実行モデル
**********


名前づけと束縛 (naming and binding)
===================================

*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.

コードブロックは、実行フレーム (*execution frame*) 上で実行されます。
実行フレームには、 (デバッグに使われる) 管理情報が収められています。ま
た、現在のコードブロックの実行が完了した際に、どのようにプログラムの実
行を継続するかを決定しています。

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))

ある名前がコードブロック内で使われると、その名前を最も近傍から囲うよう
なスコープ (最内スコープ: nearest enclosing scope) を使って束縛の解決
を行います。こうしたスコープからなる、あるコードブロック内で参照できる
スコープ全ての集合は、ブロックの環境(*environment*)と呼ばれます。

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".

代入文や import 文はいずれも、クラスや関数定義、モジュールレベル (トッ
プレベルのコードブロック) 内で起こります。

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.

あるモジュールの名前空間は、そのモジュールが最初に import された時に自
動的に作成されます。スクリプトの主モジュール (main module) は常に
"__main__" と呼ばれます。

"global" 文は、同じブロックの束縛操作と同じスコープを持ちます。ある自
由変数の最内スコープに global 文がある場合、その自由変数はグローバル変
数とみなされます。

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.


動的な機能とのやりとり
----------------------

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.


例外
====

例外とは、コードブロックの通常の制御フローを中断して、エラーやその他の
例外的な状況を処理できるようにするための手段です。例外はエラーが検出さ
れた時点で *送出 (raise)* されます; 例外は、エラーが発生部の周辺のコー
ドブロックか、エラーが発生したコードブロック直接または間接的に呼び出し
ているコードブロックで *処理 (handle)* することができます。

Python インタプリタは、ランタイムエラー (ゼロ除算など) が検出されると
例外を送出します。Python プログラムから、 "raise" 文を使って明示的に例
外を送出することもできます。例外ハンドラ (exception handler) は、
"try" … "except" 文で指定することができます。 "try" 文の "finally" 節
を使うとクリーンアップコード (cleanup code) を指定できます。このコード
は例外は処理しませんが、先行するコードブロックで例外が起きても起きなく
ても実行されます。

Python は、エラー処理に 「プログラムの終了 (termination)」 モデルを用
いています: 例外ハンドラは、プログラムに何が発生したかを把握することが
でき、ハンドラの外側のレベルに処理を継続することはできますが、(問題の
あったコード部分を最初から実行しなおすのでない限り) エラーの原因を修復
したり、実行に失敗した操作をやり直すことはできません。

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".

例外は、クラスインスタンスによって識別されます。 "except" 節はインスタ
ンスのクラスにもとづいて選択されます: これはインスタンスのクラスか、そ
のベースクラスを参照します。このインスタンスはハンドラによって受け取ら
れ、例外条件に関する追加情報を伝えることができます。

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.

注釈: 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.

"try" 文については、 try 文 節、 "raise" 文については raise 文 節も参
照してください。

-[ 脚注 ]-

[1] この制限は、上記の操作によって実行されるコードが、モジュールを
    コン パイルしたときには利用できないために起こります。
