8. Compound statements¶

Compound statements contain (groups of) other statements; they affect or controlthe execution of those other statements in some way. In general, compoundstatements span multiple lines, although in simple incarnations a whole compoundstatement may be contained in one line.

The if, while and for statements implementtraditional control flow constructs. try specifies exceptionhandlers and/or cleanup code for a group of statements, while thewith statement allows the execution of initialization andfinalization code around a block of code. Function and class definitions arealso syntactically compound statements.

A compound statement consists of one or more ‘clauses.’ A clause consists of aheader and a ‘suite.’ The clause headers of a particular compound statement areall at the same indentation level. Each clause header begins with a uniquelyidentifying keyword and ends with a colon. A suite is a group of statementscontrolled by a clause. A suite can be one or more semicolon-separated simplestatements on the same line as the header, following the header’s colon, or itcan be one or more indented statements on subsequent lines. Only the latterform of a suite can contain nested compound statements; the following is illegal,mostly because it wouldn’t be clear to which if clause a followingelse clause would belong:

if test1: if test2: print(x)

Also note that the semicolon binds tighter than the colon in this context, sothat in the following example, either all or none of the print() calls areexecuted:

if x < y < z: print(x); print(y); print(z)

Summarizing:

compound_stmt ::=  if_stmt                   | while_stmt                   | for_stmt                   | try_stmt                   | with_stmt                   | funcdef                   | classdef                   | async_with_stmt                   | async_for_stmt                   | async_funcdefsuite         ::=  stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENTstatement     ::=  stmt_list NEWLINE | compound_stmtstmt_list     ::=  simple_stmt (";" simple_stmt)* [";"]

Note that statements always end in a NEWLINE possibly followed by aDEDENT. Also note that optional continuation clauses always begin with akeyword that cannot start a statement, thus there are no ambiguities (the‘dangling else’ problem is solved in Python by requiring nestedif statements to be indented).

The formatting of the grammar rules in the following sections places each clauseon a separate line for clarity.

8.1. The if statement¶

The if statement is used for conditional execution:

if_stmt ::=  "if" assignment_expression ":" suite             ("elif" assignment_expression ":" suite)*             ["else" ":" suite]

It selects exactly one of the suites by evaluating the expressions one by oneuntil one is found to be true (see section Boolean operations for the definition oftrue and false); then that suite is executed (and no other part of theif statement is executed or evaluated). If all expressions arefalse, the suite of the else clause, if present, is executed.

8.2. The while statement¶

The while statement is used for repeated execution as long as anexpression is true:

while_stmt ::=  "while" assignment_expression ":" suite                ["else" ":" suite]

This repeatedly tests the expression and, if it is true, executes the firstsuite; if the expression is false (which may be the first time it is tested) thesuite of the else clause, if present, is executed and the loopterminates.

A break statement executed in the first suite terminates the loopwithout executing the else clause’s suite. A continuestatement executed in the first suite skips the rest of the suite and goes backto testing the expression.

8.3. The for statement¶

The for statement is used to iterate over the elements of a sequence(such as a string, tuple or list) or other iterable object:

for_stmt ::=  "for" target_list "in" expression_list ":" suite              ["else" ":" suite]

The expression list is evaluated once; it should yield an iterable object. Aniterator is created for the result of the expression_list. The suite isthen executed once for each item provided by the iterator, in the order returnedby the iterator. Each item in turn is assigned to the target list using thestandard rules for assignments (see Assignment statements), and then the suite isexecuted. When the items are exhausted (which is immediately when the sequenceis empty or an iterator raises a StopIteration exception), the suite inthe else clause, if present, is executed, and the loop terminates.

A break statement executed in the first suite terminates the loopwithout executing the else clause’s suite. A continuestatement executed in the first suite skips the rest of the suite and continueswith the next item, or with the else clause if there is no nextitem.

