7. Simple statements
********************

A simple statement is comprised within a single logical line. Several
simple statements may occur on a single line separated by semicolons.
The syntax for simple statements is:

   simple_stmt ::= expression_stmt
                   | assert_stmt
                   | assignment_stmt
                   | augmented_assignment_stmt
                   | annotated_assignment_stmt
                   | pass_stmt
                   | del_stmt
                   | return_stmt
                   | yield_stmt
                   | raise_stmt
                   | break_stmt
                   | continue_stmt
                   | import_stmt
                   | future_stmt
                   | global_stmt
                   | nonlocal_stmt


7.1. Expression statements
==========================

Expression statements are used (mostly interactively) to compute and
write a value, or (usually) to call a procedure (a function that
returns no meaningful result; in Python, procedures return the value
"None").  Other uses of expression statements are allowed and
occasionally useful.  The syntax for an expression statement is:

   expression_stmt ::= starred_expression

An expression statement evaluates the expression list (which may be a
single expression).

In interactive mode, if the value is not "None", it is converted to a
string using the built-in "repr()" function and the resulting string
is written to standard output on a line by itself (except if the
result is "None", so that procedure calls do not cause any output.)


7.2. Assignment statements
==========================

Assignment statements are used to (re)bind names to values and to
modify attributes or items of mutable objects:

   assignment_stmt ::= (target_list "=")+ (starred_expression | yield_expression)
   target_list     ::= target ("," target)* [","]
   target          ::= identifier
              | "(" [target_list] ")"
              | "[" [target_list] "]"
              | attributeref
              | subscription
              | slicing
              | "*" target

(See section Primaries for the syntax definitions for *attributeref*,
*subscription*, and *slicing*.)

An assignment statement evaluates the expression list (remember that
this can be a single expression or a comma-separated list, the latter
yielding a tuple) and assigns the single resulting object to each of
the target lists, from left to right.

Assignment is defined recursively depending on the form of the target
(list). When a target is part of a mutable object (an attribute
reference, subscription or slicing), the mutable object must
ultimately perform the assignment and decide about its validity, and
may raise an exception if the assignment is unacceptable.  The rules
observed by various types and the exceptions raised are given with the
definition of the object types (see section The standard type
hierarchy).

Assignment of an object to a target list, optionally enclosed in
parentheses or square brackets, is recursively defined as follows.

* 如果目标列表为后面不带逗号、可以包含于圆括号内的单一目标，则将对象
  赋 值给该目标。

* Else: The object must be an iterable with the same number of items
  as there are targets in the target list, and the items are assigned,
  from left to right, to the corresponding targets.

  * If the target list contains one target prefixed with an
    asterisk, called a "starred" target: The object must be an
    iterable with at least as many items as there are targets in the
    target list, minus one.  The first items of the iterable are
    assigned, from left to right, to the targets before the starred
    target.  The final items of the iterable are assigned to the
    targets after the starred target.  A list of the remaining items
    in the iterable is then assigned to the starred target (the list
    can be empty).

  * Else: The object must be an iterable with the same number of
    items as there are targets in the target list, and the items are
    assigned, from left to right, to the corresponding targets.

Assignment of an object to a single target is recursively defined as
follows.

* If the target is an identifier (name):

  * If the name does not occur in a "global" or "nonlocal" statement
    in the current code block: the name is bound to the object in the
    current local namespace.

  * Otherwise: the name is bound to the object in the global
    namespace or the outer namespace determined by "nonlocal",
    respectively.

  The name is rebound if it was already bound.  This may cause the
  reference count for the object previously bound to the name to reach
  zero, causing the object to be deallocated and its destructor (if it
  has one) to be called.

* If the target is an attribute reference: The primary expression in
  the reference is evaluated.  It should yield an object with
  assignable attributes; if this is not the case, "TypeError" is
  raised.  That object is then asked to assign the assigned object to
  the given attribute; if it cannot perform the assignment, it raises
  an exception (usually but not necessarily "AttributeError").

  注意：如果该对象为类实例并且属性引用在赋值运算符的两侧都出现，则右侧
  表达式 "a.x" 可以访问实例属性或（如果实例属性不存在）类属性。 左侧目
  标 "a.x" 将总是设定为实例属性，并在必要时创建该实例属性。 因此 "a.x"
  的两次出现不一定指向相同的属性：如果右侧表达式指向一个类属性，则左侧
  会创建一个新的实例属性作为赋值的目标:

     class Cls:
         x = 3             # class variable
     inst = Cls()
     inst.x = inst.x + 1   # writes inst.x as 4 leaving Cls.x as 3

  This description does not necessarily apply to descriptor
  attributes, such as properties created with "property()".

