8. Instruções compostas¶
Instruções compostas contém (grupos de) outras instruções; Elas afetam ou controlam a execução dessas outras instruções de alguma maneira. Em geral, instruções compostas abrangem múltiplas linhas, no entanto em algumas manifestações simples uma instrução composta inteira pode estar contida em uma linha.
As instruções if, while e for implementam construções tradicionais de controle do fluxo de execução. try especifica tratadores de exceção e/ou código de limpeza para uma instrução ou grupo de instruções, enquanto a palavra reservada with permite a execução de código de inicialização e finalização em volta de um bloco de código. Definições de função e classe também são sintaticamente instruções compostas.
Uma instrução composta consiste em uma ou mais “cláusulas”. Uma cláusula consiste em um cabeçalho e um “conjunto”. Os cabeçalhos das cláusulas de uma instrução composta específica estão todos no mesmo nível de indentação. Cada cabeçalho de cláusula começa com uma palavra reservada de identificação exclusiva e termina com dois pontos. Um conjunto é um grupo de instruções controladas por uma cláusula. Um conjunto pode ser uma ou mais instruções simples separadas por ponto e vírgula na mesma linha do cabeçalho, após os dois pontos do cabeçalho, ou pode ser uma ou mais instruções indentadas nas linhas subsequentes. Somente a última forma de conjunto pode conter instruções compostas aninhadas; o seguinte é ilegal, principalmente porque não ficaria claro a qual cláusula if a seguinte cláusula else pertenceria:
if test1: if test2: print(x)
Observe também que o ponto e vírgula é mais vinculado que os dois pontos neste contexto, de modo que no exemplo a seguir, todas ou nenhuma das chamadas print() são executadas:
if x < y < z: print(x); print(y); print(z)
Resumindo:
compound_stmt ::=if_stmt|while_stmt|for_stmt|try_stmt|with_stmt|funcdef|classdef|async_with_stmt|async_for_stmt|async_funcdefsuite ::=stmt_listNEWLINE | NEWLINE INDENTstatement+ DEDENT statement ::=stmt_listNEWLINE |compound_stmtstmt_list ::=simple_stmt(";"simple_stmt)* [";"]
Note que instruções sempre terminam em uma NEWLINE possivelmente seguida por uma DEDENT. Note também que cláusulas opcionais de continuação sempre começam com uma palavra reservada que não pode iniciar uma instrução, desta forma não há ambiguidades (o problema do “else pendurado” é resolvido em Python obrigando que instruções if aninhadas tenham indentação)
A formatação das regras de gramática nas próximas seções põe cada cláusula em uma linha separada para as tornar mais claras.
8.1. A instrução if¶
A instrução if é usada para execução condicional:
if_stmt ::= "if"assignment_expression":"suite("elif"assignment_expression":"suite)* ["else" ":"suite]
Ele seleciona exatamente um dos conjuntos avaliando as expressões uma por uma até que uma seja considerada verdadeira (veja a seção Operações booleanas para a definição de verdadeiro e falso); então esse conjunto é executado (e nenhuma outra parte da instrução if é executada ou avaliada). Se todas as expressões forem falsas, o conjunto da cláusula else, se presente, é executado.
8.2. A instrução while¶
A instrução while é usada para execução repetida desde que uma expressão seja verdadeira:
while_stmt ::= "while"assignment_expression":"suite["else" ":"suite]
Isto testa repetidamente a expressão e, se for verdadeira, executa o primeiro conjunto; se a expressão for falsa (o que pode ser a primeira vez que ela é testada) o conjunto da cláusula else, se presente, é executado e o laço termina.
Uma instrução break executada no primeiro conjunto termina o loop sem executar o conjunto da cláusula else. Uma instrução continue executada no primeiro conjunto ignora o resto do conjunto e volta a testar a expressão.
8.3. A instrução for¶
A instrução for é usada para iterar sobre os elementos de uma sequência (como uma string, tupla ou lista) ou outro objeto iterável:
for_stmt ::= "for"target_list"in"expression_list":"suite["else" ":"suite]
The expression list is evaluated once; it should yield an iterable object. An
iterator is created for the result of the expression_list. The suite is
then executed once for each item provided by the iterator, in the order returned
by the iterator. Each item in turn is assigned to the target list using the
standard rules for assignments (see Instruções de atribuição), and then the suite is
executed. When the items are exhausted (which is immediately when the sequence
is empty or an iterator raises a StopIteration exception), the suite in
the else clause, if present, is executed, and the loop terminates.
