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
A compound statement consists 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 a 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`
57
suite: `stmt_list` NEWLINE | NEWLINE INDENT `statement`+ DEDENT
58
statement: `stmt_list` NEWLINE | `compound_stmt`
59
stmt_list: `simple_stmt` (";" `simple_stmt`)* [";"]
66
Note that statements always end in a ``NEWLINE`` possibly followed by a
67
``DEDENT``. Also note that optional continuation clauses always begin with a
68
keyword that cannot start a statement, thus there are no ambiguities (the
69
'dangling :keyword:`else`' problem is solved in Python by requiring nested
70
:keyword:`if` statements to be indented).
72
The formatting of the grammar rules in the following sections places each clause
73
on a separate line for clarity.
80
The :keyword:`if` statement
81
===========================
90
The :keyword:`if` statement is used for conditional execution:
93
if_stmt: "if" `expression` ":" `suite`
94
: ( "elif" `expression` ":" `suite` )*
95
: ["else" ":" `suite`]
97
It selects exactly one of the suites by evaluating the expressions one by one
98
until one is found to be true (see section :ref:`booleans` for the definition of
99
true and false); then that suite is executed (and no other part of the
100
:keyword:`if` statement is executed or evaluated). If all expressions are
101
false, the suite of the :keyword:`else` clause, if present, is executed.
106
The :keyword:`while` statement
107
==============================
112
pair: loop; statement
115
The :keyword:`while` statement is used for repeated execution as long as an
119
while_stmt: "while" `expression` ":" `suite`
120
: ["else" ":" `suite`]
122
This repeatedly tests the expression and, if it is true, executes the first
123
suite; if the expression is false (which may be the first time it is tested) the
124
suite of the :keyword:`else` clause, if present, is executed and the loop
131
A :keyword:`break` statement executed in the first suite terminates the loop
132
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
133
statement executed in the first suite skips the rest of the suite and goes back
134
to testing the expression.
139
The :keyword:`for` statement
140
============================
147
pair: loop; statement
153
The :keyword:`for` statement is used to iterate over the elements of a sequence
154
(such as a string, tuple or list) or other iterable object:
157
for_stmt: "for" `target_list` "in" `expression_list` ":" `suite`
158
: ["else" ":" `suite`]
160
The expression list is evaluated once; it should yield an iterable object. An
161
iterator is created for the result of the ``expression_list``. The suite is
162
then executed once for each item provided by the iterator, in the order returned
163
by the iterator. Each item in turn is assigned to the target list using the
164
standard rules for assignments (see :ref:`assignment`), and then the suite is
165
executed. When the items are exhausted (which is immediately when the sequence
166
is empty or an iterator raises a :exc:`StopIteration` exception), the suite in
167
the :keyword:`else` clause, if present, is executed, and the loop terminates.
173
A :keyword:`break` statement executed in the first suite terminates the loop
174
without executing the :keyword:`else` clause's suite. A :keyword:`continue`
175
statement executed in the first suite skips the rest of the suite and continues
176
with the next item, or with the :keyword:`else` clause if there is no next
179
The for-loop makes assignments to the variables(s) in the target list.
180
This overwrites all previous assignments to those variables including
181
those made in the suite of the for-loop::
185
i = 5 # this will not affect the for-loop
186
# because i will be overwritten with the next
193
Names in the target list are not deleted when the loop is finished, but if the
194
sequence is empty, they will not have been assigned to at all by the loop. Hint:
195
the built-in function :func:`range` returns an iterator of integers suitable to
196
emulate the effect of Pascal's ``for i := a to b do``; e.g., ``list(range(3))``
197
returns the list ``[0, 1, 2]``.
