, evaluation of the atom yields
that object. When a name is not bound, an attempt to evaluate it
raises a `NameError` exception.
---
guido@cwi.nl


Literals

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---
## Literals
Python knows string and numeric literals:
```text
literal: stringliteral | integer | longinteger | floatnumber
```
Evaluation of a literal yields an object of the given type (string,
integer, long integer, floating point number) with the given value.
The value may be approximated in the case of floating point literals.
See section node14.html#literals for details.
All literals correspond to immutable data types, and hence the
object's identity is less important than its value. Multiple
evaluations of literals with the same value (either the same
occurrence in the program text or a different occurrence) may obtain
the same object or a different object with the same value.
(In the original implementation, all literals in the same code block
with the same type and value yield the same object.)
---
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Parenthesized forms

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---
## Parenthesized forms
A parenthesized form is an optional condition list enclosed in
parentheses:
```text
parenth_form: "(" [condition_list] ")"
```
A parenthesized condition list yields whatever that condition list
yields.
An empty pair of parentheses yields an empty tuple object. Since
tuples are immutable, the rules for literals apply here.
(Note that tuples are not formed by the parentheses, but rather by use
of the comma operator. The exception is the empty tuple, for which
parentheses are required --- allowing unparenthesized ``nothing''
in expressions would cause ambiguities and allow common typos to
pass uncaught.)
---
guido@cwi.nl


List displays

---
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---
## List displays
A list display is a possibly empty series of conditions enclosed in
square brackets:
```text
list_display: "[" [condition_list] "]"
```
A list display yields a new list object.
If it has no condition list, the list object has no items. Otherwise,
the elements of the condition list are evaluated from left to right
and inserted in the list object in that order.
---
guido@cwi.nl


Dictionary displays

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---
## Dictionary displays
A dictionary display is a possibly empty series of key/datum pairs
enclosed in curly braces:
```text
dict_display: "{" [key_datum_list] "}"
key_datum_list: key_datum ("," key_datum)* [","]
key_datum: condition ":" condition
```
A dictionary display yields a new dictionary object.
The key/datum pairs are evaluated from left to right to define the
entries of the dictionary: each key object is used as a key into the
dictionary to store the corresponding datum.
Restrictions on the types of the key values are listed earlier in
section node21.html#types.
Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key
value prevails.
---
guido@cwi.nl


String conversions

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---
## String conversions
A string conversion is a condition list enclosed in reverse (or
backward) quotes:
```text
string_conversion: "`" condition_list "`"
```
A string conversion evaluates the contained condition list and
converts the resulting object into a string according to rules
specific to its type.
If the object is a string, a number, `None`, or a tuple, list or
dictionary containing only objects whose type is one of these, the
resulting string is a valid Python expression which can be passed to
the built-in function `eval()` to yield an expression with the
same value (or an approximation, if floating point numbers are
involved).
(In particular, converting a string adds quotes around it and converts
``funny'' characters to escape sequences that are safe to print.)
It is illegal to attempt to convert recursive objects (e.g. lists or
dictionaries that contain a reference to themselves, directly or
indirectly.)
The built-in function `repr()` performs exactly the same
conversion in its argument as enclosing it it reverse quotes does.
The built-in function `str()` performs a similar but more
user-friendly conversion.
---
guido@cwi.nl


Lexical analysis

---
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---
# Lexical analysis
A Python program is read by a parser. Input to the parser is a
stream of tokens, generated by the lexical analyzer. This
chapter describes how the lexical analyzer breaks a file into tokens.
---
- Line structure (node5.html#SECTION00310000000000000000)
- Comments (node6.html#SECTION00311000000000000000)
Explicit line joining (node7.html#SECTION00312000000000000000)
Implicit line joining (node8.html#SECTION00313000000000000000)
Blank lines (node9.html#SECTION00314000000000000000)
Indentation (node10.html#SECTION00315000000000000000)
Other tokens (node11.html#SECTION00320000000000000000)
Identifiers (node12.html#SECTION00330000000000000000)
- Keywords (node13.html#SECTION00331000000000000000)
Literals (node14.html#SECTION00340000000000000000)
- String literals (node15.html#SECTION00341000000000000000)
Numeric literals (node16.html#SECTION00342000000000000000)
Operators (node17.html#SECTION00350000000000000000)
Delimiters (node18.html#SECTION00360000000000000000)
---
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Primaries

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# Primaries
Primaries represent the most tightly bound operations of the language.
Their syntax is:
```text
primary: atom | attributeref | subscription | slicing | call
```
---
- Attribute references (node41.html#SECTION00631000000000000000)
Subscriptions (node42.html#SECTION00632000000000000000)
Slicings (node43.html#SECTION00633000000000000000)
Calls (node44.html#SECTION00634000000000000000)
---
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Attribute references

---
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---
## Attribute references
An attribute reference is a primary followed by a period and a name:
```text
attributeref: primary "." identifier
```
The primary must evaluate to an object of a type that supports
attribute references, e.g. a module or a list. This object is then
asked to produce the attribute whose name is the identifier. If this
attribute is not available, the exception `AttributeError` is
raised. Otherwise, the type and value of the object produced is
determined by the object. Multiple evaluations of the same attribute
reference may yield different objects.
---
guido@cwi.nl


Subscriptions

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---
## Subscriptions
A subscription selects an item of a sequence (string, tuple or list)
or mapping (dictionary) object:
```text
subscription: primary "[" condition "]"
```
The primary must evaluate to an object of a sequence or mapping type.
If it is a mapping, the condition must evaluate to an object whose
value is one of the keys of the mapping, and the subscription selects
the value in the mapping that corresponds to that key.
If it is a sequence, the condition must evaluate to a plain integer.
If this value is negative, the length of the sequence is added to it
(so that, e.g. `x[-1]` selects the last item of `x`.)
The resulting value must be a nonnegative integer smaller than the
number of items in the sequence, and the subscription selects the item
whose index is that value (counting from zero).
A string's items are characters. A character is not a separate data
type but a string of exactly one character.
---
guido@cwi.nl