The for-loop makes assignments to the variables in the target list.This overwrites all previous assignments to those variables includingthose made in the suite of the for-loop:

for i in range(10):    print(i)    i = 5             # this will not affect the for-loop                      # because i will be overwritten with the next                      # index in the range

Names in the target list are not deleted when the loop is finished, but if thesequence is empty, they will not have been assigned to at all by the loop. Hint:the built-in function range() returns an iterator of integers suitable toemulate the effect of Pascal’s for i := a to b do; e.g., list(range(3))returns the list [0, 1, 2].

Note

There is a subtlety when the sequence is being modified by the loop (this canonly occur for mutable sequences, e.g. lists). An internal counter is usedto keep track of which item is used next, and this is incremented on eachiteration. When this counter has reached the length of the sequence the loopterminates. 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 theindex of the current item which has already been treated). Likewise, if thesuite inserts an item in the sequence before the current item, the currentitem will be treated again the next time through the loop. This can lead tonasty bugs that can be avoided by making a temporary copy using a slice ofthe whole sequence, e.g.,

for x in a[:]:    if x < 0: a.remove(x)

8.4. The try statement¶

The try statement specifies exception handlers and/or cleanup codefor a group of statements:

try_stmt  ::=  try1_stmt | try2_stmttry1_stmt ::=  "try" ":" suite               ("except" [expression ["as" identifier]] ":" suite)+               ["else" ":" suite]               ["finally" ":" suite]try2_stmt ::=  "try" ":" suite               "finally" ":" suite

The except clause(s) specify one or more exception handlers. When noexception occurs in the try clause, no exception handler is executed.When an exception occurs in the try suite, a search for an exceptionhandler is started. This search inspects the except clauses in turn until oneis found that matches the exception. An expression-less except clause, ifpresent, must be last; it matches any exception. For an except clause with anexpression, that expression is evaluated, and the clause matches the exceptionif the resulting object is “compatible” with the exception. An object iscompatible with an exception if it is the class or a base class of the exceptionobject or a tuple containing an item compatible with the exception.

If no except clause matches the exception, the search for an exception handlercontinues in the surrounding code and on the invocation stack. 1

If the evaluation of an expression in the header of an except clause raises anexception, the original search for a handler is canceled and a search starts forthe new exception in the surrounding code and on the call stack (it is treatedas if the entire try statement raised the exception).

When a matching except clause is found, the exception is assigned to the targetspecified after the as keyword in that except clause, if present, andthe except clause’s suite is executed. All except clauses must have anexecutable block. When the end of this block is reached, execution continuesnormally after the entire try statement. (This means that if two nestedhandlers exist for the same exception, and the exception occurs in the tryclause of the inner handler, the outer handler will not handle the exception.)

When an exception has been assigned using as target, it is cleared at theend of the except clause. This is as if

except E as N:    foo

was translated to

except E as N:    try:        foo    finally:        del N

This means the exception must be assigned to a different name to be able torefer to it after the except clause. Exceptions are cleared because with thetraceback attached to them, they form a reference cycle with the stack frame,keeping all locals in that frame alive until the next garbage collection occurs.

Before an except clause’s suite is executed, details about the exception arestored in the sys module and can be accessed via sys.exc_info().sys.exc_info() returns a 3-tuple consisting of the exception class, theexception instance and a traceback object (see section The standard type hierarchy) identifyingthe point in the program where the exception occurred. sys.exc_info()values are restored to their previous values (before the call) when returningfrom a function that handled an exception.

The optional else clause is executed if the control flow leaves thetry suite, no exception was raised, and no return,continue, or break statement was executed. Exceptions inthe else clause are not handled by the preceding exceptclauses.

If finally is present, it specifies a ‘cleanup’ handler. Thetry clause is executed, including any except andelse clauses. If an exception occurs in any of the clauses and isnot handled, the exception is temporarily saved. The finally clauseis executed. If there is a saved exception it is re-raised at the end of thefinally clause. If the finally clause raises anotherexception, the saved exception is set as the context of the new exception.If the finally clause executes a return, breakor continue statement, the saved exception is discarded:

>>> def f():...     try:...         1/0...     finally:...         return 42...>>> f()42

The exception information is not available to the program during execution ofthe finally clause.

When a return, break or continue statement isexecuted in the try suite of a try…finallystatement, the finally clause is also executed ‘on the way out.’

