[Django] Django에서 쓰이는 Context Manager

Context Manager의 개념은 여기 <-  에서 확인 하실수 있습니다.

 

실제로 소스코드에서 쓰는 Context Manager는 TestCase에서 settings 값을 변경하는 TestContextDecorator에 구현이 되어 있습니다.

 

Testcase.settings→ override_settings→TestContextDecorator

class TestContextDecorator:
    """
    A base class that can either be used as a context manager during tests
    or as a test function or unittest.TestCase subclass decorator to perform
    temporary alterations.

    `attr_name`: attribute assigned the return value of enable() if used as
                 a class decorator.

    `kwarg_name`: keyword argument passing the return value of enable() if
                  used as a function decorator.
    """

    def __init__(self, attr_name=None, kwarg_name=None):
        self.attr_name = attr_name
        self.kwarg_name = kwarg_name

    def enable(self):
        raise NotImplementedError

    def disable(self):
        raise NotImplementedError

    def __enter__(self):
        return self.enable()

    def __exit__(self, exc_type, exc_value, traceback):
        self.disable()

class override_settings(TestContextDecorator):
    # ...
    def enable(self):
        # ...
        # This gets called by __enter__
        for key, new_value in self.options.items():
            setattr(override, key, new_value)
        self.wrapped = settings._wrapped
        settings._wrapped = override
        for key, new_value in self.options.items():
            setting_changed.send(sender=settings._wrapped.__class__,
                                 setting=key, value=new_value, enter=True)

    def disable(self):
        # ...
        # This gets called by __exit__
        for key in self.options:
            new_value = getattr(settings, key, None)
            setting_changed.send(sender=settings._wrapped.__class__,
                                 setting=key, value=new_value, enter=False)

또한 Django의 Transaction의 데코레이터 에서도 사용됩니다.

@transaction.atomic
def do_something():
    # this must run in a transaction
    # ...

# class Atomic is implemented later
def atomic(using=None, savepoint=True):
    # Bare decorator: @atomic -- although the first argument is called
    # `using`, it's actually the function being decorated.
    if callable(using):
        return Atomic(DEFAULT_DB_ALIAS, savepoint)(using)
    # Decorator: @atomic(...) or context manager: with atomic(...): ...
    else:
        return Atomic(using, savepoint)

class Atomic(ContextDecorator):
    # There is a lot of complicated corner cases and error handling.
    # See the gory details in django/django/db/transaction.py
    def __init__(self, using, savepoint):
        self.using = using
        self.savepoint = savepoint

    def __enter__(self):
        connection = get_connection(self.using)
        # ...
        # sid = connection.savepoint()
        # connection.savepoint_ids.append(sid)

    def __exit__(self, exc_type, exc_value, traceback):
        # Skip the gory details
        # ...
        sid = connection.savepoint_ids.pop()
        if sid is not None:
            try:
                connection.savepoint_commit(sid)
            except DatabaseError:
                connection.savepoint_rollback(sid)