202
single: loop; over mutable sequence
203
single: mutable sequence; loop over
205
There is a subtlety when the sequence is being modified by the loop (this can
206
only occur for mutable sequences, i.e. lists). An internal counter is used
207
to keep track of which item is used next, and this is incremented on each
208
iteration. When this counter has reached the length of the sequence the loop
209
terminates. This means that if the suite deletes the current (or a previous)
210
item from the sequence, the next item will be skipped (since it gets the
211
index of the current item which has already been treated). Likewise, if the
212
suite inserts an item in the sequence before the current item, the current
213
item will be treated again the next time through the loop. This can lead to
214
nasty bugs that can be avoided by making a temporary copy using a slice of
215
the whole sequence, e.g., ::
218
if x < 0: a.remove(x)
225
The :keyword:`try` statement
226
============================
232
.. index:: keyword: except
234
The :keyword:`try` statement specifies exception handlers and/or cleanup code
235
for a group of statements:
238
try_stmt: try1_stmt | try2_stmt
239
try1_stmt: "try" ":" `suite`
240
: ("except" [`expression` ["as" `identifier`]] ":" `suite`)+
241
: ["else" ":" `suite`]
242
: ["finally" ":" `suite`]
243
try2_stmt: "try" ":" `suite`
244
: "finally" ":" `suite`
247
The :keyword:`except` clause(s) specify one or more exception handlers. When no
248
exception occurs in the :keyword:`try` clause, no exception handler is executed.
249
When an exception occurs in the :keyword:`try` suite, a search for an exception
250
handler is started. This search inspects the except clauses in turn until one
251
is found that matches the exception. An expression-less except clause, if
252
present, must be last; it matches any exception. For an except clause with an
253
expression, that expression is evaluated, and the clause matches the exception
254
if the resulting object is "compatible" with the exception. An object is
255
compatible with an exception if it is the class or a base class of the exception
256
object or a tuple containing an item compatible with the exception.
258
If no except clause matches the exception, the search for an exception handler
259
continues in the surrounding code and on the invocation stack. [#]_
261
If the evaluation of an expression in the header of an except clause raises an
262
exception, the original search for a handler is canceled and a search starts for
263
the new exception in the surrounding code and on the call stack (it is treated
264
as if the entire :keyword:`try` statement raised the exception).
266
When a matching except clause is found, the exception is assigned to the target
267
specified after the :keyword:`as` keyword in that except clause, if present, and
268
the except clause's suite is executed. All except clauses must have an
269
executable block. When the end of this block is reached, execution continues
270
normally after the entire try statement. (This means that if two nested
271
handlers exist for the same exception, and the exception occurs in the try
272
clause of the inner handler, the outer handler will not handle the exception.)
274
When an exception has been assigned using ``as target``, it is cleared at the
275
end of the except clause. This is as if ::
288
This means the exception must be assigned to a different name to be able to
289
refer to it after the except clause. Exceptions are cleared because with the
290
traceback attached to them, they form a reference cycle with the stack frame,
291
keeping all locals in that frame alive until the next garbage collection occurs.
297
Before an except clause's suite is executed, details about the exception are
298
stored in the :mod:`sys` module and can be accessed via :func:`sys.exc_info`.
299
:func:`sys.exc_info` returns a 3-tuple consisting of the exception class, the
300
exception instance and a traceback object (see section :ref:`types`) identifying
301
the point in the program where the exception occurred. :func:`sys.exc_info`
302
values are restored to their previous values (before the call) when returning
303
from a function that handled an exception.
311
The optional :keyword:`else` clause is executed if and when control flows off
312
the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
313
clause are not handled by the preceding :keyword:`except` clauses.
315
.. index:: keyword: finally
317
If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
318
:keyword:`try` clause is executed, including any :keyword:`except` and
319
:keyword:`else` clauses. If an exception occurs in any of the clauses and is
320
not handled, the exception is temporarily saved. The :keyword:`finally` clause
321
is executed. If there is a saved exception it is re-raised at the end of the
322
:keyword:`finally` clause. If the :keyword:`finally` clause raises another
323
exception, the saved exception is set as the context of the new exception.
324
If the :keyword:`finally` clause executes a :keyword:`return` or :keyword:`break`
325
statement, the saved exception is discarded::
336
The exception information is not available to the program during execution of
337
the :keyword:`finally` clause.