Slicings

---
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---
## Slicings
A slicing (or slice) selects a range of items in a sequence (string,
tuple or list) object:
```text
slicing: primary "[" [condition] ":" [condition] "]"
```
The primary must evaluate to a sequence object. The lower and upper
bound expressions, if present, must evaluate to plain integers;
defaults are zero and the sequence's length, respectively. If either
bound is negative, the sequence's length is added to it. The slicing
now selects all items with index k such that
`i <= k < j` where i
and j are the specified lower and upper bounds. This may be an
empty sequence. It is not an error if i or j lie outside the
range of valid indexes (such items don't exist so they aren't
selected).
---
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Calls

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---
## Calls
A call calls a callable object (e.g. a function) with a possibly empty
series of arguments:
```text
call: primary "(" [condition_list] ")"
```
The primary must evaluate to a callable object (user-defined
functions, built-in functions, methods of built-in objects, class
objects, and methods of class instances are callable). If it is a
class, the argument list must be empty; otherwise, the arguments are
evaluated.
A call always returns some value, possibly `None`, unless it
raises an exception. How this value is computed depends on the type
of the callable object. If it is:
---
guido@cwi.nl


Unary arithmetic operations

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---
# Unary arithmetic operations
All unary arithmetic (and bit-wise) operations have the same priority:
```text
u_expr: primary | "-" u_expr | "+" u_expr | "~" u_expr
```
The unary `"-"` (minus) operator yields the negation of its
numeric argument.
The unary `"+"` (plus) operator yields its numeric argument
unchanged.
The unary `"~"` (invert) operator yields the bit-wise inversion
of its plain or long integer argument. The bit-wise inversion of
`x` is defined as `-(x+1)`.
In all three cases, if the argument does not have the proper type,
a `TypeError` exception is raised.
---
guido@cwi.nl


Binary arithmetic operations

---
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---
# Binary arithmetic operations
The binary arithmetic operations have the conventional priority
levels. Note that some of these operations also apply to certain
non-numeric types. There is no ``power'' operator, so there are only
two levels, one for multiplicative operators and one for additive
operators:
```text
m_expr: u_expr | m_expr "*" u_expr
| m_expr "/" u_expr | m_expr "%" u_expr
a_expr: m_expr | aexpr "+" m_expr | aexpr "-" m_expr
```
The `"*"` (multiplication) operator yields the product of its
arguments. The arguments must either both be numbers, or one argument
must be a plain integer and the other must be a sequence. In the
former case, the numbers are converted to a common type and then
multiplied together. In the latter case, sequence repetition is
performed; a negative repetition factor yields an empty sequence.
The `"/"` (division) operator yields the quotient of its
arguments. The numeric arguments are first converted to a common
type. Plain or long integer division yields an integer of the same
type; the result is that of mathematical division with the `floor'
function applied to the result. Division by zero raises the
`ZeroDivisionError` exception.
The `"%"` (modulo) operator yields the remainder from the
division of the first argument by the second. The numeric arguments
are first converted to a common type. A zero right argument raises
the `ZeroDivisionError` exception. The arguments may be floating
point numbers, e.g. `3.14 % 0.7` equals `0.34`. The modulo
operator always yields a result with the same sign as its second
operand (or zero); the absolute value of the result is strictly
smaller than the second operand.
The integer division and modulo operators are connected by the
following identity: `x == (x/y)*y + (x%y)`. Integer division and
modulo are also connected with the built-in function `divmod()`:
`divmod(x, y) == (x/y, x%y)`. These identities don't hold for
floating point numbers; there a similar identity holds where
`x/y` is replaced by `floor(x/y)`).
The `"+"` (addition) operator yields the sum of its arguments.
The arguments must either both be numbers, or both sequences of the
same type. In the former case, the numbers are converted to a common
type and then added together. In the latter case, the sequences are
concatenated.
The `"-"` (subtraction) operator yields the difference of its
arguments. The numeric arguments are first converted to a common
type.
---
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---
---
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Shifting operations

---
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---
# Shifting operations
The shifting operations have lower priority than the arithmetic
operations:
```text
shift_expr: a_expr | shift_expr ( "<<" | ">>" ) a_expr
```
These operators accept plain or long integers as arguments. The
arguments are converted to a common type. They shift the first
argument to the left or right by the number of bits given by the
second argument.
A right shift by n bits is defined as division by
`pow(2, n )`. A left shift by n bits is defined as
multiplication with `pow(2, n )`; for plain integers there is
no overflow check so this drops bits and flips the sign if the result
is not less than `pow(2,31)` in absolute value.
Negative shift counts raise a `ValueError` exception.
---
guido@cwi.nl


Binary bit-wise operations

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---
# Binary bit-wise operations
Each of the three bitwise operations has a different priority level:
```text
and_expr: shift_expr | and_expr "&" shift_expr
xor_expr: and_expr | xor_expr "^" and_expr
or_expr: xor_expr | or_expr "|" xor_expr
```
The `"&"` operator yields the bitwise AND of its arguments, which
must be plain or long integers. The arguments are converted to a
common type.
The `"^"` operator yields the bitwise XOR (exclusive OR) of its
arguments, which must be plain or long integers. The arguments are
converted to a common type.
The `"|"` operator yields the bitwise (inclusive) OR of its
arguments, which must be plain or long integers. The arguments are
converted to a common type.
---
guido@cwi.nl


