7. 复合语句
***********

复合语句是包含其它语句（语句组）的语句；它们会以某种方式影响或控制所包
含其它语句的执行。 通常，复合语句会跨越多行，虽然在某些简单形式下整个
复合语句也可能包含于一行之内。

The "if", "while" and "for" statements implement traditional control
flow constructs.  "try" specifies exception handlers and/or cleanup
code for a group of statements.  Function and class definitions are
also syntactically compound statements.

Compound statements consist of one or more ‘clauses.’  A clause
consists of a header and a ‘suite.’  The clause headers of a
particular compound statement are all at the same indentation level.
Each clause header begins with a uniquely identifying keyword and ends
with a colon.  A suite is a group of statements controlled by a
clause.  A suite can be one or more semicolon-separated simple
statements on the same line as the header, following the header’s
colon, or it can be one or more indented statements on subsequent
lines.  Only the latter form of suite can contain nested compound
statements; the following is illegal, mostly because it wouldn’t be
clear to which "if" clause a following "else" clause would belong:

   if test1: if test2: print x

Also note that the semicolon binds tighter than the colon in this
context, so that in the following example, either all or none of the
"print" statements are executed:

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

总结:

   compound_stmt ::= if_stmt
                     | while_stmt
                     | for_stmt
                     | try_stmt
                     | with_stmt
                     | funcdef
                     | classdef
                     | decorated
   suite         ::= stmt_list NEWLINE | NEWLINE INDENT statement+ DEDENT
   statement     ::= stmt_list NEWLINE | compound_stmt
   stmt_list     ::= simple_stmt (";" simple_stmt)* [";"]

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

为了保证清晰，以下各节中语法规则采用将每个子句都放在单独行中的格式。


7.1. The "if" statement
=======================

"if" 语句用于有条件的执行:

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

它通过对表达式逐个求值直至找到一个真值（请参阅 布尔运算 了解真值与假值
的定义）在子句体中选择唯一匹配的一个；然后执行该子句体（而且 "if" 语句
的其他部分不会被执行或求值）。 如果所有表达式均为假值，则如果 "else"
子句体如果存在就会被执行。


7.2. The "while" statement
==========================

"while" 语句用于在表达式保持为真的情况下重复地执行:

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

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

A "break" statement executed in the first suite terminates the loop
without executing the "else" clause’s suite.  A "continue" statement
executed in the first suite skips the rest of the suite and goes back
to testing the expression.


7.3. The "for" statement
========================

"for" 语句用于对序列（例如字符串、元组或列表）或其他可迭代对象中的元素
进行迭代:

   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 of ascending indices.  Each
item in turn is assigned to the target list using the standard rules
for assignments, and then the suite is executed.  When the items are
exhausted (which is immediately when the sequence is empty), the suite
in the "else" clause, if present, is executed, and the loop
terminates.

A "break" statement executed in the first suite terminates the loop
without executing the "else" clause’s suite.  A "continue" statement
executed in the first suite skips the rest of the suite and continues
with the next item, or with the "else" clause if there was no next
item.

The suite may assign to the variable(s) in the target list; this does
not affect the next item assigned to it.

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

注解: 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)


7.4. The "try" statement
========================

"try" 语句可为一组语句指定异常处理器和/或清理代码:

   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

在 2.5 版更改: In previous versions of Python,
"try"…"except"…"finally" did not work. "try"…"except" had to be nested
in "try"…"finally".

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 compatible with the exception.

如果没有 except 子句与异常相匹配，则会在周边代码和发起调用栈上继续搜索
异常处理器。 [1]

如果在对 except 子句头中的表达式求值时引发了异常，则原来对处理器的搜索
会被取消，并在周边代码和调用栈上启动对新异常的搜索（它会被视作是整个
"try" 语句所引发的异常）。

When a matching except clause is found, the exception is assigned to
the target specified 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.)

Before an except clause’s suite is executed, details about the
exception are assigned to three variables in the "sys" module:
"sys.exc_type" receives the object identifying the exception;
"sys.exc_value" receives the exception’s parameter;
"sys.exc_traceback" receives a traceback object (see section 标准类型
层级结构) identifying the point in the program where the exception
occurred. These details are also available through the
"sys.exc_info()" function, which returns a tuple "(exc_type,
exc_value, exc_traceback)".  Use of the corresponding variables is
deprecated in favor of this function, since their use is unsafe in a
threaded program.  As of Python 1.5, the variables are restored to
their previous values (before the call) when returning from a function
that handled an exception.

The optional "else" clause is executed if the control flow leaves the
"try" suite, no exception was raised, and no "return", "continue", or
"break" statement was executed.  Exceptions in the "else" clause are
not handled by the preceding "except" clauses.

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 or
executes a "return" or "break" statement, the saved exception is
discarded:

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

在 "finally" 子句执行期间，程序不能获取异常信息。

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.’ A "continue" statement is illegal in
the "finally" clause. (The reason is a problem with the current
implementation — this restriction may be lifted in the future).

