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_funcdef
   suite         ::= stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
   statement     ::= stmt_list NEWLINE | compound_stmt
   stmt_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_stmt
   try1_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 it is the class or a base class of the exception
object, or a tuple containing an item that is the class or a 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.

Alterado na versão 3.8: Prior to Python 3.8, a "continue" statement
was illegal in the "finally" clause due to a problem with the
implementation.


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)* ":" suite
   with_item ::= expression ["as" target]

A execução da instrução "with" com um "item" ocorre da seguinte
maneira:

1. The context expression (the expression given in the "with_item") is
   evaluated 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 value
   from "__enter__()" is assigned to it.

   Nota:

     The "with" statement 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.

6. O conjunto é executado.

7. 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, three
   "None" arguments 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 the "with"
   statement.

   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.

Ver também:

  **PEP 343** - A instrução "with"
     A especificação, o histórico e os exemplos para a instrução
     Python "with".


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] ":" suite
   decorators                ::= decorator+
   decorator                 ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
   dotted_name               ::= identifier ("." identifier)*
   parameter_list            ::= defparameter ("," defparameter)* "," "/" ["," [parameter_list_no_posonly]]
                        | parameter_list_no_posonly
   parameter_list_no_posonly ::= defparameter ("," defparameter)* ["," [parameter_list_starargs]]
                                 | parameter_list_starargs
   parameter_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".

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] ":" suite
   inheritance ::= "(" [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.

**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* is able to call asynchronous code in its
*iter* implementation, and *asynchronous iterator* can call
asynchronous code in its *next* method.

The "async for" statement allows convenient iteration over
asynchronous iterators.

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)
   aexit = type(manager).__aexit__
   aenter = type(manager).__aenter__
   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 "finally" clause 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*.