* If the target is a subscription: The primary expression in the
  reference is evaluated.  It should yield either a mutable sequence
  object (such as a list) or a mapping object (such as a dictionary).
  Next, the subscript expression is evaluated.

  If the primary is a mutable sequence object (such as a list), the
  subscript must yield an integer.  If it is negative, the sequence's
  length is added to it.  The resulting value must be a nonnegative
  integer less than the sequence's length, and the sequence is asked
  to assign the assigned object to its item with that index.  If the
  index is out of range, "IndexError" is raised (assignment to a
  subscripted sequence cannot add new items to a list).

  If the primary is a mapping object (such as a dictionary), the
  subscript must have a type compatible with the mapping's key type,
  and the mapping is then asked to create a key/datum pair which maps
  the subscript to the assigned object.  This can either replace an
  existing key/value pair with the same key value, or insert a new
  key/value pair (if no key with the same value existed).

  For user-defined objects, the "__setitem__()" method is called with
  appropriate arguments.

* If the target is a slicing: The primary expression in the
  reference is evaluated.  It should yield a mutable sequence object
  (such as a list).  The assigned object should be a sequence object
  of the same type.  Next, the lower and upper bound expressions are
  evaluated, insofar they are present; defaults are zero and the
  sequence's length.  The bounds should evaluate to integers. If
  either bound is negative, the sequence's length is added to it.  The
  resulting bounds are clipped to lie between zero and the sequence's
  length, inclusive.  Finally, the sequence object is asked to replace
  the slice with the items of the assigned sequence.  The length of
  the slice may be different from the length of the assigned sequence,
  thus changing the length of the target sequence, if the target
  sequence allows it.

**CPython implementation detail:** In the current implementation, the
syntax for targets is taken to be the same as for expressions, and
invalid syntax is rejected during the code generation phase, causing
less detailed error messages.

Although the definition of assignment implies that overlaps between
the left-hand side and the right-hand side are 'simultaneous' (for
example "a, b = b, a" swaps two variables), overlaps *within* the
collection of assigned-to variables occur left-to-right, sometimes
resulting in confusion.  For instance, the following program prints
"[0, 2]":

   x = [0, 1]
   i = 0
   i, x[i] = 1, 2         # i is updated, then x[i] is updated
   print(x)

也參考:

  **PEP 3132** - Extended Iterable Unpacking
     The specification for the "*target" feature.


7.2.1. Augmented assignment statements
--------------------------------------

Augmented assignment is the combination, in a single statement, of a
binary operation and an assignment statement:

   augmented_assignment_stmt ::= augtarget augop (expression_list | yield_expression)
   augtarget                 ::= identifier | attributeref | subscription | slicing
   augop                     ::= "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**="
             | ">>=" | "<<=" | "&=" | "^=" | "|="

(See section Primaries for the syntax definitions of the last three
symbols.)

An augmented assignment evaluates the target (which, unlike normal
assignment statements, cannot be an unpacking) and the expression
list, performs the binary operation specific to the type of assignment
on the two operands, and assigns the result to the original target.
The target is only evaluated once.

An augmented assignment expression like "x += 1" can be rewritten as
"x = x + 1" to achieve a similar, but not exactly equal effect. In the
augmented version, "x" is only evaluated once. Also, when possible,
the actual operation is performed *in-place*, meaning that rather than
creating a new object and assigning that to the target, the old object
is modified instead.

Unlike normal assignments, augmented assignments evaluate the left-
hand side *before* evaluating the right-hand side.  For example, "a[i]
+= f(x)" first looks-up "a[i]", then it evaluates "f(x)" and performs
the addition, and lastly, it writes the result back to "a[i]".

With the exception of assigning to tuples and multiple targets in a
single statement, the assignment done by augmented assignment
statements is handled the same way as normal assignments. Similarly,
with the exception of the possible *in-place* behavior, the binary
operation performed by augmented assignment is the same as the normal
binary operations.

For targets which are attribute references, the same caveat about
class and instance attributes applies as for regular assignments.


