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: ``exc_type``, the
289 exception class; ``exc_value``, the exception instance; ``exc_traceback``, a
290 traceback object (see section :ref:`types`) identifying the point in the program
291 where the exception occurred. :func:`sys.exc_info` values are restored to their
292 previous values (before the call) when returning from a function that handled an
301 The optional :keyword:`else` clause is executed if and when control flows off
302 the end of the :keyword:`try` clause. [#]_ Exceptions in the :keyword:`else`
303 clause are not handled by the preceding :keyword:`except` clauses.
305 .. index:: keyword: finally
307 If :keyword:`finally` is present, it specifies a 'cleanup' handler. The
308 :keyword:`try` clause is executed, including any :keyword:`except` and
309 :keyword:`else` clauses. If an exception occurs in any of the clauses and is
310 not handled, the exception is temporarily saved. The :keyword:`finally` clause
311 is executed. If there is a saved exception, it is re-raised at the end of the
312 :keyword:`finally` clause. If the :keyword:`finally` clause raises another
313 exception or executes a :keyword:`return` or :keyword:`break` statement, the
314 saved exception is lost. The exception information is not available to the
315 program during execution of the :keyword:`finally` clause.
322 When a :keyword:`return`, :keyword:`break` or :keyword:`continue` statement is
323 executed in the :keyword:`try` suite of a :keyword:`try`...\ :keyword:`finally`
324 statement, the :keyword:`finally` clause is also executed 'on the way out.' A
325 :keyword:`continue` statement is illegal in the :keyword:`finally` clause. (The
326 reason is a problem with the current implementation --- this restriction may be
327 lifted in the future).
329 Additional information on exceptions can be found in section :ref:`exceptions`,
330 and information on using the :keyword:`raise` statement to generate exceptions
331 may be found in section :ref:`raise`.
337 The :keyword:`with` statement
338 =============================
340 .. index:: statement: with
342 The :keyword:`with` statement is used to wrap the execution of a block with
343 methods defined by a context manager (see section :ref:`context-managers`).
344 This allows common :keyword:`try`...\ :keyword:`except`...\ :keyword:`finally`
345 usage patterns to be encapsulated for convenient reuse.
348 with_stmt: "with" with_item ("," with_item)* ":" `suite`
349 with_item: `expression` ["as" `target`]
351 The execution of the :keyword:`with` statement with one "item" proceeds as follows:
353 #. The context expression is evaluated to obtain a context manager.
355 #. The context manager's :meth:`__enter__` method is invoked.
357 #. If a target was included in the :keyword:`with` statement, the return value
358 from :meth:`__enter__` is assigned to it.
362 The :keyword:`with` statement guarantees that if the :meth:`__enter__`
363 method returns without an error, then :meth:`__exit__` will always be
364 called. Thus, if an error occurs during the assignment to the target
365 list, it will be treated the same as an error occurring within the suite
366 would be. See step 5 below.
368 #. The suite is executed.
370 #. The context manager's :meth:`__exit__` method is invoked. If an exception
371 caused the suite to be exited, its type, value, and traceback are passed as
372 arguments to :meth:`__exit__`. Otherwise, three :const:`None` arguments are
375 If the suite was exited due to an exception, and the return value from the
376 :meth:`__exit__` method was false, the exception is reraised. If the return
377 value was true, the exception is suppressed, and execution continues with the
378 statement following the :keyword:`with` statement.
380 If the suite was exited for any reason other than an exception, the return
381 value from :meth:`__exit__` is ignored, and execution proceeds at the normal
382 location for the kind of exit that was taken.
384 With more than one item, the context managers are processed as if multiple
385 :keyword:`with` statements were nested::
387 with A() as a, B() as b:
396 .. versionchanged:: 3.1
397 Support for multiple context expressions.
401 :pep:`0343` - The "with" statement
402 The specification, background, and examples for the Python :keyword:`with`
414 pair: function; definition
417 object: user-defined function
422 A function definition defines a user-defined function object (see section
426 funcdef: [`decorators`] "def" `funcname` "(" [`parameter_list`] ")" ["->" `expression`] ":" `suite`
427 decorators: `decorator`+
428 decorator: "@" `dotted_name` ["(" [`argument_list` [","]] ")"] NEWLINE
429 dotted_name: `identifier` ("." `identifier`)*
430 parameter_list: (`defparameter` ",")*
431 : ( "*" [`parameter`] ("," `defparameter`)*
432 : [, "**" `parameter`]
434 : | `defparameter` [","] )
435 parameter: `identifier` [":" `expression`]
436 defparameter: `parameter` ["=" `expression`]
437 funcname: `identifier`
440 A function definition is an executable statement. Its execution binds the
441 function name in the current local namespace to a function object (a wrapper
442 around the executable code for the function). This function object contains a
443 reference to the current global namespace as the global namespace to be used
444 when the function is called.
446 The function definition does not execute the function body; this gets executed
447 only when the function is called. [#]_
452 A function definition may be wrapped by one or more :term:`decorator` expressions.
453 Decorator expressions are evaluated when the function is defined, in the scope
454 that contains the function definition. The result must be a callable, which is
455 invoked with the function object as the only argument. The returned value is
456 bound to the function name instead of the function object. Multiple decorators
457 are applied in nested fashion. For example, the following code ::
466 func = f1(arg)(f2(func))
468 .. index:: triple: default; parameter; value
470 When one or more parameters have the form *parameter* ``=`` *expression*, the
471 function is said to have "default parameter values." For a parameter with a
472 default value, the corresponding argument may be omitted from a call, in which
473 case the parameter's default value is substituted. If a parameter has a default
474 value, all following parameters up until the "``*``" must also have a default
475 value --- this is a syntactic restriction that is not expressed by the grammar.
