7
.. index:: pair: compound; statement
9
Compound statements contain (groups of) other statements; they affect or control
10
the execution of those other statements in some way. In general, compound
11
statements span multiple lines, although in simple incarnations a whole compound
12
statement may be contained in one line.
14
The :keyword:`if`, :keyword:`while` and :keyword:`for` statements implement
15
traditional control flow constructs. :keyword:`try` specifies exception
16
handlers and/or cleanup code for a group of statements, while the
17
:keyword:`with` statement allows the execution of initialization and
18
finalization code around a block of code. Function and class definitions are
19
also syntactically compound statements.
25
Compound statements consist of one or more 'clauses.' A clause consists of a
26
header and a 'suite.' The clause headers of a particular compound statement are
27
all at the same indentation level. Each clause header begins with a uniquely
28
identifying keyword and ends with a colon. A suite is a group of statements
29
controlled by a clause. A suite can be one or more semicolon-separated simple
30
statements on the same line as the header, following the header's colon, or it
31
can be one or more indented statements on subsequent lines. Only the latter
32
form of suite can contain nested compound statements; the following is illegal,
33
mostly because it wouldn't be clear to which :keyword:`if` clause a following
34
:keyword:`else` clause would belong::
36
if test1: if test2: print(x)
38
Also note that the semicolon binds tighter than the colon in this context, so
39
that in the following example, either all or none of the :func:`print` calls are
42
if x < y < z: print(x); print(y); print(z)
47
compound_stmt: `if_stmt`
54
suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
55
statement: `stmt_list` NEWLINE | `compound_stmt`
56
stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
63
Note that statements always end in a ``NEWLINE`` possibly followed by a
64
``DEDENT``. Also note that optional continuation clauses always begin with a
65
keyword that cannot start a statement, thus there are no ambiguities (the
66
'dangling :keyword:`else`' problem is solved in Python by requiring nested
67
:keyword:`if` statements to be indented).
69
The formatting of the grammar rules in the following sections places each clause
70
on a separate line for clarity.
77
The :keyword:`if` statement
78
===========================
87
The :keyword:`if` statement is used for conditional execution:
90
if_stmt: "if" `expression` ":" `suite`
91
: ( "elif" `expression` ":" `suite` )*
92
: ["else" ":" `suite`]
94
It selects exactly one of the suites by evaluating the expressions one by one
95
until one is found to be true (see section :ref:`booleans` for the definition of
96
true and false); then that suite is executed (and no other part of the
97
:keyword:`if` statement is executed or evaluated). If all expressions are
98
false, the suite of the :keyword:`else` clause, if present, is executed.
103
The :keyword:`while` statement
104
==============================
109
pair: loop; statement
112
The :keyword:`while` statement is used for repeated execution as long as an
116
while_stmt: "while" `expression` ":" `suite`
117
: ["else" ":" `suite`]
119
This repeatedly tests the expression and, if it is true, executes the first
120
suite; if the expression is false (which may be the first time it is tested) the
121
suite of the :keyword:`else` clause, if present, is executed and the loop
128
A :keyword:`break` statement executed in the first suite terminates the loop
129
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
130
statement executed in the first suite skips the rest of the suite and goes back
131
to testing the expression.
136
The :keyword:`for` statement
137
============================
144
pair: loop; statement
150
The :keyword:`for` statement is used to iterate over the elements of a sequence
151
(such as a string, tuple or list) or other iterable object:
154
for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
155
: ["else" ":" `suite`]
157
The expression list is evaluated once; it should yield an iterable object. An
158
iterator is created for the result of the ``expression_list``. The suite is
159
then executed once for each item provided by the iterator, in the order of
160
ascending indices. Each item in turn is assigned to the target list using the
161
standard rules for assignments (see :ref:`assignment`), and then the suite is
162
executed. When the items are exhausted (which is immediately when the sequence
163
is empty or an iterator raises a :exc:`StopIteration` exception), the suite in
164
the :keyword:`else` clause, if present, is executed, and the loop terminates.
170
A :keyword:`break` statement executed in the first suite terminates the loop
171
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
172
statement executed in the first suite skips the rest of the suite and continues
173
with the next item, or with the :keyword:`else` clause if there was no next
176
The suite may assign to the variable(s) in the target list; this does not affect
177
the next item assigned to it.
182
Names in the target list are not deleted when the loop is finished, but if the
183
sequence is empty, it will not have been assigned to at all by the loop. Hint:
184
the built-in function :func:`range` returns an iterator of integers suitable to
185
emulate the effect of Pascal's ``for i := a to b do``; e.g., ``list(range(3))``
186
returns the list ``[0, 1, 2]``.