7.2.2. Annotated assignment statements
--------------------------------------

*标注* 赋值就是在单个语句中将变量或属性标注和可选的赋值语句合为一体:

   annotated_assignment_stmt ::= augtarget ":" expression
                                 ["=" (starred_expression | yield_expression)]

与普通 Assignment statements 的差别在于仅允许单个目标。

For simple names as assignment targets, if in class or module scope,
the annotations are evaluated and stored in a special class or module
attribute "__annotations__" that is a dictionary mapping from variable
names (mangled if private) to evaluated annotations. This attribute is
writable and is automatically created at the start of class or module
body execution, if annotations are found statically.

For expressions as assignment targets, the annotations are evaluated
if in class or module scope, but not stored.

If a name is annotated in a function scope, then this name is local
for that scope. Annotations are never evaluated and stored in function
scopes.

If the right hand side is present, an annotated assignment performs
the actual assignment before evaluating annotations (where
applicable). If the right hand side is not present for an expression
target, then the interpreter evaluates the target except for the last
"__setitem__()" or "__setattr__()" call.

也參考:

  **PEP 526** - Syntax for Variable Annotations
     The proposal that added syntax for annotating the types of
     variables (including class variables and instance variables),
     instead of expressing them through comments.

  **PEP 484** - Type hints
     The proposal that added the "typing" module to provide a standard
     syntax for type annotations that can be used in static analysis
     tools and IDEs.

3.8 版更變: 现在带有标注的赋值允许在右边以同样的表达式作为常规赋值。
之前，某些表达式（例如未加圆括号的元组表达式）会导致语法错误。


7.3. "assert" 语句
==================

Assert statements are a convenient way to insert debugging assertions
into a program:

   assert_stmt ::= "assert" expression ["," expression]

The simple form, "assert expression", is equivalent to

   if __debug__:
       if not expression: raise AssertionError

The extended form, "assert expression1, expression2", is equivalent to

   if __debug__:
       if not expression1: raise AssertionError(expression2)

以上等价形式假定 "__debug__" 和 "AssertionError" 指向具有指定名称的内
置变量。 在当前实现中，内置变量 "__debug__" 在正常情况下为 "True"，在
请求优化时为 "False" (对应命令行选项为 "-O")。 如果在编译时请求优化，
当前代码生成器不会为 assert 语句发出任何代码。 请注意不必在错误信息中
包含失败表达式的源代码；它会被作为栈追踪的一部分被显示。

Assignments to "__debug__" are illegal.  The value for the built-in
variable is determined when the interpreter starts.


7.4. "pass" 语句
================

   pass_stmt ::= "pass"

"pass" is a null operation --- when it is executed, nothing happens.
It is useful as a placeholder when a statement is required
syntactically, but no code needs to be executed, for example:

   def f(arg): pass    # a function that does nothing (yet)

   class C: pass       # a class with no methods (yet)


7.5. "del" 语句
===============

   del_stmt ::= "del" target_list

Deletion is recursively defined very similar to the way assignment is
defined. Rather than spelling it out in full details, here are some
hints.

Deletion of a target list recursively deletes each target, from left
to right.

Deletion of a name removes the binding of that name from the local or
global namespace, depending on whether the name occurs in a "global"
statement in the same code block.  If the name is unbound, a
"NameError" exception will be raised.

Deletion of attribute references, subscriptions and slicings is passed
to the primary object involved; deletion of a slicing is in general
equivalent to assignment of an empty slice of the right type (but even
this is determined by the sliced object).

3.2 版更變: Previously it was illegal to delete a name from the local
namespace if it occurs as a free variable in a nested block.


7.6. "return" 语句
==================

   return_stmt ::= "return" [expression_list]

"return" may only occur syntactically nested in a function definition,
not within a nested class definition.

If an expression list is present, it is evaluated, else "None" is
substituted.

"return" leaves the current function call with the expression list (or
"None") as return value.

当 "return" 将控制流传出一个带有 "finally" 子句的 "try" 语句时，该
"finally" 子句会先被执行然后再真正离开该函数。

In a generator function, the "return" statement indicates that the
generator is done and will cause "StopIteration" to be raised. The
returned value (if any) is used as an argument to construct
"StopIteration" and becomes the "StopIteration.value" attribute.

