8.1.Theif statement¶

Theif 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 sectionBoolean 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 theelse clause, if present, is executed.

8.2.Thewhile statement¶

Thewhile 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 theelse clause, if present, is executed and the loopterminates.

Abreak statement executed in the first suite terminates the loopwithout executing theelse clause’s suite. Acontinuestatement executed in the first suite skips the rest of the suite and goes backto testing the expression.

8.3.Thefor statement¶

Thefor 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 theexpression_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 (seeAssignment statements), and then the suite isexecuted. When the items are exhausted (which is immediately when the sequenceis empty or an iterator raises aStopIteration exception), the suite intheelse clause, if present, is executed, and the loop terminates.

Abreak statement executed in the first suite terminates the loopwithout executing theelse clause’s suite. Acontinuestatement executed in the first suite skips the rest of the suite and continueswith the next item, or with theelse 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:

foriinrange(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 functionrange() returns an iterator of integers suitable toemulate the effect of Pascal’sfori:=atobdo; e.g.,list(range(3))returns the list[0,1,2].

forxina[:]:ifx<0:a.remove(x)

8.4.Thetry statement¶

Thetry 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

Theexcept clause(s) specify one or more exception handlers. When noexception occurs in thetry clause, no exception handler is executed.When an exception occurs in thetry 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 the object is the class or anon-virtual base class of the exception object,or a tuple containing an item that is the class or a non-virtual base classof the exception object.

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 entiretry statement raised the exception).

When a matching except clause is found, the exception is assigned to the targetspecified after theas 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 usingastarget, it is cleared at theend of the except clause. This is as if

exceptEasN:foo

was translated to

exceptEasN:try:foofinally:delN

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 thesys module and can be accessed viasys.exc_info().sys.exc_info() returns a 3-tuple consisting of the exception class, theexception instance and a traceback object (see sectionThe 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 optionalelse clause is executed if the control flow leaves thetry suite, no exception was raised, and noreturn,continue, orbreak statement was executed. Exceptions intheelse clause are not handled by the precedingexceptclauses.

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

>>>deff():...try:...1/0...finally:...return42...>>>f()42

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

When areturn,break orcontinue statement isexecuted in thetry suite of atry…finallystatement, thefinally clause is also executed ‘on the way out.’

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

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

Additional information on exceptions can be found in sectionExceptions,and information on using theraise statement to generate exceptionsmay be found in sectionThe raise statement.

8.5.Thewith statement¶

Thewith statement is used to wrap the execution of a block withmethods defined by a context manager (see sectionWith Statement Context Managers).This allows commontry…except…finallyusage patterns to be encapsulated for convenient reuse.

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

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

1. The context expression (the expression given in thewith_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 thewith statement, the return valuefrom__enter__() is assigned to it.

   Note

   Thewith 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, threeNone 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 thewith 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:

withEXPRESSIONasTARGET:SUITE

is semantically equivalent to:

manager=(EXPRESSION)enter=type(manager).__enter__exit=type(manager).__exit__value=enter(manager)hit_except=Falsetry:TARGET=valueSUITEexcept:hit_except=Trueifnotexit(manager,*sys.exc_info()):raisefinally:ifnothit_except:exit(manager,None,None,None)

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

withA()asa,B()asb:SUITE

is semantically equivalent to:

withA()asa:withB()asb:SUITE

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 ::=  "@"assignment_expression NEWLINEparameter_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 moredecorator 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)@f2deffunc():pass

is roughly equivalent to

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

except that the original function is not temporarily bound to the namefunc.

When one or moreparameters have the formparameter=expression, the function is said to have “default parameter values.” For aparameter with a default value, the correspondingargument 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.:

defwhats_on_the_telly(penguin=None):ifpenguinisNone:penguin=[]penguin.append("property of the zoo")returnpenguin

Function call semantics are described in more detail in sectionCalls. Afunction call always assigns values to all parameters mentioned in the parameterlist, either from positional 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 passedby keyword arguments. Parameters before “/” are positional-only parametersand may only be passed by positional arguments.

Parameters may have anannotation 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 theannotations 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 insectionLambdas. 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.

8.7.Class definitions¶

A class definition defines a class object (see sectionThe 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 (seeMetaclasses 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 classobject; hence,

classFoo:pass

is equivalent to

classFoo(object):pass

The class’s suite is then executed in a new execution frame (seeNaming 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 usingmetaclasses.

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

@f1(arg)@f2classFoo:pass

is roughly equivalent to

classFoo: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 withself.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.

8.8.Coroutines¶

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

asyncdeffunc(param1,param2):do_stuff()awaitsome_coroutine()

async_for_stmt ::=  "async"for_stmt

asyncforTARGETinITER:SUITEelse:SUITE2

iter=(ITER)iter=type(iter).__aiter__(iter)running=Truewhilerunning:try:TARGET=awaittype(iter).__anext__(iter)exceptStopAsyncIteration:running=Falseelse:SUITEelse:SUITE2

async_with_stmt ::=  "async"with_stmt

asyncwithEXPRESSIONasTARGET:SUITE

manager=(EXPRESSION)aenter=type(manager).__aenter__aexit=type(manager).__aexit__value=awaitaenter(manager)hit_except=Falsetry:TARGET=valueSUITEexcept:hit_except=Trueifnotawaitaexit(manager,*sys.exc_info()):raisefinally:ifnothit_except:awaitaexit(manager,None,None,None)