191
single: loop; over mutable sequence
192
single: mutable sequence; loop over
194
There is a subtlety when the sequence is being modified by the loop (this can
195
only occur for mutable sequences, i.e. lists). An internal counter is used
196
to keep track of which item is used next, and this is incremented on each
197
iteration. When this counter has reached the length of the sequence the loop
198
terminates. This means that if the suite deletes the current (or a previous)
199
item from the sequence, the next item will be skipped (since it gets the
200
index of the current item which has already been treated). Likewise, if the
201
suite inserts an item in the sequence before the current item, the current
202
item will be treated again the next time through the loop. This can lead to
203
nasty bugs that can be avoided by making a temporary copy using a slice of
204
the whole sequence, e.g., ::
207
if x < 0: a.remove(x)
214
The :keyword:`try` statement
215
============================
221
.. index:: keyword: except
223
The :keyword:`try` statement specifies exception handlers and/or cleanup code
224
for a group of statements:
227
try_stmt: try1_stmt | try2_stmt
228
try1_stmt: "try" ":" `suite`
229
: ("except" [`expression` ["as" `target`]] ":" `suite`)+
230
: ["else" ":" `suite`]
231
: ["finally" ":" `suite`]
232
try2_stmt: "try" ":" `suite`
233
: "finally" ":" `suite`
236
The :keyword:`except` clause(s) specify one or more exception handlers. When no
237
exception occurs in the :keyword:`try` clause, no exception handler is executed.
238
When an exception occurs in the :keyword:`try` suite, a search for an exception
239
handler is started. This search inspects the except clauses in turn until one
240
is found that matches the exception. An expression-less except clause, if
241
present, must be last; it matches any exception. For an except clause with an
242
expression, that expression is evaluated, and the clause matches the exception
243
if the resulting object is "compatible" with the exception. An object is
244
compatible with an exception if it is the class or a base class of the exception
245
object or a tuple containing an item compatible with the exception.
247
If no except clause matches the exception, the search for an exception handler
248
continues in the surrounding code and on the invocation stack. [#]_
250
If the evaluation of an expression in the header of an except clause raises an
251
exception, the original search for a handler is canceled and a search starts for
252
the new exception in the surrounding code and on the call stack (it is treated
253
as if the entire :keyword:`try` statement raised the exception).
255
When a matching except clause is found, the exception is assigned to the target
256
specified after the :keyword:`as` keyword in that except clause, if present, and
257
the except clause's suite is executed. All except clauses must have an
258
executable block. When the end of this block is reached, execution continues
259
normally after the entire try statement. (This means that if two nested
260
handlers exist for the same exception, and the exception occurs in the try
261
clause of the inner handler, the outer handler will not handle the exception.)
263
When an exception has been assigned using ``as target``, it is cleared at the
264
end of the except clause. This is as if ::
277
This means the exception must be assigned to a different name to be able to
278
refer to it after the except clause. Exceptions are cleared because with the
279
traceback attached to them, they form a reference cycle with the stack frame,
280
keeping all locals in that frame alive until the next garbage collection occurs.
286
Before an except clause's suite is executed, details about the exception are
287
stored in the :mod:`sys` module and can be access via :func:`sys.exc_info`.
288
:func:`sys.exc_info` returns a 3-tuple consisting of the exception class, the
289
exception instance and a traceback object (see section :ref:`types`) identifying
290
the point in the program where the exception occurred. :func:`sys.exc_info`
291
values are restored to their previous values (before the call) when returning
292
from a function that handled an exception.
300
The optional :keyword:`else` clause is executed if and when control flows off
301
the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
302
clause are not handled by the preceding :keyword:`except` clauses.
304
.. index:: keyword: finally
306
If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
307
:keyword:`try` clause is executed, including any :keyword:`except` and
308
:keyword:`else` clauses. If an exception occurs in any of the clauses and is
309
not handled, the exception is temporarily saved. The :keyword:`finally` clause
310
is executed. If there is a saved exception it is re-raised at the end of the
311
:keyword:`finally` clause. If the :keyword:`finally` clause raises another
312
exception, the saved exception is set as the context of the new exception.
313
If the :keyword:`finally` clause executes a :keyword:`return` or :keyword:`break`
314
statement, the saved exception is discarded::
325
The exception information is not available to the program during execution of
326
the :keyword:`finally` clause.
333
When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
334
executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
335
statement, the :keyword:`finally` clause is also executed 'on the way out.' A
336
:keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
337
reason is a problem with the current implementation --- this restriction may be
338
lifted in the future).