Uma instrução break executada no primeiro conjunto termina o loop sem executar o conjunto da cláusula else. Uma instrução continue executada no primeiro conjunto pula o resto do conjunto e continua com o próximo item, ou com a cláusula else se não houver próximo item.
O laço for faz atribuições às variáveis na lista de destino. Isso substitui todas as atribuições anteriores a essas variáveis, incluindo aquelas feitas no conjunto do laço for:
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 the
sequence 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 to
emulate the effect of Pascal’s for i := a to b do; e.g., list(range(3))
returns the list [0, 1, 2].
Nota
There is a subtlety when the sequence is being modified by the loop (this can only occur for mutable sequences, e.g. lists). An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. When this counter has reached the length of the sequence the loop terminates. 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 the index of the current item which has already been treated). Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g.,
for x in a[:]:
if x < 0: a.remove(x)
8.4. A instrução try¶
The try statement specifies exception handlers and/or cleanup code
for 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 no
exception occurs in the try clause, no exception handler is executed.
When an exception occurs in the try suite, a search for an exception
handler is started. This search inspects the except clauses in turn until one
is found that matches the exception. An expression-less except clause, if
present, must be last; it matches any exception. For an except clause with an
expression, that expression is evaluated, and the clause matches the exception
if the resulting object is “compatible” with the exception. An object is
compatible with an exception if the object is the class or a
non-virtual base class of the exception object,
or a tuple containing an item that is the class or a non-virtual base class
of the exception object.
If no except clause matches the exception, the search for an exception handler continues in the surrounding code and on the invocation stack. 1
If the evaluation of an expression in the header of an except clause raises an
exception, the original search for a handler is canceled and a search starts for
the new exception in the surrounding code and on the call stack (it is treated
as if the entire try statement raised the exception).
When a matching except clause is found, the exception is assigned to the target
specified after the as keyword in that except clause, if present, and
the except clause’s suite is executed. All except clauses must have an
executable block. When the end of this block is reached, execution continues
normally after the entire try statement. (This means that if two nested
handlers exist for the same exception, and the exception occurs in the try
clause 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 the
end of the except clause. This is as if
except E as N:
foo
fosse traduzido para
except E as N:
try:
foo
finally:
del N
This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback 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 are
stored in the sys module and can be accessed via sys.exc_info().
sys.exc_info() returns a 3-tuple consisting of the exception class, the
exception instance and a traceback object (see section A hierarquia de tipos padrão) identifying
the point in the program where the exception occurred. sys.exc_info()
values are restored to their previous values (before the call) when returning
from a function that handled an exception.
A cláusula opcional else é executada se o fluxo de controle deixar o conjunto try, nenhuma exceção foi levantada e nenhuma instrução return, continue ou break foi executada. Exceções na cláusula else não são manipuladas pelas cláusulas except precedentes.
If finally is present, it specifies a ‘cleanup’ handler. The
try clause is executed, including any except and
else clauses. If an exception occurs in any of the clauses and is
not handled, the exception is temporarily saved. The finally clause
is executed. If there is a saved exception it is re-raised at the end of the
finally clause. If the finally clause raises another
exception, the saved exception is set as the context of the new exception.
If the finally clause executes a return, break
or 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 of
the finally clause.
When a return, break or continue statement is
executed in the try suite of a try…finally
statement, the finally clause is also executed ‘on the way out.’
The return value of a function is determined by the last return
statement executed. Since the finally clause always executes, a
return statement executed in the finally clause will
always be the last one executed:
>>> def foo():
... try:
... return 'try'
... finally:
... return 'finally'
...
>>> foo()
'finally'
Informações adicionais sobre exceções podem ser encontradas na seção Exceções, e informações sobre como usar a instrução raise para gerar exceções podem ser encontradas na seção A instrução raise.
8.5. A instrução with¶
A instrução with é usada para envolver em um invólucro a execução de um bloco com métodos definidos por um gerenciador de contexto (veja a seção Gerenciadores de contexto da instrução with). Isso permite que padrões comuns de uso de try…except…finally sejam encapsulados para reutilização conveniente.
with_stmt ::= "with"with_item(","with_item)* ":"suitewith_item ::=expression["as"target]
A execução da instrução with com um “item” ocorre da seguinte maneira:
The context expression (the expression given in the
with_item) is evaluated to obtain a context manager.The context manager’s
__enter__()is loaded for later use.The context manager’s
__exit__()is loaded for later use.The context manager’s
__enter__()method is invoked.If a target was included in the
withstatement, the return value from__enter__()is assigned to it.Nota
The
withstatement guarantees that if the__enter__()method returns without an error, then__exit__()will always be called. 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 would be. See step 6 below.O conjunto é executado.