在一个异步生成器函数中，一个空的 "return" 语句表示异步生成器已完成并将
导致 "StopAsyncIteration" 被引发。 一个非空的 "return" 语句在异步生成
器函数中会导致语法错误。


7.7. "yield" 语句
=================

   yield_stmt ::= yield_expression

A "yield" statement is semantically equivalent to a yield expression.
The yield statement can be used to omit the parentheses that would
otherwise be required in the equivalent yield expression statement.
For example, the yield statements

   yield <expr>
   yield from <expr>

are equivalent to the yield expression statements

   (yield <expr>)
   (yield from <expr>)

Yield expressions and statements are only used when defining a
*generator* function, and are only used in the body of the generator
function.  Using yield in a function definition is sufficient to cause
that definition to create a generator function instead of a normal
function.

For full details of "yield" semantics, refer to the Yield expressions
section.


7.8. "raise" 语句
=================

   raise_stmt ::= "raise" [expression ["from" expression]]

If no expressions are present, "raise" re-raises the last exception
that was active in the current scope.  If no exception is active in
the current scope, a "RuntimeError" exception is raised indicating
that this is an error.

Otherwise, "raise" evaluates the first expression as the exception
object.  It must be either a subclass or an instance of
"BaseException". If it is a class, the exception instance will be
obtained when needed by instantiating the class with no arguments.

The *type* of the exception is the exception instance's class, the
*value* is the instance itself.

A traceback object is normally created automatically when an exception
is raised and attached to it as the "__traceback__" attribute, which
is writable. You can create an exception and set your own traceback in
one step using the "with_traceback()" exception method (which returns
the same exception instance, with its traceback set to its argument),
like so:

   raise Exception("foo occurred").with_traceback(tracebackobj)

The "from" clause is used for exception chaining: if given, the second
*expression* must be another exception class or instance, which will
then be attached to the raised exception as the "__cause__" attribute
(which is writable).  If the raised exception is not handled, both
exceptions will be printed:

   >>> try:
   ...     print(1 / 0)
   ... except Exception as exc:
   ...     raise RuntimeError("Something bad happened") from exc
   ...
   Traceback (most recent call last):
     File "<stdin>", line 2, in <module>
   ZeroDivisionError: division by zero

   The above exception was the direct cause of the following exception:

   Traceback (most recent call last):
     File "<stdin>", line 4, in <module>
   RuntimeError: Something bad happened

A similar mechanism works implicitly if an exception is raised inside
an exception handler or a "finally" clause: the previous exception is
then attached as the new exception's "__context__" attribute:

   >>> try:
   ...     print(1 / 0)
   ... except:
   ...     raise RuntimeError("Something bad happened")
   ...
   Traceback (most recent call last):
     File "<stdin>", line 2, in <module>
   ZeroDivisionError: division by zero

   During handling of the above exception, another exception occurred:

   Traceback (most recent call last):
     File "<stdin>", line 4, in <module>
   RuntimeError: Something bad happened

Exception chaining can be explicitly suppressed by specifying "None"
in the "from" clause:

   >>> try:
   ...     print(1 / 0)
   ... except:
   ...     raise RuntimeError("Something bad happened") from None
   ...
   Traceback (most recent call last):
     File "<stdin>", line 4, in <module>
   RuntimeError: Something bad happened

Additional information on exceptions can be found in section
Exceptions, and information about handling exceptions is in section
try 语句.

3.3 版更變: "None" is now permitted as "Y" in "raise X from Y".

3.3 版新加入: The "__suppress_context__" attribute to suppress
automatic display of the exception context.


7.9. "break" 语句
=================

   break_stmt ::= "break"

"break" may only occur syntactically nested in a "for" or "while"
loop, but not nested in a function or class definition within that
loop.

它会终结最近的外层循环，如果循环有可选的 "else" 子句，也会跳过该子句。

If a "for" loop is terminated by "break", the loop control target
keeps its current value.

当 "break" 将控制流传出一个带有 "finally" 子句的 "try" 语句时，该
"finally" 子句会先被执行然后再真正离开该循环。


7.10. "continue" 语句
=====================

   continue_stmt ::= "continue"

"continue" 在语法上只会出现于 "for" 或 "while" 循环所嵌套的代码中，但
不会出现于该循环内部的函数或类定义中。 它会继续执行最近的外层循环的下
一个轮次。

当 "continue" 将控制流传出一个带有 "finally" 子句的 "try" 语句时，该
"finally" 子句会先被执行然后再真正开始循环的下一个轮次。


7.11. "import" 语句
===================

   import_stmt     ::= "import" module ["as" identifier] ("," module ["as" identifier])*
                   | "from" relative_module "import" identifier ["as" identifier]
                   ("," identifier ["as" identifier])*
                   | "from" relative_module "import" "(" identifier ["as" identifier]
                   ("," identifier ["as" identifier])* [","] ")"
                   | "from" module "import" "*"
   module          ::= (identifier ".")* identifier
   relative_module ::= "."* module | "."+

The basic import statement (no "from" clause) is executed in two
steps:

1. find a module, loading and initializing it if necessary

2. define a name or names in the local namespace for the scope
   where the "import" statement occurs.