477 **Default parameter values are evaluated when the function definition is
478 executed.** This means that the expression is evaluated once, when the function
479 is defined, and that that same "pre-computed" value is used for each call. This
480 is especially important to understand when a default parameter is a mutable
481 object, such as a list or a dictionary: if the function modifies the object
482 (e.g. by appending an item to a list), the default value is in effect modified.
483 This is generally not what was intended. A way around this is to use ``None``
484 as the default, and explicitly test for it in the body of the function, e.g.::
486 def whats_on_the_telly(penguin=None):
489 penguin.append("property of the zoo")
496 Function call semantics are described in more detail in section :ref:`calls`. A
497 function call always assigns values to all parameters mentioned in the parameter
498 list, either from position arguments, from keyword arguments, or from default
499 values. If the form "``*identifier``" is present, it is initialized to a tuple
500 receiving any excess positional parameters, defaulting to the empty tuple. If
501 the form "``**identifier``" is present, it is initialized to a new dictionary
502 receiving any excess keyword arguments, defaulting to a new empty dictionary.
503 Parameters after "``*``" or "``*identifier``" are keyword-only parameters and
504 may only be passed used keyword arguments.
506 .. index:: pair: function; annotations
508 Parameters may have annotations of the form "``: expression``" following the
509 parameter name. Any parameter may have an annotation even those of the form
510 ``*identifier`` or ``**identifier``. Functions may have "return" annotation of
511 the form "``-> expression``" after the parameter list. These annotations can be
512 any valid Python expression and are evaluated when the function definition is
513 executed. Annotations may be evaluated in a different order than they appear in
514 the source code. The presence of annotations does not change the semantics of a
515 function. The annotation values are available as values of a dictionary keyed
516 by the parameters' names in the :attr:`__annotations__` attribute of the
519 .. index:: pair: lambda; form
521 It is also possible to create anonymous functions (functions not bound to a
522 name), for immediate use in expressions. This uses lambda forms, described in
523 section :ref:`lambda`. Note that the lambda form is merely a shorthand for a
524 simplified function definition; a function defined in a ":keyword:`def`"
525 statement can be passed around or assigned to another name just like a function
526 defined by a lambda form. The ":keyword:`def`" form is actually more powerful
527 since it allows the execution of multiple statements and annotations.
529 **Programmer's note:** Functions are first-class objects. A "``def``" form
530 executed inside a function definition defines a local function that can be
531 returned or passed around. Free variables used in the nested function can
532 access the local variables of the function containing the def. See section
533 :ref:`naming` for details.
544 pair: class; definition
547 pair: execution; frame
551 A class definition defines a class object (see section :ref:`types`):
553 .. XXX need to document PEP 3115 changes here (new metaclasses)
556 classdef: [`decorators`] "class" `classname` [`inheritance`] ":" `suite`
557 inheritance: "(" [`expression_list`] ")"
558 classname: `identifier`
561 A class definition is an executable statement. It first evaluates the
562 inheritance list, if present. Each item in the inheritance list should evaluate
563 to a class object or class type which allows subclassing. The class's suite is
564 then executed in a new execution frame (see section :ref:`naming`), using a
565 newly created local namespace and the original global namespace. (Usually, the
566 suite contains only function definitions.) When the class's suite finishes
567 execution, its execution frame is discarded but its local namespace is
568 saved. [#]_ A class object is then created using the inheritance list for the
569 base classes and the saved local namespace for the attribute dictionary. The
570 class name is bound to this class object in the original local namespace.
572 Classes can also be decorated; as with functions, ::
581 Foo = f1(arg)(f2(Foo))
583 **Programmer's note:** Variables defined in the class definition are class
584 variables; they are shared by instances. Instance variables can be set in a
585 method with ``self.name = value``. Both class and instance variables are
586 accessible through the notation "``self.name``", and an instance variable hides
587 a class variable with the same name when accessed in this way. Class variables
588 can be used as defaults for instance variables, but using mutable values there
589 can lead to unexpected results. Descriptors can be used to create instance
590 variables with different implementation details.
592 .. XXX add link to descriptor docs above
596 :pep:`3129` - Class Decorators
598 Class definitions, like function definitions, may be wrapped by one or more
599 :term:`decorator` expressions. The evaluation rules for the decorator
600 expressions are the same as for functions. The result must be a class object,
601 which is then bound to the class name.
604 .. rubric:: Footnotes
606 .. [#] The exception is propagated to the invocation stack only if there is no
607 :keyword:`finally` clause that negates the exception.
609 .. [#] Currently, control "flows off the end" except in the case of an exception
610 or the execution of a :keyword:`return`, :keyword:`continue`, or
611 :keyword:`break` statement.
613 .. [#] A string literal appearing as the first statement in the function body is
614 transformed into the function's ``__doc__`` attribute and therefore the
615 function's :term:`docstring`.
617 .. [#] A string literal appearing as the first statement in the class body is
618 transformed into the namespace's ``__doc__`` item and therefore the class's