344
When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
345
executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
346
statement, the :keyword:`finally` clause is also executed 'on the way out.' A
347
:keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
348
reason is a problem with the current implementation --- this restriction may be
349
lifted in the future).
351
The return value of a function is determined by the last :keyword:`return`
352
statement executed. Since the :keyword:`finally` clause always executes, a
353
:keyword:`return` statement executed in the :keyword:`finally` clause will
354
always be the last one executed::
365
Additional information on exceptions can be found in section :ref:`exceptions`,
366
and information on using the :keyword:`raise` statement to generate exceptions
367
may be found in section :ref:`raise`.
373
The :keyword:`with` statement
374
=============================
378
single: as; with statement
380
The :keyword:`with` statement is used to wrap the execution of a block with
381
methods defined by a context manager (see section :ref:`context-managers`).
382
This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
383
usage patterns to be encapsulated for convenient reuse.
386
with_stmt: "with" with_item ("," with_item)* ":" `suite`
387
with_item: `expression` ["as" `target`]
389
The execution of the :keyword:`with` statement with one "item" proceeds as follows:
391
#. The context expression (the expression given in the :token:`with_item`) is
392
evaluated to obtain a context manager.
394
#. The context manager's :meth:`__exit__` is loaded for later use.
396
#. The context manager's :meth:`__enter__` method is invoked.
398
#. If a target was included in the :keyword:`with` statement, the return value
399
from :meth:`__enter__` is assigned to it.
403
The :keyword:`with` statement guarantees that if the :meth:`__enter__`
404
method returns without an error, then :meth:`__exit__` will always be
405
called. Thus, if an error occurs during the assignment to the target list,
406
it will be treated the same as an error occurring within the suite would
407
be. See step 6 below.
409
#. The suite is executed.
411
#. The context manager's :meth:`__exit__` method is invoked. If an exception
412
caused the suite to be exited, its type, value, and traceback are passed as
413
arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
416
If the suite was exited due to an exception, and the return value from the
417
:meth:`__exit__` method was false, the exception is reraised. If the return
418
value was true, the exception is suppressed, and execution continues with the
419
statement following the :keyword:`with` statement.
421
If the suite was exited for any reason other than an exception, the return
422
value from :meth:`__exit__` is ignored, and execution proceeds at the normal
423
location for the kind of exit that was taken.
425
With more than one item, the context managers are processed as if multiple
426
:keyword:`with` statements were nested::
428
with A() as a, B() as b:
437
.. versionchanged:: 3.1
438
Support for multiple context expressions.
442
:pep:`0343` - The "with" statement
443
The specification, background, and examples for the Python :keyword:`with`
448
single: parameter; function definition
458
pair: function; definition
461
object: user-defined function
466
A function definition defines a user-defined function object (see section
470
funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite`
471
decorators: `decorator`+
472
decorator: "@" `dotted_name` ["(" [`parameter_list` [","]] ")"] NEWLINE
473
dotted_name: `identifier` ("." `identifier`)*
474
parameter_list: (`defparameter` ",")*
475
: | "*" [`parameter`] ("," `defparameter`)* ["," "**" `parameter`]
477
: | `defparameter` [","] )
478
parameter: `identifier` [":" `expression`]
479
defparameter: `parameter` ["=" `expression`]
480
funcname: `identifier`
483
A function definition is an executable statement. Its execution binds the
484
function name in the current local namespace to a function object (a wrapper
485
around the executable code for the function). This function object contains a
486
reference to the current global namespace as the global namespace to be used
487
when the function is called.
489
The function definition does not execute the function body; this gets executed
490
only when the function is called. [#]_
495
A function definition may be wrapped by one or more :term:`decorator` expressions.
496
Decorator expressions are evaluated when the function is defined, in the scope
497
that contains the function definition. The result must be a callable, which is
498
invoked with the function object as the only argument. The returned value is
499
bound to the function name instead of the function object. Multiple decorators
500
are applied in nested fashion. For example, the following code ::
509
func = f1(arg)(f2(func))
512
triple: default; parameter; value
513
single: argument; function definition
515
When one or more :term:`parameters <parameter>` have the form *parameter* ``=``
516
*expression*, the function is said to have "default parameter values." For a
517
parameter with a default value, the corresponding :term:`argument` may be
518
omitted from a call, in which
519
case the parameter's default value is substituted. If a parameter has a default
520
value, all following parameters up until the "``*``" must also have a default
521
value --- this is a syntactic restriction that is not expressed by the grammar.