The return value of a function is determined by the last returnstatement executed. Since the finally clause always executes, areturn statement executed in the finally clause willalways be the last one executed:

>>> def foo():...     try:...         return 'try'...     finally:...         return 'finally'...>>> foo()'finally'

Additional information on exceptions can be found in section Exceptions,and information on using the raise statement to generate exceptionsmay be found in section The raise statement.

Changed in version 3.8: Prior to Python 3.8, a continue statement was illegal in thefinally clause due to a problem with the implementation.

8.5. The with statement¶

The with statement is used to wrap the execution of a block withmethods defined by a context manager (see section With Statement Context Managers).This allows common try…except…finallyusage patterns to be encapsulated for convenient reuse.

with_stmt ::=  "with" with_item ("," with_item)* ":" suitewith_item ::=  expression ["as" target]

The execution of the with statement with one “item” proceeds as follows:

1. The context expression (the expression given in the with_item) isevaluated to obtain a context manager.

2. The context manager’s __enter__() is loaded for later use.

3. The context manager’s __exit__() is loaded for later use.

4. The context manager’s __enter__() method is invoked.

5. If a target was included in the with statement, the return valuefrom __enter__() is assigned to it.

   Note

   The with statement guarantees that if the __enter__()method returns without an error, then __exit__() will always becalled. Thus, if an error occurs during the assignment to the target list,it will be treated the same as an error occurring within the suite wouldbe. See step 6 below.

6. The suite is executed.

7. The context manager’s __exit__() method is invoked. If an exceptioncaused the suite to be exited, its type, value, and traceback are passed asarguments to __exit__(). Otherwise, three None arguments aresupplied.

   If the suite was exited due to an exception, and the return value from the__exit__() method was false, the exception is reraised. If the returnvalue was true, the exception is suppressed, and execution continues with thestatement following the with statement.

   If the suite was exited for any reason other than an exception, the returnvalue from __exit__() is ignored, and execution proceeds at the normallocation for the kind of exit that was taken.

The following code:

with EXPRESSION as TARGET:    SUITE

is semantically equivalent to:

manager = (EXPRESSION)enter = type(manager).__enter__exit = type(manager).__exit__value = enter(manager)hit_except = Falsetry:    TARGET = value    SUITEexcept:    hit_except = True    if not exit(manager, *sys.exc_info()):        raisefinally:    if not hit_except:        exit(manager, None, None, None)

With more than one item, the context managers are processed as if multiplewith statements were nested:

with A() as a, B() as b:    SUITE

is semantically equivalent to:

with A() as a:    with B() as b:        SUITE

Changed in version 3.1: Support for multiple context expressions.

See also

PEP 343 - The “with” statement

The specification, background, and examples for the Python withstatement.

8.6. Function definitions¶

A function definition defines a user-defined function object (see sectionThe standard type hierarchy):

funcdef                   ::=  [decorators] "def" funcname "(" [parameter_list] ")"                               ["->" expression] ":" suitedecorators                ::=  decorator+decorator                 ::=  "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINEdotted_name               ::=  identifier ("." identifier)*parameter_list            ::=  defparameter ("," defparameter)* "," "/" ["," [parameter_list_no_posonly]]                                 | parameter_list_no_posonlyparameter_list_no_posonly ::=  defparameter ("," defparameter)* ["," [parameter_list_starargs]]                               | parameter_list_starargsparameter_list_starargs   ::=  "*" [parameter] ("," defparameter)* ["," ["**" parameter [","]]]                               | "**" parameter [","]parameter                 ::=  identifier [":" expression]defparameter              ::=  parameter ["=" expression]funcname                  ::=  identifier

A function definition is an executable statement. Its execution binds thefunction name in the current local namespace to a function object (a wrapperaround the executable code for the function). This function object contains areference to the current global namespace as the global namespace to be usedwhen the function is called.

The function definition does not execute the function body; this gets executedonly when the function is called. 2

A function definition may be wrapped by one or more decorator expressions.Decorator expressions are evaluated when the function is defined, in the scopethat contains the function definition. The result must be a callable, which isinvoked with the function object as the only argument. The returned value isbound to the function name instead of the function object. Multiple decoratorsare applied in nested fashion. For example, the following code

@f1(arg)@f2def func(): pass

is roughly equivalent to

def func(): passfunc = f1(arg)(f2(func))

except that the original function is not temporarily bound to the name func.

