4. Execution model
******************


4.1. Structure of a program
===========================

Python 程序是由代码块构成的。 *代码块* 是被作为一个单元来执行的一段
Python 程序文本。 以下几个都是代码块：模块、函数体和类定义。 交互式输
入的每条命令都是一个代码块。 一个脚本文件（作为标准输入发送给解释器或
是作为命令行参数发送给解释器的文件）也是一个代码块。 一条脚本命令（通
过 "-c" 选项在解释器命令行中指定的命令）也是一个代码块。  传递给内置函
数 "eval()" 和 "exec()" 的字符串参数也是代码块。

A code block is executed in an *execution frame*.  A frame contains
some administrative information (used for debugging) and determines
where and how execution continues after the code block's execution has
completed.


4.2. Naming and binding
=======================


4.2.1. Binding of names
-----------------------

*Names* refer to objects.  Names are introduced by name binding
operations.

以下构造会绑定名称：传给函数的正式形参，"import" 语句，类与函数定义（
这会在定义的代码块中绑定类或函数名称）以及发生以标识符为目标的赋值，
"for" 循环的开头，或 "with" 语句和 "except" 子句的 "as" 之后。
"import" 语句的 "from ... import *" 形式会绑定在被导入模块中定义的所有
名称，那些以下划线开头的除外。 这种形式仅在模块层级上被使用。

A target occurring in a "del" statement is also considered bound for
this purpose (though the actual semantics are to unbind the name).

Each assignment or import statement occurs within a block defined by a
class or function definition or at the module level (the top-level
code block).

If a name is bound in a block, it is a local variable of that block,
unless declared as "nonlocal" or "global".  If a name is bound at the
module level, it is a global variable.  (The variables of the module
code block are local and global.)  If a variable is used in a code
block but not defined there, it is a *free variable*.

Each occurrence of a name in the program text refers to the *binding*
of that name established by the following name resolution rules.


4.2.2. Resolution of names
--------------------------

A *scope* defines the visibility of a name within a block.  If a local
variable is defined in a block, its scope includes that block.  If the
definition occurs in a function block, the scope extends to any blocks
contained within the defining one, unless a contained block introduces
a different binding for the name.

When a name is used in a code block, it is resolved using the nearest
enclosing scope.  The set of all such scopes visible to a code block
is called the block's *environment*.

When a name is not found at all, a "NameError" exception is raised. If
the current scope is a function scope, and the name refers to a local
variable that has not yet been bound to a value at the point where the
name is used, an "UnboundLocalError" exception is raised.
"UnboundLocalError" is a subclass of "NameError".

If a name binding operation occurs anywhere within a code block, all
uses of the name within the block are treated as references to the
current block.  This can lead to errors when a name is used within a
block before it is bound.  This rule is subtle.  Python lacks
declarations and allows name binding operations to occur anywhere
within a code block.  The local variables of a code block can be
determined by scanning the entire text of the block for name binding
operations.

如果 "global" 语句出现在一个代码块中，则所有对该语句所指定名称的使用都
是在最高层级命名空间内对该名称绑定的引用。 名称在最高层级命名内的解析
是通过全局命名空间，也就是包含该代码块的模块的命名空间，以及内置命名空
间即 "builtins" 模块的命名空间。 全局命名空间会先被搜索。 如果未在其中
找到指定名称，再搜索内置命名空间。  "global" 语句必须位于所有对其所指
定名称的使用之前。

The "global" statement has the same scope as a name binding operation
in the same block.  If the nearest enclosing scope for a free variable
contains a global statement, the free variable is treated as a global.

The "nonlocal" statement causes corresponding names to refer to
previously bound variables in the nearest enclosing function scope.
"SyntaxError" is raised at compile time if the given name does not
exist in any enclosing function scope.

The namespace for a module is automatically created the first time a
module is imported.  The main module for a script is always called
"__main__".

Class definition blocks and arguments to "exec()" and "eval()" are
special in the context of name resolution. A class definition is an
executable statement that may use and define names. These references
follow the normal rules for name resolution with an exception that
unbound local variables are looked up in the global namespace. The
namespace of the class definition becomes the attribute dictionary of
the class. The scope of names defined in a class block is limited to
the class block; it does not extend to the code blocks of methods --
this includes comprehensions and generator expressions since they are
implemented using a function scope.  This means that the following
will fail:

   class A:
       a = 42
       b = list(a + i for i in range(10))


4.2.3. Builtins and restricted execution
----------------------------------------

**CPython implementation detail:** Users should not touch
"__builtins__"; it is strictly an implementation detail.  Users
wanting to override values in the builtins namespace should "import"
the "builtins" module and modify its attributes appropriately.

The builtins namespace associated with the execution of a code block
is actually found by looking up the name "__builtins__" in its global
namespace; this should be a dictionary or a module (in the latter case
the module's dictionary is used).  By default, when in the "__main__"
module, "__builtins__" is the built-in module "builtins"; when in any
other module, "__builtins__" is an alias for the dictionary of the
"builtins" module itself.


4.2.4. Interaction with dynamic features
----------------------------------------

Name resolution of free variables occurs at runtime, not at compile
time. This means that the following code will print 42:

   i = 10
   def f():
       print(i)
   i = 42
   f()

The "eval()" and "exec()" functions do not have access to the full
environment for resolving names.  Names may be resolved in the local
and global namespaces of the caller.  Free variables are not resolved
in the nearest enclosing namespace, but in the global namespace.  [1]
The "exec()" and "eval()" functions have optional arguments to
override the global and local namespace.  If only one namespace is
specified, it is used for both.


4.3. Exceptions
===============

Exceptions are a means of breaking out of the normal flow of control
of a code block in order to handle errors or other exceptional
conditions.  An exception is *raised* at the point where the error is
detected; it may be *handled* by the surrounding code block or by any
code block that directly or indirectly invoked the code block where
the error occurred.

The Python interpreter raises an exception when it detects a run-time
error (such as division by zero).  A Python program can also
explicitly raise an exception with the "raise" statement. Exception
handlers are specified with the "try" ... "except" statement.  The
"finally" clause of such a statement can be used to specify cleanup
code which does not handle the exception, but is executed whether an
exception occurred or not in the preceding code.

Python uses the "termination" model of error handling: an exception
handler can find out what happened and continue execution at an outer
level, but it cannot repair the cause of the error and retry the
failing operation (except by re-entering the offending piece of code
from the top).

当一个异常完全未被处理时，解释器会终止程序的执行，或者返回交互模式的主
循环。 无论是哪种情况，它都会打印栈回溯信息，除非是当异常为
"SystemExit" 的时候。

Exceptions are identified by class instances.  The "except" clause is
selected depending on the class of the instance: it must reference the
class of the instance or a base class thereof.  The instance can be
received by the handler and can carry additional information about the
exceptional condition.

備註: Exception messages are not part of the Python API.  Their
  contents may change from one version of Python to the next without
  warning and should not be relied on by code which will run under
  multiple versions of the interpreter.

See also the description of the "try" statement in section try 语句
and "raise" statement in section raise 语句.

-[ 註解 ]-

[1] This limitation occurs because the code that is executed by
    these operations is not available at the time the module is
    compiled.