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'

有关异常的更多信息可以在 异常 一节找到，有关使用 "raise" 语句生成异常
的信息可以在 The raise statement 一节找到。


7.5. The "with" statement
=========================

2.5 新版功能.

"with" 语句用于包装带有使用上下文管理器 (参见 with 语句上下文管理器 一
节) 定义的方法的代码块的执行。 这允许对普通的 "try"…"except"…"finally"
使用模式进行封装以方便地重用。

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

带有一个“项目”的 "with" 语句的执行过程如下:

1. 对上下文表达式 (在 "with_item" 中给出的表达式) 求值以获得一个上
   下文 管理器。

2. 载入上下文管理器的 "__exit__()" 以便后续使用。

3. 发起调用上下文管理器的 "__enter__()" 方法。

4. 如果 "with" 语句中包含一个目标，来自 "__enter__()" 的返回值将被
   赋值 给它。

   注解: "with" 语句会保证如果 "__enter__()" 方法返回时未发生错误，
     则 "__exit__()" 将总是被调用。 因此，如果在对目标列表赋值期间发生
     错 误，则会将其视为在语句体内部发生的错误。 参见下面的第 6 步。

5. 执行语句体。

6. 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.

   如果语句体由于异常以外的任何原因退出，则来自 "__exit__()" 的返回值
   会被忽略，并会在该类退出正常的发生位置继续执行。

如果有多个项目，则会视作存在多个 "with" 语句嵌套来处理多个上下文管理器
:

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

等价于

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

注解: In Python 2.5, the "with" statement is only allowed when the
  "with_statement" feature has been enabled.  It is always enabled in
  Python 2.6.

在 2.7 版更改: 支持多个上下文表达式。

参见:

  **PEP 343** - “with” 语句
     Python "with" 语句的规范描述、背景和示例。


7.6. 函数定义
=============

函数定义就是对用户自定义函数的定义（参见 标准类型层级结构 一节）:

   decorated      ::= decorators (classdef | funcdef)
   decorators     ::= decorator+
   decorator      ::= "@" dotted_name ["(" [argument_list [","]] ")"] NEWLINE
   funcdef        ::= "def" funcname "(" [parameter_list] ")" ":" suite
   dotted_name    ::= identifier ("." identifier)*
   parameter_list ::= (defparameter ",")*
                      (  "*" identifier ["," "**" identifier]
                      | "**" identifier
                      | defparameter [","] )
   defparameter   ::= parameter ["=" expression]
   sublist        ::= parameter ("," parameter)* [","]
   parameter      ::= identifier | "(" sublist ")"
   funcname       ::= identifier

函数定义是一条可执行语句。 它执行时会在当前局部命名空间中将函数名称绑
定到一个函数对象（函数可执行代码的包装器）。 这个函数对象包含对当前全
局命名空间的引用，作为函数被调用时所使用的全局命名空间。

函数定义并不会执行函数体；只有当函数被调用时才会执行此操作。 [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 equivalent to:

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

When one or more top-level *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 must also have a default value — this is a syntactic
restriction that is not expressed by the grammar.

**Default parameter values are evaluated 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 调用.
A function call always assigns values to all parameters mentioned in
the parameter list, either from position 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 dictionary
receiving any excess keyword arguments, defaulting to a new empty
dictionary.

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 lambda 表达式.  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.

**Programmer’s note:** Functions are first-class objects.  A “"def"”
form 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 命名与绑定 for details.


7.7. 类定义
===========

类定义就是对类对象的定义 (参见 标准类型层级结构 一节):

   classdef    ::= "class" classname [inheritance] ":" suite
   inheritance ::= "(" [expression_list] ")"
   classname   ::= identifier

A class definition is an executable statement.  It first evaluates the
inheritance list, if present.  Each item in the inheritance list
should evaluate to a class object or class type which allows
subclassing.  The class’s suite is then executed in a new execution
frame (see section 命名与绑定), using a newly created local namespace
and the original global namespace. (Usually, the suite contains only
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.

**Programmer’s note:** Variables defined in the class definition are
class variables; they are shared by all instances.  To create instance
variables, they can be set in a method with "self.name = value".  Both
class and instance variables are accessible through the notation
“"self.name"”, and an instance variable hides a class variable with
the same name when accessed in this way. Class variables can be used
as defaults for instance variables, but using mutable values there can
lead to unexpected results.  For *new-style class*es, descriptors can
be used to create instance variables with different implementation
details.

Class definitions, like function definitions, may be wrapped by one or
more *decorator* expressions.  The evaluation rules for the decorator
expressions are the same as for functions.  The result must be a class
object, which is then bound to the class name.

-[ 脚注 ]-

[1] 异常会被传播给发起调用栈，除非存在一个 "finally" 子句正好引发
    了另 一个异常。 新引发的异常将导致旧异常的丢失。

[2] 作为函数体的第一条语句出现的字符串字面值会被转换为函数的
    "__doc__" 属性，也就是该函数的 *docstring*。

[3] 作为类体的第一条语句出现的字符串字面值会被转换为命名空间的
    "__doc__" 条目，也就是该类的 *docstring*。