523
**Default parameter values are evaluated from left to right when the function
524
definition is executed.** This means that the expression is evaluated once, when
525
the function is defined, and that the same "pre-computed" value is used for each
526
call. This is especially important to understand when a default parameter is a
527
mutable object, such as a list or a dictionary: if the function modifies the
528
object (e.g. by appending an item to a list), the default value is in effect
529
modified. This is generally not what was intended. A way around this is to use
530
``None`` as the default, and explicitly test for it in the body of the function,
533
def whats_on_the_telly(penguin=None):
536
penguin.append("property of the zoo")
543
Function call semantics are described in more detail in section :ref:`calls`. A
544
function call always assigns values to all parameters mentioned in the parameter
545
list, either from position arguments, from keyword arguments, or from default
546
values. If the form "``*identifier``" is present, it is initialized to a tuple
547
receiving any excess positional parameters, defaulting to the empty tuple. If
548
the form "``**identifier``" is present, it is initialized to a new dictionary
549
receiving any excess keyword arguments, defaulting to a new empty dictionary.
550
Parameters after "``*``" or "``*identifier``" are keyword-only parameters and
551
may only be passed used keyword arguments.
553
.. index:: pair: function; annotations
555
Parameters may have annotations of the form "``: expression``" following the
556
parameter name. Any parameter may have an annotation even those of the form
557
``*identifier`` or ``**identifier``. Functions may have "return" annotation of
558
the form "``-> expression``" after the parameter list. These annotations can be
559
any valid Python expression and are evaluated when the function definition is
560
executed. Annotations may be evaluated in a different order than they appear in
561
the source code. The presence of annotations does not change the semantics of a
562
function. The annotation values are available as values of a dictionary keyed
563
by the parameters' names in the :attr:`__annotations__` attribute of the
566
.. index:: pair: lambda; expression
568
It is also possible to create anonymous functions (functions not bound to a
569
name), for immediate use in expressions. This uses lambda expressions, described in
570
section :ref:`lambda`. Note that the lambda expression is merely a shorthand for a
571
simplified function definition; a function defined in a ":keyword:`def`"
572
statement can be passed around or assigned to another name just like a function
573
defined by a lambda expression. The ":keyword:`def`" form is actually more powerful
574
since it allows the execution of multiple statements and annotations.
576
**Programmer's note:** Functions are first-class objects. A "``def``" statement
577
executed inside a function definition defines a local function that can be
578
returned or passed around. Free variables used in the nested function can
579
access the local variables of the function containing the def. See section
580
:ref:`naming` for details.
584
:pep:`3107` - Function Annotations
585
The original specification for function annotations.
596
pair: class; definition
599
pair: execution; frame
603
A class definition defines a class object (see section :ref:`types`):
606
classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
607
inheritance: "(" [`parameter_list`] ")"
608
classname: `identifier`
610
A class definition is an executable statement. The inheritance list usually
611
gives a list of base classes (see :ref:`metaclasses` for more advanced uses), so
612
each item in the list should evaluate to a class object which allows
613
subclassing. Classes without an inheritance list inherit, by default, from the
614
base class :class:`object`; hence, ::
624
The class's suite is then executed in a new execution frame (see :ref:`naming`),
625
using a newly created local namespace and the original global namespace.
626
(Usually, the suite contains mostly function definitions.) When the class's
627
suite finishes execution, its execution frame is discarded but its local
628
namespace is saved. [#]_ A class object is then created using the inheritance
629
list for the base classes and the saved local namespace for the attribute
630
dictionary. The class name is bound to this class object in the original local
633
Class creation can be customized heavily using :ref:`metaclasses <metaclasses>`.