When one or more parameters have the form parameter =expression, the function is said to have “default parameter values.” For aparameter with a default value, the corresponding argument may beomitted from a call, in whichcase the parameter’s default value is substituted. If a parameter has a defaultvalue, all following parameters up until the “*” must also have a defaultvalue — this is a syntactic restriction that is not expressed by the grammar.

Default parameter values are evaluated from left to right when the functiondefinition is executed. This means that the expression is evaluated once, whenthe function is defined, and that the same “pre-computed” value is used for eachcall. This is especially important to understand when a default parameter is amutable object, such as a list or a dictionary: if the function modifies theobject (e.g. by appending an item to a list), the default value is in effectmodified. This is generally not what was intended. A way around this is to useNone as the default, and explicitly test for it in the body of the function,e.g.:

def whats_on_the_telly(penguin=None):    if penguin is None:        penguin = []    penguin.append("property of the zoo")    return penguin

Function call semantics are described in more detail in section Calls. Afunction call always assigns values to all parameters mentioned in the parameterlist, either from position arguments, from keyword arguments, or from defaultvalues. If the form “*identifier” is present, it is initialized to a tuplereceiving any excess positional parameters, defaulting to the empty tuple.If the form “**identifier” is present, it is initialized to a newordered mapping receiving any excess keyword arguments, defaulting to anew empty mapping of the same type. Parameters after “*” or“*identifier” are keyword-only parameters and may only be passedused keyword arguments.

Parameters may have an annotation of the form “: expression”following the parameter name. Any parameter may have an annotation, even those of the form*identifier or **identifier. Functions may have “return” annotation ofthe form “-> expression” after the parameter list. These annotations can beany valid Python expression. The presence of annotations does not change thesemantics of a function. The annotation values are available as values ofa dictionary keyed by the parameters’ names in the __annotations__attribute of the function object. If the annotations import from__future__ is used, annotations are preserved as strings at runtime whichenables postponed evaluation. Otherwise, they are evaluated when the functiondefinition is executed. In this case annotations may be evaluated ina different order than they appear in the source code.

It is also possible to create anonymous functions (functions not bound to aname), for immediate use in expressions. This uses lambda expressions, described insection Lambdas. Note that the lambda expression is merely a shorthand for asimplified function definition; a function defined in a “def”statement can be passed around or assigned to another name just like a functiondefined by a lambda expression. The “def” form is actually more powerfulsince it allows the execution of multiple statements and annotations.

Programmer’s note: Functions are first-class objects. A “def” statementexecuted inside a function definition defines a local function that can bereturned or passed around. Free variables used in the nested function canaccess the local variables of the function containing the def. See sectionNaming and binding for details.

See also

PEP 3107 - Function Annotations

The original specification for function annotations.

PEP 484 - Type Hints

Definition of a standard meaning for annotations: type hints.

PEP 526 - Syntax for Variable Annotations

Ability to type hint variable declarations, including classvariables and instance variables

PEP 563 - Postponed Evaluation of Annotations

Support for forward references within annotations by preservingannotations in a string form at runtime instead of eager evaluation.

8.7. Class definitions¶

A class definition defines a class object (see section The standard type hierarchy):

classdef    ::=  [decorators] "class" classname [inheritance] ":" suiteinheritance ::=  "(" [argument_list] ")"classname   ::=  identifier

A class definition is an executable statement. The inheritance list usuallygives a list of base classes (see Metaclasses for more advanced uses), soeach item in the list should evaluate to a class object which allowssubclassing. Classes without an inheritance list inherit, by default, from thebase class object; hence,

class Foo:    pass

is equivalent to

class Foo(object):    pass

The class’s suite is then executed in a new execution frame (see Naming and binding),using a newly created local namespace and the original global namespace.(Usually, the suite contains mostly function definitions.) When the class’ssuite finishes execution, its execution frame is discarded but its localnamespace is saved. 3 A class object is then created using the inheritancelist for the base classes and the saved local namespace for the attributedictionary. The class name is bound to this class object in the original localnamespace.