Comparisons

---
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---
# Comparisons
Contrary to C, all comparison operations in Python have the same
priority, which is lower than that of any arithmetic, shifting or
bitwise operation. Also contrary to C, expressions like
`a < b < c` have the interpretation that is conventional in
mathematics:
```text
comparison: or_expr (comp_operator or_expr)*
comp_operator: "<"|">"|"=="|">="|"<="|"<>"|"!="|"is" ["not"]|["not"] "in"
```
Comparisons yield integer values: 1 for true, 0 for false.
Comparisons can be chained arbitrarily, e.g. `x < y <= z` is
equivalent to `x < y and y <= z`, except that `y` is
evaluated only once (but in both cases `z` is not evaluated at all
when `x < y` is found to be false).
Formally, if a, b, c, ..., y, z are
expressions and opa, opb, ..., opy are comparison
operators, then a opa b opb c ...y opy z is equivalent
to a opa b `and` b opb c `and` ...`and`
y opy z, except that each expression is evaluated at most once.
Note that a opa b opb c doesn't imply any kind of comparison
between a and c, so that e.g. `x < y > z` is
perfectly legal (though perhaps not pretty).
The forms `<>` and `!=` are equivalent; for consistency with
C, `!=` is preferred; where `!=` is mentioned below
`<>` is also implied.
The operators "<", ">", "==", ">=", "<=", and "!=" compare
the values of two objects. The objects needn't have the same type.
If both are numbers, they are coverted to a common type. Otherwise,
objects of different types always compare unequal, and are
ordered consistently but arbitrarily.
(This unusual definition of comparison is done to simplify the
definition of operations like sorting and the `in` and
`not` `in` operators.)
Comparison of objects of the same type depends on the type:
- Numbers are compared arithmetically.
Strings are compared lexicographically using the numeric equivalents
(the result of the built-in function `ord`) of their characters.
Tuples and lists are compared lexicographically using comparison of
corresponding items.
Mappings (dictionaries) are compared through lexicographic
comparison of their sorted (key, value) lists.
footnode.html#937
Most other types compare unequal unless they are the same object;
the choice whether one object is considered smaller or larger than
another one is made arbitrarily but consistently within one
execution of a program.
The operators `in` and `not in` test for sequence
membership: if y is a sequence, `x in y` is
true if and only if there exists an index i such that
`x = y [ i ]`.
`x not in y` yields the inverse truth value. The
exception `TypeError` is raised when y is not a sequence,
or when y is a string and x is not a string of length one.
footnode.html#827
The operators `is` and `is not` test for object identity:
x `is` y is true if and only if x and y
are the same object. x `is not` y yields the inverse
truth value.
---
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---
---
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Line structure

---
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---
# Line structure
A Python program is divided in a number of logical lines. The end of
a logical line is represented by the token NEWLINE. Statements cannot
cross logical line boundaries except where NEWLINE is allowed by the
syntax (e.g. between statements in compound statements).
---
- Comments (node6.html#SECTION00311000000000000000)
Explicit line joining (node7.html#SECTION00312000000000000000)
Implicit line joining (node8.html#SECTION00313000000000000000)
Blank lines (node9.html#SECTION00314000000000000000)
Indentation (node10.html#SECTION00315000000000000000)
---
guido@cwi.nl


Boolean operations

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---
# Boolean operations
Boolean operations have the lowest priority of all Python operations:
```text
condition: or_test | lambda_form
or_test: and_test | or_test "or" and_test
and_test: not_test | and_test "and" not_test
not_test: comparison | "not" not_test
lambda_form: "lambda" [parameter_list]: condition
```
In the context of Boolean operations, and also when conditions are
used by control flow statements, the following values are interpreted
as false: `None`, numeric zero of all types, empty sequences
(strings, tuples and lists), and empty mappings (dictionaries). All
other values are interpreted as true.
The operator `not` yields 1 if its argument is false, 0 otherwise.
The condition x `and` y first evaluates x; if
x is false, its value is returned; otherwise, y is
evaluated and the resulting value is returned.
The condition x `or` y first evaluates x; if
x is true, its value is returned; otherwise, y is
evaluated and the resulting value is returned.
(Note that `and` and `or` do not restrict the value and type
they return to 0 and 1, but rather return the last evaluated argument.
This is sometimes useful, e.g. if `s` is a string that should be
replaced by a default value if it is empty, the expression
`s or 'foo'` yields the desired value. Because `not` has to
invent a value anyway, it does not bother to return a value of the
same type as its argument, so e.g. `not 'foo'` yields `0`,
not `''`.)
Lambda forms (lambda expressions) have the same syntactic position as
conditions. They are a shorthand to create anonymous functions; the
expression lambda arguments: condition
yields a function object that behaves virtually identical to one
defined with
def name (arguments): return condition.
See section node72.html#function for the syntax of
parameter lists. Note that functions created with lambda forms cannot
contain statements.
---
guido@cwi.nl


Expression lists and condition lists

---
node52.html node31.html node50.html node1.html node79.html
---
# Expression lists and condition lists
```text
expr_list: or_expr ("," or_expr)* [","]
cond_list: condition ("," condition)* [","]
```
The only difference between expression lists and condition lists is
the lowest priority of operators that can be used in them without
being enclosed in parentheses; condition lists allow all operators,
while expression lists don't allow comparisons and Boolean operators
(they do allow bitwise and shift operators though).
Expression lists are used in expression statements and assignments;
condition lists are used everywhere else where a list of
comma-separated values is required.
An expression (condition) list containing at least one comma yields a
tuple. The length of the tuple is the number of expressions
(conditions) in the list. The expressions (conditions) are evaluated
from left to right. (Condition lists are used syntactically is a few
places where no tuple is constructed but a list of values is needed
nevertheless.)
The trailing comma is required only to create a single tuple (a.k.a. a
singleton); it is optional in all other cases. A single
expression (condition) without a trailing comma doesn't create a
tuple, but rather yields the value of that expression (condition).
(To create an empty tuple, use an empty pair of parentheses:
`()`.)
---
guido@cwi.nl


