4. 执行模型
***********


4.1. 命名与绑定
===============

*Names* refer to objects.  Names are introduced by name binding
operations. Each occurrence of a name in the program text refers to
the *binding* of that name established in the innermost function block
containing the use.

A *block* is a piece of Python program text that is executed as a
unit. The following are blocks: a module, a function body, and a class
definition. Each command typed interactively is a block.  A script
file (a file given as standard input to the interpreter or specified
on the interpreter command line the first argument) is a code block.
A script command (a command specified on the interpreter command line
with the ‘**-c**’ option) is a code block.  The file read by the
built-in function "execfile()" is a code block.  The string argument
passed to the built-in function "eval()" and to the "exec" statement
is a code block. The expression read and evaluated by the built-in
function "input()" is a code block.

代码块在 *执行帧* 中被执行。 一个帧会包含某些管理信息（用于调试）并决
定代码块执行完成后应前往何处以及如何继续执行。

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.  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 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))

当一个名称在代码块中被使用时，会由包含它的最近作用域来解析。 对一个代
码块可见的所有这种作用域的集合称为该代码块的 *环境*。

If a name is bound in a block, it is a local variable of that block.
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*.

When a name is not found at all, a "NameError" exception is raised.
If the name refers to a local variable that has not been bound, a
"UnboundLocalError" exception is raised.  "UnboundLocalError" is a
subclass of "NameError".

The following constructs bind names: formal parameters to functions,
"import" statements, class and function definitions (these bind the
class or function name in the defining block), and targets that are
identifiers if occurring in an assignment, "for" loop header, in the
second position of an "except" clause header or after "as" in a "with"
statement.  The "import" statement of the form "from ... import *"
binds all names defined in the imported module, except those beginning
with an underscore.  This form may only be used at the module level.

A target occurring in a "del" statement is also considered bound for
this purpose (though the actual semantics are to unbind the name).  It
is illegal to unbind a name that is referenced by an enclosing scope;
the compiler will report a "SyntaxError".

每条赋值或导入语句均发生于类或函数内部定义的代码块中，或是发生于模块层
级（即最高层级的代码块）。

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.

If the global statement occurs within a block, all uses of the name
specified in the statement refer to the binding of that name in the
top-level namespace. Names are resolved in the top-level namespace by
searching the global namespace, i.e. the namespace of the module
containing the code block, and the builtins namespace, the namespace
of the module "__builtin__".  The global namespace is searched first.
If the name is not found there, the builtins namespace is searched.
The global statement must precede all uses of the name.

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 "__builtin__" (note: no
‘s’); when in any other module, "__builtins__" is an alias for the
dictionary of the "__builtin__" module itself.  "__builtins__" can be
set to a user-created dictionary to create a weak form of 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 "__builtin__" (no ‘s’) module and modify its attributes
appropriately.

模块的作用域会在模块第一次被导入时自动创建。 一个脚本的主模块总是被命
名为 "__main__"。

"global" 语句与同一代码块中名称绑定具有相同的作用域。 如果一个自由变量
的最近包含作用域中有一条 global 语句，则该自由变量也会被当作是全局变量
。

A class definition is an executable statement that may use and define
names. These references follow the normal rules for name resolution.
The namespace of the class definition becomes the attribute dictionary
of the class.  Names defined at the class scope are not visible in
methods.


4.1.1. 与动态特性的交互
-----------------------

There are several cases where Python statements are illegal when used
in conjunction with nested scopes that contain free variables.

If a variable is referenced in an enclosing scope, it is illegal to
delete the name.  An error will be reported at compile time.

If the wild card form of import — "import *" — is used in a function
and the function contains or is a nested block with free variables,
the compiler will raise a "SyntaxError".

If "exec" is used in a function and the function contains or is a
nested block with free variables, the compiler will raise a
"SyntaxError" unless the exec explicitly specifies the local namespace
for the "exec".  (In other words, "exec obj" would be illegal, but
"exec obj in ns" would be legal.)

The "eval()", "execfile()", and "input()" functions and the "exec"
statement 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" statement and
the "eval()" and "execfile()" functions have optional arguments to
override the global and local namespace.  If only one namespace is
specified, it is used for both.


4.2. 异常
=========

异常是中断代码块的正常控制流程以便处理错误或其他异常条件的一种方式。
异常会在错误被检测到的位置 *引发*，它可以被当前包围代码块或是任何直接
或间接发起调用发生错误的代码块的其他代码块所 *处理*。

Python 解析器会在检测到运行时错误（例如零作为被除数）的时候引发异常。
Python 程序也可以通过 "raise" 语句显式地引发异常。 异常处理是通过
"try" … "except" 语句来指定的。 该语句的 "finally" 子句可被用来指定清
理代码，它并不处理异常，而是无论之前的代码是否发生异常都会被执行。

Python 的错误处理采用的是“终止”模型：异常处理器可以找出发生了什么问题
，并在外层继续执行，但它不能修复错误的根源并重试失败的操作（除非通过从
顶层重新进入出错的代码片段）。

When an exception is not handled at all, the interpreter terminates
execution of the program, or returns to its interactive main loop.  In
either case, it prints a stack backtrace, except when the exception is
"SystemExit".

异常是通过类实例来标识的。 "except" 子句会依据实例的类来选择：它必须引
用实例的类或是其所属的基类。 实例可通过处理器被接收，并可携带有关异常
条件的附加信息。

Exceptions can also be identified by strings, in which case the
"except" clause is selected by object identity.  An arbitrary value
can be raised along with the identifying string which can be passed to
the handler.

注解: Messages to exceptions 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.

另请参看  The try statement 小节中对 "try" 语句的描述以及 The raise
statement 小节中对 "raise" 语句的描述。

-[ 脚注 ]-

[1] 出现这样的限制是由于通过这些操作执行的代码在模块被编译的时候并
    不可 用。