340
Additional information on exceptions can be found in section :ref:`exceptions`,
341
and information on using the :keyword:`raise` statement to generate exceptions
342
may be found in section :ref:`raise`.
348
The :keyword:`with` statement
349
=============================
351
.. index:: statement: with
353
The :keyword:`with` statement is used to wrap the execution of a block with
354
methods defined by a context manager (see section :ref:`context-managers`).
355
This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
356
usage patterns to be encapsulated for convenient reuse.
359
with_stmt: "with" with_item ("," with_item)* ":" `suite`
360
with_item: `expression` ["as" `target`]
362
The execution of the :keyword:`with` statement with one "item" proceeds as follows:
364
#. The context expression (the expression given in the :token:`with_item`) is
365
evaluated to obtain a context manager.
367
#. The context manager's :meth:`__exit__` is loaded for later use.
369
#. The context manager's :meth:`__enter__` method is invoked.
371
#. If a target was included in the :keyword:`with` statement, the return value
372
from :meth:`__enter__` is assigned to it.
376
The :keyword:`with` statement guarantees that if the :meth:`__enter__`
377
method returns without an error, then :meth:`__exit__` will always be
378
called. Thus, if an error occurs during the assignment to the target list,
379
it will be treated the same as an error occurring within the suite would
380
be. See step 6 below.
382
#. The suite is executed.
384
#. The context manager's :meth:`__exit__` method is invoked. If an exception
385
caused the suite to be exited, its type, value, and traceback are passed as
386
arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
389
If the suite was exited due to an exception, and the return value from the
390
:meth:`__exit__` method was false, the exception is reraised. If the return
391
value was true, the exception is suppressed, and execution continues with the
392
statement following the :keyword:`with` statement.
394
If the suite was exited for any reason other than an exception, the return
395
value from :meth:`__exit__` is ignored, and execution proceeds at the normal
396
location for the kind of exit that was taken.
398
With more than one item, the context managers are processed as if multiple
399
:keyword:`with` statements were nested::
401
with A() as a, B() as b:
410
.. versionchanged:: 3.1
411
Support for multiple context expressions.
415
:pep:`0343` - The "with" statement
416
The specification, background, and examples for the Python :keyword:`with`
421
single: parameter; function definition
431
pair: function; definition
434
object: user-defined function
439
A function definition defines a user-defined function object (see section
443
funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite`
444
decorators: `decorator`+
445
decorator: "@" `dotted_name` ["(" [`parameter_list` [","]] ")"] NEWLINE
446
dotted_name: `identifier` ("." `identifier`)*
447
parameter_list: (`defparameter` ",")*
448
: ( "*" [`parameter`] ("," `defparameter`)* ["," "**" `parameter`]
450
: | `defparameter` [","] )
451
parameter: `identifier` [":" `expression`]
452
defparameter: `parameter` ["=" `expression`]
453
funcname: `identifier`
456
A function definition is an executable statement. Its execution binds the
457
function name in the current local namespace to a function object (a wrapper
458
around the executable code for the function). This function object contains a
459
reference to the current global namespace as the global namespace to be used
460
when the function is called.
462
The function definition does not execute the function body; this gets executed
463
only when the function is called. [#]_
468
A function definition may be wrapped by one or more :term:`decorator` expressions.
469
Decorator expressions are evaluated when the function is defined, in the scope
470
that contains the function definition. The result must be a callable, which is
471
invoked with the function object as the only argument. The returned value is
472
bound to the function name instead of the function object. Multiple decorators
473
are applied in nested fashion. For example, the following code ::
482
func = f1(arg)(f2(func))
485
triple: default; parameter; value
486
single: argument; function definition
488
When one or more :term:`parameters <parameter>` have the form *parameter* ``=``
489
*expression*, the function is said to have "default parameter values." For a
490
parameter with a default value, the corresponding :term:`argument` may be
491
omitted from a call, in which
492
case the parameter's default value is substituted. If a parameter has a default
493
value, all following parameters up until the "``*``" must also have a default
494
value --- this is a syntactic restriction that is not expressed by the grammar.
496
**Default parameter values are evaluated from left to right when the function
497
definition is executed.** This means that the expression is evaluated once, when
498
the function is defined, and that the same "pre-computed" value is used for each
499
call. This is especially important to understand when a default parameter is a
500
mutable object, such as a list or a dictionary: if the function modifies the
501
object (e.g. by appending an item to a list), the default value is in effect
502
modified. This is generally not what was intended. A way around this is to use
503
``None`` as the default, and explicitly test for it in the body of the function,
506
def whats_on_the_telly(penguin=None):
509
penguin.append("property of the zoo")
516
Function call semantics are described in more detail in section :ref:`calls`. A
517
function call always assigns values to all parameters mentioned in the parameter
518
list, either from position arguments, from keyword arguments, or from default
519
values. If the form "``*identifier``" is present, it is initialized to a tuple
520
receiving any excess positional parameters, defaulting to the empty tuple. If
521
the form "``**identifier``" is present, it is initialized to a new dictionary
522
receiving any excess keyword arguments, defaulting to a new empty dictionary.