Summary

---
node53.html node31.html node51.html node1.html node79.html
---
# Summary
The following table summarizes the operator precedences in Python,
from lowest precedence (least binding) to highest precedence (most
binding). Operators in the same box have the same precedence. Unless
the syntax is explicitly given, operators are binary. Operators in
the same box group left to right (except for comparisons, which
chain from left to right --- see above).
---
guido@cwi.nl


Simple statements

---
node54.html ref.html node52.html node1.html node79.html
---
# Simple statements
Simple statements are comprised within a single logical line.
Several simple statements may occur on a single line separated
by semicolons. The syntax for simple statements is:
```text
simple_stmt: expression_stmt
| assignment_stmt
| pass_stmt
| del_stmt
| print_stmt
| return_stmt
| raise_stmt
| break_stmt
| continue_stmt
| import_stmt
| global_stmt
| access_stmt
| exec_stmt
```
---
- Expression statements (node54.html#SECTION00710000000000000000)
Assignment statements (node55.html#SECTION00720000000000000000)
The pass statement (node56.html#SECTION00730000000000000000)
The del statement (node57.html#SECTION00740000000000000000)
The print statement (node58.html#SECTION00750000000000000000)
The return statement (node59.html#SECTION00760000000000000000)
The raise statement (node60.html#SECTION00770000000000000000)
The break statement (node61.html#SECTION00780000000000000000)
The continue statement (node62.html#SECTION00790000000000000000)
The import statement (node63.html#SECTION007100000000000000000)
The global statement (node64.html#SECTION007110000000000000000)
The access statement (node65.html#SECTION007120000000000000000)
The exec statement (node66.html#SECTION007130000000000000000)
---
guido@cwi.nl


Expression statements

---
node55.html node53.html node53.html node1.html node79.html
---
# Expression statements
Expression statements are used (mostly interactively) to compute and
write a value, or (usually) to call a procedure (a function that
returns no meaningful result; in Python, procedures return the value
`None`):
```text
expression_stmt: expression_list
```
An expression statement evaluates the expression list (which may be a
single expression). If the value is not `None`, it is converted
to a string using the rules for string conversions (expressions in
reverse quotes), and the resulting string is written to standard
output (see section node58.html#print) on a line by itself.
(The exception for `None` is made so that procedure calls, which
are syntactically equivalent to expressions, do not cause any output.
A tuple with only `None` items is written normally.)
---
guido@cwi.nl


Assignment statements

---
node56.html node53.html node54.html node1.html node79.html
---
# Assignment statements
Assignment statements are used to (re)bind names to values and to
modify attributes or items of mutable objects:
```text
assignment_stmt: (target_list "=")+ expression_list
target_list: target ("," target)* [","]
target: identifier | "(" target_list ")" | "[" target_list "]"
| attributeref | subscription | slicing
```
(See section node40.html#primaries for the syntax definitions for the last
three symbols.)
An assignment statement evaluates the expression list (remember that
this can be a single expression or a comma-separated list, the latter
yielding a tuple) and assigns the single resulting object to each of
the target lists, from left to right.
Assignment is defined recursively depending on the form of the target
(list). When a target is part of a mutable object (an attribute
reference, subscription or slicing), the mutable object must
ultimately perform the assignment and decide about its validity, and
may raise an exception if the assignment is unacceptable. The rules
observed by various types and the exceptions raised are given with the
definition of the object types (see section node21.html#types).
Assignment of an object to a target list is recursively defined as
follows.
- If the target list is a single target: the object is assigned to that
target.
If the target list is a comma-separated list of targets: the object
must be a tuple with the same number of items as the list contains
targets, and the items are assigned, from left to right, to the
corresponding targets.
Assignment of an object to a single target is recursively defined as
follows.
(In the current implementation, the syntax for targets is taken
to be the same as for expressions, and invalid syntax is rejected
during the code generation phase, causing less detailed error
messages.)
WARNING: Although the definition of assignment implies that overlaps
between the left-hand side and the right-hand side are `safe' (e.g.
`a, b = b, a` swaps two variables), overlaps within the
collection of assigned-to variables are not safe! For instance, the
following program prints [0, 2]@:
```text
x = [0, 1]
i = 0
i, x[i] = 1, 2
print x
```
---
node56.html node53.html node54.html node1.html node79.html
---
---
guido@cwi.nl


The pass statement

---
node57.html node53.html node55.html node1.html node79.html
---
# The pass statement
```text
pass_stmt: "pass"
```
`pass` is a null operation --- when it is executed, nothing
happens. It is useful as a placeholder when a statement is
required syntactically, but no code needs to be executed, for example:
```text
def f(arg): pass # a function that does nothing (yet)

class C: pass # an class with no methods (yet)
```
---
guido@cwi.nl


The del statement

---
node58.html node53.html node56.html node1.html node79.html
---
# The del statement
```text
del_stmt: "del" target_list
```
Deletion is recursively defined very similar to the way assignment is
defined. Rather that spelling it out in full details, here are some
hints.
Deletion of a target list recursively deletes each target, from left
to right.
Deletion of a name removes the binding of that name (which must exist)
from the local or global name space, depending on whether the name
occurs in a `global` statement in the same code block.
Deletion of attribute references, subscriptions and slicings
is passed to the primary object involved; deletion of a slicing
is in general equivalent to assignment of an empty slice of the
right type (but even this is determined by the sliced object).
---
guido@cwi.nl


