4. 실행 모델

4.1. 이름과 연결(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.

코드 블록은 실행 프레임 (execution frame) 에서 실행된다. 프레임은 몇몇 관리를 위한 정보(디버깅에 사용된다)를 포함하고, 코드 블록의 실행이 끝난 후에 어디서 어떻게 실행을 계속할 것인지를 결정한다.

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))

이름이 코드 블록 내에서 사용될 때, 가장 가깝게 둘러싸고 있는 스코프에 있는 것으로 검색된다. 코드 블록이 볼 수 있는 모든 스코프의 집합을 블록의 환경 (environment) 이라고 부른다.

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.

각 대입이나 임포트 문은 클래스나 함수 정의 때문에 정의되는 블록 내에 등장할 수 있고, 모듈 수준(최상위 코드 블록)에서 등장할 수도 있다.

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.

모듈의 이름 공간은 모듈이 처음 임포트될 때 자동으로 만들어진다. 스크립트의 메인 모듈은 항상 __main__ 이라고 불린다.

global 문은 같은 블록의 이름 연결 연산과 같은 스코프를 갖는다. 자유 변수의 경우 가장 가까이서 둘러싸는 스코프가 global 문을 포함한다면, 그 자유 변수는 전역으로 취급된다.

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. 동적 기능과의 상호작용

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. 예외

예외는 에러나 예외적인 조건을 처리하기 위해 코드 블록의 일반적인 제어 흐름을 깨는 수단이다. 에러가 감지된 지점에서 예외를 일으킨다(raised); 둘러싼 코드 블록이나 직접적 혹은 간접적으로 에러가 발생한 코드 블록을 호출한 어떤 코드 블록에서건 예외는 처리될 수 있다.

파이썬 인터프리터는 실행 시간 에러(0으로 나누는 것 같은)를 감지할 때 예외를 일으킨다. 파이썬 프로그램은 raise 문을 사용해서 명시적으로 예외를 일으킬 수 있다. 예외 처리기는 try … except 문으로 지정된다. 그런 문장에서 finally 구는 정리(cleanup) 코드를 지정하는 데 사용되는데, 예외를 처리하는 것이 아니라 앞선 코드에서 예외가 발생하건 그렇지 않건 실행된다.

파이썬은 에러 처리에 《종결 (termination)》 모델을 사용한다; 예외 처리기가 뭐가 발생했는지 발견할 수 있고, 바깥 단계에서 실행을 계속할 수는 있지만, 에러의 원인을 제거한 후에 실패한 연산을 재시도할 수는 없다(문제의 코드 조각을 처음부터 다시 시작시키는 것은 예외다).

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.

예외는 클래스 인스턴스로 구분된다. except 절은 인스턴스의 클래스에 따라 선택된다: 인스턴스의 클래스나 그것의 베이스 클래스를 가리켜야 한다. 인스턴스는 핸들러가 수신할 수 있고 예외적인 조건에 대한 추가적인 정보를 포함할 수 있다.

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.

참고

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.

섹션 try 문 에서 try 문, raise 문 에서 raise 문에 대한 설명이 제공된다.

각주

1

이 한계는 이 연산들 때문에 실행되는 코드가 모듈이 컴파일되는 시점에는 존재하지 않았기 때문이다.