The context manager’s
__exit__()method is invoked. If an exception caused the suite to be exited, its type, value, and traceback are passed as arguments to__exit__(). Otherwise, threeNonearguments are supplied.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 return value was true, the exception is suppressed, and execution continues with the statement following thewithstatement.If the suite was exited for any reason other than an exception, the return value from
__exit__()is ignored, and execution proceeds at the normal location for the kind of exit that was taken.
O seguinte código:
with EXPRESSION as TARGET:
SUITE
é semanticamente equivalente a:
manager = (EXPRESSION)
enter = type(manager).__enter__
exit = type(manager).__exit__
value = enter(manager)
hit_except = False
try:
TARGET = value
SUITE
except:
hit_except = True
if not exit(manager, *sys.exc_info()):
raise
finally:
if not hit_except:
exit(manager, None, None, None)
Com mais de um item, os gerenciadores de contexto são processados como se várias instruções with estivessem aninhadas:
with A() as a, B() as b:
SUITE
é semanticamente equivalente a:
with A() as a:
with B() as b:
SUITE
Alterado na versão 3.1: Suporte para múltiplas expressões de contexto.
8.6. Definições de função¶
A function definition defines a user-defined function object (see section A hierarquia de tipos padrão):
funcdef ::= [decorators] "def"funcname"(" [parameter_list] ")" ["->"expression] ":"suitedecorators ::=decorator+ decorator ::= "@"assignment_expressionNEWLINE 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 the function name in the current local namespace to a function object (a wrapper around the executable code for the function). This function object contains a reference to the current global namespace as the global namespace to be used when the function is called.
The function definition does not execute the function body; this gets executed only 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 scope that contains the function definition. The result must be a callable, which is invoked with the function object as the only argument. The returned value is bound to the function name instead of the function object. Multiple decorators are applied in nested fashion. For example, the following code
@f1(arg)
@f2
def func(): pass
is roughly equivalent to
def func(): pass
func = f1(arg)(f2(func))
except that the original function is not temporarily bound to the name func.
Alterado na versão 3.9: Functions may be decorated with any valid assignment_expression.
Previously, the grammar was much more restrictive; see PEP 614 for
details.
When one or more parameters have the form parameter =
expression, the function is said to have “default parameter values.” For a
parameter with a default value, the corresponding argument may be
omitted from a call, in which
case the parameter’s default value is substituted. If a parameter has a default
value, all following parameters up until the “*” must also have a default
value — this is a syntactic restriction that is not expressed by the grammar.
Default parameter values are evaluated from left to right when the function
definition is executed. This means that the expression is evaluated once, when
the function is defined, and that the same “pre-computed” value is used for each
call. This is especially important to understand when a default parameter is a
mutable object, such as a list or a dictionary: if the function modifies the
object (e.g. by appending an item to a list), the default value is in effect
modified. This is generally not what was intended. A way around this is to use
None 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 Chamadas. A
function call always assigns values to all parameters mentioned in the parameter
list, either from positional arguments, from keyword arguments, or from default
values. If the form “*identifier” is present, it is initialized to a tuple
receiving any excess positional parameters, defaulting to the empty tuple.
If the form “**identifier” is present, it is initialized to a new
ordered mapping receiving any excess keyword arguments, defaulting to a
new empty mapping of the same type. Parameters after “*” or
“*identifier” are keyword-only parameters and may only be passed
by keyword arguments. Parameters before “/” are positional-only parameters
and may only be passed by positional arguments.
Alterado na versão 3.8: The / function parameter syntax may be used to indicate positional-only
parameters. See PEP 570 for details.
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 of
the form “-> expression” after the parameter list. These annotations can be
any valid Python expression. The presence of annotations does not change the
semantics of a function. The annotation values are available as values of
a 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 which
enables postponed evaluation. Otherwise, they are evaluated when the function
definition is executed. In this case annotations may be evaluated in
a different order than they appear in the source code.
It is also possible to create anonymous functions (functions not bound to a
name), for immediate use in expressions. This uses lambda expressions, described in
section Lambdas. Note that the lambda expression is merely a shorthand for a
simplified function definition; a function defined in a “def”
statement can be passed around or assigned to another name just like a function
defined by a lambda expression. The “def” form is actually more powerful
since it allows the execution of multiple statements and annotations.
Programmer’s note: Functions are first-class objects. A “def” statement
executed inside a function definition defines a local function that can be
returned or passed around. Free variables used in the nested function can
access the local variables of the function containing the def. See section
Nomeação e ligação for details.
Ver também
- PEP 3107 - Function Annotations
The original specification for function annotations.