635
Classes can also be decorated: just like when decorating functions, ::
644
Foo = f1(arg)(f2(Foo))
646
The evaluation rules for the decorator expressions are the same as for function
647
decorators. The result must be a class object, which is then bound to the class
650
**Programmer's note:** Variables defined in the class definition are class
651
attributes; they are shared by instances. Instance attributes can be set in a
652
method with ``self.name = value``. Both class and instance attributes are
653
accessible through the notation "``self.name``", and an instance attribute hides
654
a class attribute with the same name when accessed in this way. Class
655
attributes can be used as defaults for instance attributes, but using mutable
656
values there can lead to unexpected results. :ref:`Descriptors <descriptors>`
657
can be used to create instance variables with different implementation details.
662
:pep:`3115` - Metaclasses in Python 3
663
:pep:`3129` - Class Decorators
669
.. versionadded:: 3.5
671
.. index:: statement: async def
674
Coroutine function definition
675
-----------------------------
678
async_funcdef: [`decorators`] "async" "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite`
684
Execution of Python coroutines can be suspended and resumed at many points
685
(see :term:`coroutine`). In the body of a coroutine, any ``await`` and
686
``async`` identifiers become reserved keywords; :keyword:`await` expressions,
687
:keyword:`async for` and :keyword:`async with` can only be used in
690
Functions defined with ``async def`` syntax are always coroutine functions,
691
even if they do not contain ``await`` or ``async`` keywords.
693
It is a :exc:`SyntaxError` to use :keyword:`yield` expressions in
694
``async def`` coroutines.
696
An example of a coroutine function::
698
async def func(param1, param2):
700
await some_coroutine()
703
.. index:: statement: async for
706
The :keyword:`async for` statement
707
----------------------------------
710
async_for_stmt: "async" `for_stmt`
712
An :term:`asynchronous iterable` is able to call asynchronous code in its
713
*iter* implementation, and :term:`asynchronous iterator` can call asynchronous
714
code in its *next* method.
716
The ``async for`` statement allows convenient iteration over asynchronous
721
async for TARGET in ITER:
726
Is semantically equivalent to::
729
iter = await type(iter).__aiter__(iter)
733
TARGET = await type(iter).__anext__(iter)
734
except StopAsyncIteration:
741
See also :meth:`__aiter__` and :meth:`__anext__` for details.
743
It is a :exc:`SyntaxError` to use ``async for`` statement outside of an
744
:keyword:`async def` function.
747
.. index:: statement: async with
750
The :keyword:`async with` statement
751
-----------------------------------
754
async_with_stmt: "async" `with_stmt`
756
An :term:`asynchronous context manager` is a :term:`context manager` that is
757
able to suspend execution in its *enter* and *exit* methods.
761
async with EXPR as VAR:
764
Is semantically equivalent to::
767
aexit = type(mgr).__aexit__
768
aenter = type(mgr).__aenter__(mgr)
775
if not await aexit(mgr, *sys.exc_info()):
778
await aexit(mgr, None, None, None)
780
See also :meth:`__aenter__` and :meth:`__aexit__` for details.
782
It is a :exc:`SyntaxError` to use ``async with`` statement outside of an
783
:keyword:`async def` function.
787
:pep:`492` - Coroutines with async and await syntax
790
.. rubric:: Footnotes
792
.. [#] The exception is propagated to the invocation stack unless
793
there is a :keyword:`finally` clause which happens to raise another
794
exception. That new exception causes the old one to be lost.
796
.. [#] Currently, control "flows off the end" except in the case of an exception
797
or the execution of a :keyword:`return`, :keyword:`continue`, or
798
:keyword:`break` statement.
800
.. [#] A string literal appearing as the first statement in the function body is
801
transformed into the function's ``__doc__`` attribute and therefore the
802
function's :term:`docstring`.
804
.. [#] A string literal appearing as the first statement in the class body is
805
transformed into the namespace's ``__doc__`` item and therefore the class's
added
removed
python-3.5.1-docs-html/_sources/reference/compound_stmts.txt