When the statement contains multiple clauses (separated by commas) the
two steps are carried out separately for each clause, just as though
the clauses had been separated out into individual import statements.

The details of the first step, finding and loading modules are
described in greater detail in the section on the import system, which
also describes the various types of packages and modules that can be
imported, as well as all the hooks that can be used to customize the
import system. Note that failures in this step may indicate either
that the module could not be located, *or* that an error occurred
while initializing the module, which includes execution of the
module's code.

If the requested module is retrieved successfully, it will be made
available in the local namespace in one of three ways:

* 如果模块名称之后带有 "as"，则跟在 "as" 之后的名称将直接绑定到所导
  入 的模块。

* If no other name is specified, and the module being imported is a
  top level module, the module's name is bound in the local namespace
  as a reference to the imported module

* If the module being imported is *not* a top level module, then the
  name of the top level package that contains the module is bound in
  the local namespace as a reference to the top level package. The
  imported module must be accessed using its full qualified name
  rather than directly

The "from" form uses a slightly more complex process:

1. find the module specified in the "from" clause, loading and
   initializing it if necessary;

2. for each of the identifiers specified in the "import" clauses:

   1. check if the imported module has an attribute by that name

   2. if not, attempt to import a submodule with that name and then
      check the imported module again for that attribute

   3. if the attribute is not found, "ImportError" is raised.

   4. 否则的话，将对该值的引用存入局部命名空间，如果有 "as" 子句则
      使用 其指定的名称，否则使用该属性的名称

Examples:

   import foo                 # foo imported and bound locally
   import foo.bar.baz         # foo.bar.baz imported, foo bound locally
   import foo.bar.baz as fbb  # foo.bar.baz imported and bound as fbb
   from foo.bar import baz    # foo.bar.baz imported and bound as baz
   from foo import attr       # foo imported and foo.attr bound as attr

If the list of identifiers is replaced by a star ("'*'"), all public
names defined in the module are bound in the local namespace for the
scope where the "import" statement occurs.

The *public names* defined by a module are determined by checking the
module's namespace for a variable named "__all__"; if defined, it must
be a sequence of strings which are names defined or imported by that
module.  The names given in "__all__" are all considered public and
are required to exist.  If "__all__" is not defined, the set of public
names includes all names found in the module's namespace which do not
begin with an underscore character ("'_'").  "__all__" should contain
the entire public API. It is intended to avoid accidentally exporting
items that are not part of the API (such as library modules which were
imported and used within the module).

The wild card form of import --- "from module import *" --- is only
allowed at the module level.  Attempting to use it in class or
function definitions will raise a "SyntaxError".

当指定要导入哪个模块时，你不必指定模块的绝对名称。 当一个模块或包被包
含在另一个包之中时，可以在同一个最高层级包中进行相对导入，而不必提及包
名称。 通过在 "from" 之后指定的模块或包中使用前缀点号，你可以在不指定
确切名称的情况下指明在当前包层级结构中要上溯多少级。 一个前缀点号表示
是执行导入的模块所在的当前包，两个点号表示上溯一个包层级。 三个点号表
示上溯两级，依此类推。 因此如果你执行 "from . import mod" 时所处位置为
"pkg" 包内的一个模块，则最终你将导入 "pkg.mod"。 如果你执行 "from
..subpkg2 import mod" 时所处位置为 "pkg.subpkg1" 则你将导入
"pkg.subpkg2.mod"。 有关相对导入的规范说明包含在 包相对导入 一节中。

"importlib.import_module()" is provided to support applications that
determine dynamically the modules to be loaded.


7.11.1. Future statements
-------------------------

A *future statement* is a directive to the compiler that a particular
module should be compiled using syntax or semantics that will be
available in a specified future release of Python where the feature
becomes standard.