- PEP 484 - Dicas de tipo
Definition of a standard meaning for annotations: type hints.
- PEP 526 - Sintaxe para Anotações de Variáveis
Ability to type hint variable declarations, including class variables and instance variables
- PEP 563 - Postponed Evaluation of Annotations
Support for forward references within annotations by preserving annotations in a string form at runtime instead of eager evaluation.
8.7. Definições de classe¶
A class definition defines a class object (see section A hierarquia de tipos padrão):
classdef ::= [decorators] "class"classname[inheritance] ":"suiteinheritance ::= "(" [argument_list] ")" classname ::=identifier
A class definition is an executable statement. The inheritance list usually
gives a list of base classes (see Metaclasses for more advanced uses), so
each item in the list should evaluate to a class object which allows
subclassing. Classes without an inheritance list inherit, by default, from the
base class object; hence,
class Foo:
pass
é equivalente a
class Foo(object):
pass
The class’s suite is then executed in a new execution frame (see Nomeação e ligação), using a newly created local namespace and the original global namespace. (Usually, the suite contains mostly function definitions.) When the class’s suite finishes execution, its execution frame is discarded but its local namespace is saved. 3 A class object is then created using the inheritance list for the base classes and the saved local namespace for the attribute dictionary. The class name is bound to this class object in the original local namespace.
The order in which attributes are defined in the class body is preserved
in the new class’s __dict__. Note that this is reliable only right
after the class is created and only for classes that were defined using
the definition syntax.
Class creation can be customized heavily using metaclasses.
Classes can also be decorated: just like when decorating functions,
@f1(arg)
@f2
class Foo: pass
is roughly equivalent to
class Foo: pass
Foo = f1(arg)(f2(Foo))
The evaluation rules for the decorator expressions are the same as for function decorators. The result is then bound to the class name.
Alterado na versão 3.9: Classes may be decorated with any valid assignment_expression.
Previously, the grammar was much more restrictive; see PEP 614 for
details.
Programmer’s note: Variables defined in the class definition are class
attributes; they are shared by instances. Instance attributes can be set in a
method with self.name = value. Both class and instance attributes are
accessible through the notation “self.name”, and an instance attribute hides
a class attribute with the same name when accessed in this way. Class
attributes can be used as defaults for instance attributes, but using mutable
values there can lead to unexpected results. Descriptors
can be used to create instance variables with different implementation details.
Ver também
- PEP 3115 - Metaclasses no Python 3000
The proposal that changed the declaration of metaclasses to the current syntax, and the semantics for how classes with metaclasses are constructed.
- PEP 3129 - Class Decorators
The proposal that added class decorators. Function and method decorators were introduced in PEP 318.
8.8. Corrotinas¶
Novo na versão 3.5.
8.8.1. Definição de função de corrotina¶
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 and
async identifiers become reserved keywords; await expressions,
async for and async with can only be used in
coroutine 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 body
of 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 provides an __aiter__ method that directly
returns an asynchronous iterator, which can call asynchronous code in
its __anext__ method.
The async for statement allows convenient iteration over asynchronous
iterables.
O seguinte código:
async for TARGET in ITER:
SUITE
else:
SUITE2
Is semantically equivalent to:
iter = (ITER)
iter = type(iter).__aiter__(iter)
running = True
while running:
try:
TARGET = await type(iter).__anext__(iter)
except StopAsyncIteration:
running = False
else:
SUITE
else:
SUITE2
See also __aiter__() and __anext__() for details.
It is a SyntaxError to use an async for statement outside the
body 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 is able to suspend execution in its enter and exit methods.
O seguinte código:
async with EXPRESSION as TARGET:
SUITE
é semanticamente equivalente a:
manager = (EXPRESSION)
aenter = type(manager).__aenter__
aexit = type(manager).__aexit__
value = await aenter(manager)
hit_except = False
try:
TARGET = value
SUITE
except:
hit_except = True
if not await aexit(manager, *sys.exc_info()):
raise
finally:
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 the
body of a coroutine function.
Ver também
- PEP 492 - Coroutines with async and await syntax
The proposal that made coroutines a proper standalone concept in Python, and added supporting syntax.
Notas de rodapé
- 1
The exception is propagated to the invocation stack unless there is a
finallyclause which happens to raise another exception. That new exception causes the old one to be lost.- 2
A string literal appearing as the first statement in the function body is transformed into the function’s
__doc__attribute and therefore the function’s docstring.- 3
A string literal appearing as the first statement in the class body is transformed into the namespace’s
__doc__item and therefore the class’s docstring.