The print statement

---
node59.html node53.html node57.html node1.html node79.html
---
# The print statement
```text
print_stmt: "print" [ condition ("," condition)* [","] ]
```
`print` evaluates each condition in turn and writes the resulting
object to standard output (see below). If an object is not a string,
it is first converted to a string using the rules for string
conversions. The (resulting or original) string is then written. A
space is written before each object is (converted and) written, unless
the output system believes it is positioned at the beginning of a
line. This is the case: (1) when no characters have yet been written
to standard output; or (2) when the last character written to standard
output is `\n`; or (3) when the last write operation on standard
output was not a `print` statement. (In some cases it may be
functional to write an empty string to standard output for this
reason.)
A `"\n"` character is written at the end, unless the `print`
statement ends with a comma. This is the only action if the statement
contains just the keyword `print`.
Standard output is defined as the file object named `stdout`
in the built-in module `sys`. If no such object exists,
or if it is not a writable file, a `RuntimeError` exception is raised.
(The original implementation attempts to write to the system's original
standard output instead, but this is not safe, and should be fixed.)
---
guido@cwi.nl


The return statement

---
node60.html node53.html node58.html node1.html node79.html
---
# The return statement
```text
return_stmt: "return" [condition_list]
```
`return` may only occur syntactically nested in a function
definition, not within a nested class definition.
If a condition list is present, it is evaluated, else `None`
is substituted.
`return` leaves the current function call with the condition
list (or `None`) as return value.
When `return` passes control out of a `try` statement
with a `finally` clause, that finally clause is executed
before really leaving the function.
---
guido@cwi.nl


Comments

---
node7.html node5.html node5.html node1.html node79.html
---
## Comments
A comment starts with a hash character (`#`) that is not part of
a string literal, and ends at the end of the physical line. A comment
always signifies the end of the logical line. Comments are ignored by
the syntax.
---
guido@cwi.nl


The raise statement

---
node61.html node53.html node59.html node1.html node79.html
---
# The raise statement
```text
raise_stmt: "raise" condition ["," condition]
```
`raise` evaluates its first condition, which must yield
a string, class, or instance object. If there is a second condition,
this is evaluated, else `None` is substituted. If the first
condition is a class object, then the second condition must be an
instance of that class or one of its derivatives. If the first
condition is an instance object, the second condition must be
`None`.
If the first object is a class or string, it then raises the exception
identified by the first object, with the second one (or `None`)
as its parameter. If the first object is an instance, it raises the
exception identified by the class of the object, with the instance as
its parameter.
---
guido@cwi.nl


The break statement

---
node62.html node53.html node60.html node1.html node79.html
---
# The break statement
```text
break_stmt: "break"
```
`break` may only occur syntactically nested in a `for`
or `while` loop, but not nested in a function or class definition
within that loop.
It terminates the nearest enclosing loop, skipping the optional
`else` clause if the loop has one.
If a `for` loop is terminated by `break`, the loop control
target keeps its current value.
When `break` passes control out of a `try` statement
with a `finally` clause, that finally clause is executed
before really leaving the loop.
---
guido@cwi.nl


The continue statement

---
node63.html node53.html node61.html node1.html node79.html
---
# The continue statement
```text
continue_stmt: "continue"
```
`continue` may only occur syntactically nested in a `for` or
`while` loop, but not nested in a function or class definition or
`try` statement within that loop.footnode.html#1125
It continues with the next cycle of the nearest enclosing loop.
---
guido@cwi.nl


The import statement

---
node64.html node53.html node62.html node1.html node79.html
---
# The import statement
```text
import_stmt: "import" identifier ("," identifier)*
| "from" identifier "import" identifier ("," identifier)*
| "from" identifier "import" "*"
```
Import statements are executed in two steps: (1) find a module, and
initialize it if necessary; (2) define a name or names in the local
name space (of the scope where the `import` statement occurs).
The first form (without `from`) repeats these steps for each
identifier in the list, the `from` form performs them once, with
the first identifier specifying the module name.
The system maintains a table of modules that have been initialized,
indexed by module name. (The current implementation makes this table
accessible as `sys.modules`.) When a module name is found in
this table, step (1) is finished. If not, a search for a module
definition is started. This first looks for a built-in module
definition, and if no built-in module if the given name is found, it
searches a user-specified list of directories for a file whose name is
the module name with extension `".py"`. (The current
implementation uses the list of strings `sys.path` as the search
path; it is initialized from the shell environment variable
`$PYTHONPATH`, with an installation-dependent default.)
If a built-in module is found, its built-in initialization code is
executed and step (1) is finished. If no matching file is found,
`ImportError` is raised. If a file is found, it is parsed,
yielding an executable code block. If a syntax error occurs,
`SyntaxError` is raised. Otherwise, an empty module of the given
name is created and inserted in the module table, and then the code
block is executed in the context of this module. Exceptions during
this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can
begin.
The first form of `import` statement binds the module name in the
local name space to the module object, and then goes on to import the
next identifier, if any. The `from` from does not bind the
module name: it goes through the list of identifiers, looks each one
of them up in the module found in step (1), and binds the name in the
local name space to the object thus found. If a name is not found,
`ImportError` is raised. If the list of identifiers is replaced
by a star (`*`), all names defined in the module are bound,
except those beginning with an underscore(`_`).
Names bound by import statements may not occur in `global`
statements in the same scope.
The `from` form with `*` may only occur in a module scope.
(The current implementation does not enforce the latter two
restrictions, but programs should not abuse this freedom, as future
implementations may enforce them or silently change the meaning of the
program.)
---
node64.html node53.html node62.html node1.html node79.html
---
---
guido@cwi.nl


The global statement

---
node65.html node53.html node63.html node1.html node79.html
---
# The global statement
```text
global_stmt: "global" identifier ("," identifier)*
```
The `global` statement is a declaration which holds for the
entire current scope. It means that the listed identifiers are to be
interpreted as globals. While using global names is automatic
if they are not defined in the local scope, assigning to global
names would be impossible without `global`.
Names listed in a `global` statement must not be used in the same
scope before that `global` statement is executed.
Names listed in a `global` statement must not be defined as formal
parameters or in a `for` loop control target, `class`
definition, function definition, or `import` statement.
(The current implementation does not enforce the latter two
restrictions, but programs should not abuse this freedom, as future
implementations may enforce them or silently change the meaning of the
program.)
---
guido@cwi.nl