523
Parameters after "``*``" or "``*identifier``" are keyword-only parameters and
524
may only be passed used keyword arguments.
526
.. index:: pair: function; annotations
528
Parameters may have annotations of the form "``: expression``" following the
529
parameter name. Any parameter may have an annotation even those of the form
530
``*identifier`` or ``**identifier``. Functions may have "return" annotation of
531
the form "``-> expression``" after the parameter list. These annotations can be
532
any valid Python expression and are evaluated when the function definition is
533
executed. Annotations may be evaluated in a different order than they appear in
534
the source code. The presence of annotations does not change the semantics of a
535
function. The annotation values are available as values of a dictionary keyed
536
by the parameters' names in the :attr:`__annotations__` attribute of the
539
.. index:: pair: lambda; expression
541
It is also possible to create anonymous functions (functions not bound to a
542
name), for immediate use in expressions. This uses lambda expressions, described in
543
section :ref:`lambda`. Note that the lambda expression is merely a shorthand for a
544
simplified function definition; a function defined in a ":keyword:`def`"
545
statement can be passed around or assigned to another name just like a function
546
defined by a lambda expression. The ":keyword:`def`" form is actually more powerful
547
since it allows the execution of multiple statements and annotations.
549
**Programmer's note:** Functions are first-class objects. A "``def``" statement
550
executed inside a function definition defines a local function that can be
551
returned or passed around. Free variables used in the nested function can
552
access the local variables of the function containing the def. See section
553
:ref:`naming` for details.
557
:pep:`3107` - Function Annotations
558
The original specification for function annotations.
569
pair: class; definition
572
pair: execution; frame
576
A class definition defines a class object (see section :ref:`types`):
579
classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
580
inheritance: "(" [`parameter_list`] ")"
581
classname: `identifier`
583
A class definition is an executable statement. The inheritance list usually
584
gives a list of base classes (see :ref:`metaclasses` for more advanced uses), so
585
each item in the list should evaluate to a class object which allows
586
subclassing. Classes without an inheritance list inherit, by default, from the
587
base class :class:`object`; hence, ::
597
The class's suite is then executed in a new execution frame (see :ref:`naming`),
598
using a newly created local namespace and the original global namespace.
599
(Usually, the suite contains mostly function definitions.) When the class's
600
suite finishes execution, its execution frame is discarded but its local
601
namespace is saved. [#]_ A class object is then created using the inheritance
602
list for the base classes and the saved local namespace for the attribute
603
dictionary. The class name is bound to this class object in the original local
606
Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`.
608
Classes can also be decorated: just like when decorating functions, ::
617
Foo = f1(arg)(f2(Foo))
619
The evaluation rules for the decorator expressions are the same as for function
620
decorators. The result must be a class object, which is then bound to the class
623
**Programmer's note:** Variables defined in the class definition are class
624
attributes; they are shared by instances. Instance attributes can be set in a
625
method with ``self.name = value``. Both class and instance attributes are
626
accessible through the notation "``self.name``", and an instance attribute hides
627
a class attribute with the same name when accessed in this way. Class
628
attributes can be used as defaults for instance attributes, but using mutable
629
values there can lead to unexpected results. :ref:`Descriptors <descriptors>`
630
can be used to create instance variables with different implementation details.
635
:pep:`3115` - Metaclasses in Python 3
636
:pep:`3129` - Class Decorators
639
.. rubric:: Footnotes
641
.. [#] The exception is propagated to the invocation stack unless
642
there is a :keyword:`finally` clause which happens to raise another
643
exception. That new exception causes the old one to be lost.
645
.. [#] Currently, control "flows off the end" except in the case of an exception
646
or the execution of a :keyword:`return`, :keyword:`continue`, or
647
:keyword:`break` statement.
649
.. [#] A string literal appearing as the first statement in the function body is
650
transformed into the function's ``__doc__`` attribute and therefore the
651
function's :term:`docstring`.
653
.. [#] A string literal appearing as the first statement in the class body is
654
transformed into the namespace's ``__doc__`` item and therefore the class's
added
removed
Doc/reference/compound_stmts.rst