The future statement is intended to ease migration to future versions
of Python that introduce incompatible changes to the language.  It
allows use of the new features on a per-module basis before the
release in which the feature becomes standard.

   future_stmt ::= "from" "__future__" "import" feature ["as" identifier]
                   ("," feature ["as" identifier])*
                   | "from" "__future__" "import" "(" feature ["as" identifier]
                   ("," feature ["as" identifier])* [","] ")"
   feature     ::= identifier

A future statement must appear near the top of the module.  The only
lines that can appear before a future statement are:

* the module docstring (if any),

* comments,

* blank lines, and

* other future statements.

The only feature in Python 3.7 that requires using the future
statement is "annotations".

All historical features enabled by the future statement are still
recognized by Python 3.  The list includes "absolute_import",
"division", "generators", "generator_stop", "unicode_literals",
"print_function", "nested_scopes" and "with_statement".  They are all
redundant because they are always enabled, and only kept for backwards
compatibility.

A future statement is recognized and treated specially at compile
time: Changes to the semantics of core constructs are often
implemented by generating different code.  It may even be the case
that a new feature introduces new incompatible syntax (such as a new
reserved word), in which case the compiler may need to parse the
module differently.  Such decisions cannot be pushed off until
runtime.

For any given release, the compiler knows which feature names have
been defined, and raises a compile-time error if a future statement
contains a feature not known to it.

The direct runtime semantics are the same as for any import statement:
there is a standard module "__future__", described later, and it will
be imported in the usual way at the time the future statement is
executed.

The interesting runtime semantics depend on the specific feature
enabled by the future statement.

Note that there is nothing special about the statement:

   import __future__ [as name]

That is not a future statement; it's an ordinary import statement with
no special semantics or syntax restrictions.

Code compiled by calls to the built-in functions "exec()" and
"compile()" that occur in a module "M" containing a future statement
will, by default, use the new syntax or semantics associated with the
future statement.  This can be controlled by optional arguments to
"compile()" --- see the documentation of that function for details.

A future statement typed at an interactive interpreter prompt will
take effect for the rest of the interpreter session.  If an
interpreter is started with the "-i" option, is passed a script name
to execute, and the script includes a future statement, it will be in
effect in the interactive session started after the script is
executed.

也參考:

  **PEP 236** - Back to the __future__
     The original proposal for the __future__ mechanism.


7.12. "global" 语句
===================

   global_stmt ::= "global" identifier ("," identifier)*

"global" 语句是作用于整个当前代码块的声明。 它意味着所列出的标识符将被
解读为全局变量。 要给全局变量赋值不可能不用到 "global" 关键字，不过自
由变量也可以指向全局变量而不必声明为全局变量。

在 "global" 语句中列出的名称不得在同一代码块内该 "global" 语句之前的位
置中使用。

Names listed in a "global" statement must not be defined as formal
parameters or in a "for" loop control target, "class" definition,
function definition, "import" statement, or variable annotation.

**CPython implementation detail:** The current implementation does not
enforce some of these restrictions, but programs should not abuse this
freedom, as future implementations may enforce them or silently change
the meaning of the program.

**程序员注意事项:** "global" 是对解析器的指令。 它仅对与 "global" 语句
同时被解析的代码起作用。 特别地，包含在提供给内置 "exec()" 函数字符串
或代码对象中的 "global" 语句并不会影响 *包含* 该函数调用的代码块，而包
含在这种字符串中的代码也不会受到包含该函数调用的代码中的 "global" 语句
影响。 这同样适用于 "eval()" 和 "compile()" 函数。


7.13. "nonlocal" 语句
=====================

   nonlocal_stmt ::= "nonlocal" identifier ("," identifier)*

The "nonlocal" statement causes the listed identifiers to refer to
previously bound variables in the nearest enclosing scope excluding
globals. This is important because the default behavior for binding is
to search the local namespace first.  The statement allows
encapsulated code to rebind variables outside of the local scope
besides the global (module) scope.

Names listed in a "nonlocal" statement, unlike those listed in a
"global" statement, must refer to pre-existing bindings in an
enclosing scope (the scope in which a new binding should be created
cannot be determined unambiguously).

Names listed in a "nonlocal" statement must not collide with pre-
existing bindings in the local scope.

也參考:

  **PEP 3104** - Access to Names in Outer Scopes
     The specification for the "nonlocal" statement.