The access statement

---
node66.html node53.html node64.html node1.html node79.html
---
# The access statement
```text
access_stmt: "access" ...
```
This statement will be used in the future to control access to
instance and class variables. Currently its syntax and effects are
undefined; however the keyword `access` is a reserved word for
the parser.
---
guido@cwi.nl


The exec statement

---
node67.html node53.html node65.html node1.html node79.html
---
# The exec statement
```text
exec_stmt: "exec" expression ["in" expression ["," expression]]
```
This statement supports dynamic execution of Python code. The first
expression should evaluate to either a string, an open file object, or
a code object. If it is a string, the string is parsed as a suite of
Python statements which is then executed (unless a syntax error
occurs). If it is an open file, the file is parsed until EOF and
executed. If it is a code object, it is simply executed.
In all cases, if the optional parts are omitted, the code is executed
in the current scope. If only the first expression after `in` is
specified, it should be a dictionary, which will be used for both the
global and the local variables. If two expressions are given, both
must be dictionaries and they are used for the global and local
variables, respectively.
Hints: dynamic evaluation of expressions is supported by the built-in
function `eval()`. The built-in function `vars()` returns
the current local dictionary, which may be useful to pass around for
use by `exec`.
---
guido@cwi.nl


Compound statements

---
node68.html ref.html node66.html node1.html node79.html
---
# Compound statements
Compound statements contain (groups of) other statements; they affect
or control the execution of those other statements in some way. In
general, compound statements span multiple lines, although in simple
incarnations a whole compound statement may be contained in one line.
The `if`, `while` and `for` statements implement
traditional control flow constructs. `try` specifies exception
handlers and/or cleanup code for a group of statements. Function and
class definitions are also syntactically compound statements.
Compound statements consist of one or more `clauses'. A clause
consists of a header and a `suite'. The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon. A suite is a group of statements controlled by a
clause. A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header's
colon, or it can be one or more indented statements on subsequent
lines. Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn't be
clear to which `if` clause a following `else` clause would
belong:
```text
if test1: if test2: print x
```
Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
`print` statements are executed:
```text
if x < y < z: print x; print y; print z
```
Summarizing:
```text
compound_stmt: if_stmt | while_stmt | for_stmt
| try_stmt | funcdef | classdef
suite: stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
statement: stmt_list NEWLINE | compound_stmt
stmt_list: simple_stmt (";" simple_stmt)* [";"]
```
Note that statements always end in a `NEWLINE` possibly followed
by a `DEDENT`.
Also note that optional continuation clauses always begin with a
keyword that cannot start a statement, thus there are no ambiguities
(the `dangling `else`' problem is solved in Python by requiring
nested `if` statements to be indented).
The formatting of the grammar rules in the following sections places
each clause on a separate line for clarity.
---
- The if statement (node68.html#SECTION00810000000000000000)
The while statement (node69.html#SECTION00820000000000000000)
The for statement (node70.html#SECTION00830000000000000000)
The try statement (node71.html#SECTION00840000000000000000)
Function definitions (node72.html#SECTION00850000000000000000)
Class definitions (node73.html#SECTION00860000000000000000)
---
node68.html ref.html node66.html node1.html node79.html
---
---
guido@cwi.nl


The if statement

---
node69.html node67.html node67.html node1.html node79.html
---
# The if statement
The `if` statement is used for conditional execution:
```text
if_stmt: "if" condition ":" suite
("elif" condition ":" suite)*
["else" ":" suite]
```
It selects exactly one of the suites by evaluating the conditions one
by one until one is found to be true (see section node50.html#Booleans for
the definition of true and false); then that suite is executed (and no
other part of the `if` statement is executed or evaluated). If
all conditions are false, the suite of the `else` clause, if
present, is executed.
---
guido@cwi.nl


The while statement

---
node70.html node67.html node68.html node1.html node79.html
---
# The while statement
The `while` statement is used for repeated execution as long as a
condition is true:
```text
while_stmt: "while" condition ":" suite
["else" ":" suite]
```
This repeatedly tests the condition and, if it is true, executes the
first suite; if the condition is false (which may be the first time it
is tested) the suite of the `else` clause, if present, is
executed and the loop terminates.
A `break` statement executed in the first suite terminates the
loop without executing the `else` clause's suite. A
`continue` statement executed in the first suite skips the rest
of the suite and goes back to testing the condition.
---
guido@cwi.nl


Explicit line joining

---
node8.html node5.html node6.html node1.html node79.html
---
## Explicit line joining
Two or more physical lines may be joined into logical lines using
backslash characters (`\`), as follows: when a physical line ends
in a backslash that is not part of a string literal or comment, it is
joined with the following forming a single logical line, deleting the
backslash and the following end-of-line character. For example:
```text
if 1900 < year < 2100 and 1 <= month <= 12 \
and 1 <= day <= 31 and 0 <= hour < 24 \
and 0 <= minute < 60 and 0 <= second < 60: # Looks like a valid date
return 1
```
A line ending in a backslash cannot carry a comment; a backslash does
not continue a comment (but it does continue a string literal, see
below).
---
guido@cwi.nl