The order in which attributes are defined in the class body is preservedin the new class’s __dict__. Note that this is reliable only rightafter the class is created and only for classes that were defined usingthe definition syntax.

Class creation can be customized heavily using metaclasses.

Classes can also be decorated: just like when decorating functions,

@f1(arg)@f2class Foo: pass

is roughly equivalent to

class Foo: passFoo = f1(arg)(f2(Foo))

The evaluation rules for the decorator expressions are the same as for functiondecorators. The result is then bound to the class name.

Programmer’s note: Variables defined in the class definition are classattributes; they are shared by instances. Instance attributes can be set in amethod with self.name = value. Both class and instance attributes areaccessible through the notation “self.name”, and an instance attribute hidesa class attribute with the same name when accessed in this way. Classattributes can be used as defaults for instance attributes, but using mutablevalues there can lead to unexpected results. Descriptorscan be used to create instance variables with different implementation details.

See also

PEP 3115 - Metaclasses in Python 3000

The proposal that changed the declaration of metaclasses to the currentsyntax, and the semantics for how classes with metaclasses areconstructed.

PEP 3129 - Class Decorators

The proposal that added class decorators. Function and method decoratorswere introduced in PEP 318.

8.8. Coroutines¶

New in version 3.5.

8.8.1. Coroutine function definition¶

async_funcdef ::=  [decorators] "async" "def" funcname "(" [parameter_list] ")"                   ["->" expression] ":" suite

Execution of Python coroutines can be suspended and resumed at many points(see coroutine). Inside the body of a coroutine function, await andasync identifiers become reserved keywords; await expressions,async for and async with can only be used incoroutine function bodies.

Functions defined with async def syntax are always coroutine functions,even if they do not contain await or async keywords.

It is a SyntaxError to use a yield from expression inside the bodyof a coroutine function.

An example of a coroutine function:

async def func(param1, param2):    do_stuff()    await some_coroutine()

8.8.2. The async for statement¶

async_for_stmt ::=  "async" for_stmt

An asynchronous iterable is able to call asynchronous code in itsiter implementation, and asynchronous iterator can call asynchronouscode in its next method.

The async for statement allows convenient iteration over asynchronousiterators.

The following code:

async for TARGET in ITER:    SUITEelse:    SUITE2

Is semantically equivalent to:

iter = (ITER)iter = type(iter).__aiter__(iter)running = Truewhile running:    try:        TARGET = await type(iter).__anext__(iter)    except StopAsyncIteration:        running = False    else:        SUITEelse:    SUITE2

See also __aiter__() and __anext__() for details.

It is a SyntaxError to use an async for statement outside thebody of a coroutine function.

8.8.3. The async with statement¶

async_with_stmt ::=  "async" with_stmt

An asynchronous context manager is a context manager that isable to suspend execution in its enter and exit methods.

The following code:

async with EXPRESSION as TARGET:    SUITE

is semantically equivalent to:

manager = (EXPRESSION)aexit = type(manager).__aexit__aenter = type(manager).__aenter__value = await aenter(manager)hit_except = Falsetry:    TARGET = value    SUITEexcept:    hit_except = True    if not await aexit(manager, *sys.exc_info()):        raisefinally:    if not hit_except:        await aexit(manager, None, None, None)

See also __aenter__() and __aexit__() for details.

It is a SyntaxError to use an async with statement outside thebody of a coroutine function.

See also

PEP 492 - Coroutines with async and await syntax

The proposal that made coroutines a proper standalone concept in Python,and added supporting syntax.

Footnotes

1

The exception is propagated to the invocation stack unlessthere is a finally clause which happens to raise anotherexception. That new exception causes the old one to be lost.

2

A string literal appearing as the first statement in the function body istransformed into the function’s __doc__ attribute and therefore thefunction’s docstring.

3

A string literal appearing as the first statement in the class body istransformed into the namespace’s __doc__ item and therefore the class’sdocstring.