The for statement

---
node71.html node67.html node69.html node1.html node79.html
---
# The for statement
The `for` statement is used to iterate over the elements of a
sequence (string, tuple or list):
```text
for_stmt: "for" target_list "in" condition_list ":" suite
["else" ":" suite]
```
The condition list is evaluated once; it should yield a sequence. The
suite is then executed once for each item in the sequence, in the
order of ascending indices. Each item in turn is assigned to the
target list using the standard rules for assignments, and then the
suite is executed. When the items are exhausted (which is immediately
when the sequence is empty), the suite in the `else` clause, if
present, is executed, and the loop terminates.
A `break` statement executed in the first suite terminates the
loop without executing the `else` clause's suite. A
`continue` statement executed in the first suite skips the rest
of the suite and continues with the next item, or with the `else`
clause if there was no next item.
The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.
The target list is not deleted when the loop is finished, but if the
sequence is empty, it will not have been assigned to at all by the
loop.
Hint: the built-in function `range()` returns a sequence of
integers suitable to emulate the effect of Pascal's
`for i := a to b do`;
e.g. `range(3)` returns the list `[0, 1, 2]`.
Warning: There is a subtlety when the sequence is being modified
by the loop (this can only occur for mutable sequences, i.e. lists).
An internal counter is used to keep track of which item is used next,
and this is incremented on each iteration. When this counter has
reached the length of the sequence the loop terminates. This means that
if the suite deletes the current (or a previous) item from the
sequence, the next item will be skipped (since it gets the index of
the current item which has already been treated). Likewise, if the
suite inserts an item in the sequence before the current item, the
current item will be treated again the next time through the loop.
This can lead to nasty bugs that can be avoided by making a temporary
copy using a slice of the whole sequence, e.g.
```text
for x in a[:]:
if x < 0: a.remove(x)
```
---
node71.html node67.html node69.html node1.html node79.html
---
---
guido@cwi.nl


The try statement

---
node72.html node67.html node70.html node1.html node79.html
---
# The try statement
The `try` statement specifies exception handlers and/or cleanup
code for a group of statements:
```text
try_stmt: try_exc_stmt | try_fin_stmt
try_exc_stmt: "try" ":" suite
("except" [condition ["," target]] ":" suite)+
["else" ":" suite]
try_fin_stmt: "try" ":" suite
"finally" ":" suite
```
There are two forms of `try` statement: `try...except` and
`try...finally`. These forms cannot be mixed.
The `try...except` form specifies one or more exception handlers
(the `except` clauses). When no exception occurs in the
`try` clause, no exception handler is executed. When an
exception occurs in the `try` suite, a search for an exception
handler is started. This inspects the except clauses in turn until
one is found that matches the exception. A condition-less except
clause, if present, must be last; it matches any exception. For an
except clause with a condition, that condition is evaluated, and the
clause matches the exception if the resulting object is ``compatible''
with the exception. An object is compatible with an exception if it
is either the object that identifies the exception, or (for exceptions
that are classes) it is a base class of the exception, or it is a
tuple containing an item that is compatible with the exception. Note
that the object identities must match, i.e. it must be the same
object, not just an object with the same value.
If no except clause matches the exception, the search for an exception
handler continues in the surrounding code and on the invocation stack.
If the evaluation of a condition in the header of an except clause
raises an exception, the original search for a handler is cancelled
and a search starts for the new exception in the surrounding code and
on the call stack (it is treated as if the entire `try` statement
raised the exception).
When a matching except clause is found, the exception's parameter is
assigned to the target specified in that except clause, if present,
and the except clause's suite is executed. When the end of this suite
is reached, execution continues normally after the entire try
statement. (This means that if two nested handlers exist for the same
exception, and the exception occurs in the try clause of the inner
handler, the outer handler will not handle the exception.)
Before an except clause's suite is executed, details about the
exception are assigned to three variables in the `sys` module:
`sys.exc_type` receives the object identifying the exception;
`sys.exc_value` receives the exception's parameter;
`sys.exc_traceback` receives a traceback object (see section
node21.html#traceback) identifying the point in the program where the
exception occurred.
The optional `else` clause is executed when no exception occurs
in the `try` clause. Exceptions in the `else` clause are
not handled by the preceding `except` clauses.
The `try...finally` form specifies a `cleanup' handler. The
`try` clause is executed. When no exception occurs, the
`finally` clause is executed. When an exception occurs in the
`try` clause, the exception is temporarily saved, the
`finally` clause is executed, and then the saved exception is
re-raised. If the `finally` clause raises another exception or
executes a `return`, `break` or `continue` statement,
the saved exception is lost.
When a `return` or `break` statement is executed in the
`try` suite of a `try...finally` statement, the
`finally` clause is also executed `on the way out'. A
`continue` statement is illegal in the `try` clause. (The
reason is a problem with the current implementation --- this
restriction may be lifted in the future).
---
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Function definitions

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# Function definitions
A function definition defines a user-defined function object (see
section node21.html#types):
```text
funcdef: "def" funcname "(" [parameter_list] ")" ":" suite
parameter_list: (defparameter ",")* ("*" identifier | defparameter [","])
defparameter: parameter ["=" condition]
sublist: parameter ("," parameter)* [","]
parameter: identifier | "(" sublist ")"
funcname: identifier
```
A function definition is an executable statement. Its execution binds
the function name in the current local name space to a function object
(a wrapper around the executable code for the function). This
function object contains a reference to the current global name space
as the global name space to be used when the function is called.
The function definition does not execute the function body; this gets
executed only when the function is called.
When one or more top-level parameters have the form parameter =
condition, the function is said to have ``default parameter values''.
Default parameter values are evaluated when the function definition is
executed. For a parameter with a default value, the correponding
argument may be omitted from a call, in which case the parameter's
default value is substituted. If a parameter has a default value, all
following parameters must also have a default value --- this is a
syntactic restriction that is not expressed by the grammar.
footnode.html#1238
Function call semantics are described in section node44.html#calls. When a
user-defined function is called, first missing arguments for which a
default value exists are supplied; then the arguments (a.k.a. actual
parameters) are bound to the (formal) parameters, as follows:
- If there are no formal parameters, there must be no arguments.
If the formal parameter list does not end in a star followed by an
identifier, there must be exactly as many arguments as there are
parameters in the formal parameter list (at the top level); the
arguments are assigned to the formal parameters one by one. Note that
the presence or absence of a trailing comma at the top level in either
the formal or the actual parameter list makes no difference. The
assignment to a formal parameter is performed as if the parameter
occurs on the left hand side of an assignment statement whose right
hand side's value is that of the argument.
If the formal parameter list ends in a star followed by an identifier,
preceded by zero or more comma-followed parameters, there must be at
least as many arguments as there are parameters preceding the star.
Call this number N. The first N arguments are assigned to
the corresponding formal parameters in the way descibed above. A
tuple containing the remaining arguments, if any, is then assigned to
the identifier following the star. This variable will always be a
tuple: if there are no extra arguments, its value is `()`, if
there is just one extra argument, it is a singleton tuple.
Note that the `variable length parameter list' feature only works at
the top level of the parameter list; individual parameters use a model
corresponding more closely to that of ordinary assignment. While the
latter model is generally preferable, because of the greater type
safety it offers (wrong-sized tuples aren't silently mistreated),
variable length parameter lists are a sufficiently accepted practice
in most programming languages that a compromise has been worked out.
(And anyway, assignment has no equivalent for empty argument lists.)
It is also possible to create anonymous functions (functions not bound
to a name), for immediate use in expressions. This uses lambda forms,
described in section node50.html#lambda.
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Class definitions

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# Class definitions
A class definition defines a class object (see section node21.html#types):
```text
classdef: "class" classname [inheritance] ":" suite
inheritance: "(" [condition_list] ")"
classname: identifier
```
A class definition is an executable statement. It first evaluates the
inheritance list, if present. Each item in the inheritance list
should evaluate to a class object. The class's suite is then executed
in a new execution frame (see section node29.html#execframes), using a newly
created local name space and the original global name space.
(Usually, the suite contains only function definitions.) When the
class's suite finishes execution, its execution frame is discarded but
its local name space is saved. A class object is then created using
the inheritance list for the base classes and the saved local name
space for the attribute dictionary. The class name is bound to this
class object in the original local name space.
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Top-level components

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# Top-level components
The Python interpreter can get its input from a number of sources:
from a script passed to it as standard input or as program argument,
typed in interactively, from a module source file, etc. This chapter
gives the syntax used in these cases.
---
- Complete Python programs (node75.html#SECTION00910000000000000000)
File input (node76.html#SECTION00920000000000000000)
Interactive input (node77.html#SECTION00930000000000000000)
Expression input (node78.html#SECTION00940000000000000000)
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Complete Python programs

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# Complete Python programs
While a language specification need not prescribe how the language
interpreter is invoked, it is useful to have a notion of a complete
Python program. A complete Python program is executed in a minimally
initialized environment: all built-in and standard modules are
available, but none have been initialized, except for `sys`
(various system services), `__builtin__` (built-in functions,
exceptions and `None`) and `__main__`. The latter is used
to provide the local and global name space for execution of the
complete program.
The syntax for a complete Python program is that for file input,
described in the next section.
The interpreter may also be invoked in interactive mode; in this case,
it does not read and execute a complete program but reads and executes
one statement (possibly compound) at a time. The initial environment
is identical to that of a complete program; each statement is executed
in the name space of `__main__`.
Under Unix, a complete program can be passed to the interpreter in
three forms: with the -c string command line option, as a
file passed as the first command line argument, or as standard input.
If the file or standard input is a tty device, the interpreter enters
interactive mode; otherwise, it executes the file as a complete
program.
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File input

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# File input
All input read from non-interactive files has the same form:
```text
file_input: (NEWLINE | statement)*
```
This syntax is used in the following situations:
- when parsing a complete Python program (from a file or from a string);
when parsing a module;
when parsing a string passed to the `exec` statement;
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Interactive input

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# Interactive input
Input in interactive mode is parsed using the following grammar:
```text
interactive_input: [stmt_list] NEWLINE | compound_stmt NEWLINE
```
Note that a (top-level) compound statement must be followed by a blank
line in interactive mode; this is needed to help the parser detect the
end of the input.
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Expression input

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# Expression input
There are two forms of expression input. Both ignore leading
whitespace.
The string argument to `eval()` must have the following form:
```text
eval_input: condition_list NEWLINE*
```
The input line read by `input()` must have the following form:
```text
input_input: condition_list NEWLINE
```
Note: to read `raw' input line without interpretation, you can use the
built-in function `raw_input()` or the `readline()` method
of file objects.
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Index

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## Index
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Implicit line joining

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## Implicit line joining
Expressions in parentheses, square brackets or curly braces can be
split over more than one physical line without using backslashes.
For example:
```text
month_names = ['Januari', 'Februari', 'Maart', # These are the
'April', 'Mei', 'Juni', # Dutch names
'Juli', 'Augustus', 'September', # for the months
'Oktober', 'November', 'December'] # of the year
```
Implicitly continued lines can carry comments. The indentation of the
continuation lines is not important. Blank continuation lines are
allowed.
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About this document ...

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# About this document ...
Python Reference Manual
This document was generated using the LaTeX2HTML (http://cbl.leeds.ac.uk/nikos/tex2html/doc/latex2html/latex2html.html) translator Version 95.1 (Fri Jan 20 1995) Copyright © 1993, 1994, Nikos Drakos (http://cbl.leeds.ac.uk/nikos/personal.html), Computer Based Learning Unit, University of Leeds.
The command line arguments were:
latex2html -address guido@cwi.nl -dont_include myformat ref.tex.
The translation was initiated by Guido van Rossum on Tue Apr 11 00:19:37 MDT 1995
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Blank lines

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## Blank lines
A logical line that contains only spaces, tabs, and possibly a
comment, is ignored (i.e., no NEWLINE token is generated), except that
during interactive input of statements, an entirely blank logical line
terminates a multi-line statement.
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