Class PromiseProxy

celery/local.py:281–349

Proxy that evaluates object once. :class:`Proxy` will evaluate the object each time, while the promise will only evaluate it once.

class PromiseProxy(Proxy):
    """Proxy that evaluates object once.

    :class:`Proxy` will evaluate the object each time, while the
    promise will only evaluate it once.
    """

    __slots__ = ('__pending__', '__weakref__')

    def _get_current_object(self):
        try:
            return object.__getattribute__(self, '__thing')
        except AttributeError:
            return self.__evaluate__()

    def __then__(self, fun, *args, **kwargs):
        if self.__evaluated__():
            return fun(*args, **kwargs)
        from collections import deque
        try:
            pending = object.__getattribute__(self, '__pending__')
        except AttributeError:
            pending = None
        if pending is None:
            pending = deque()
            object.__setattr__(self, '__pending__', pending)
        pending.append((fun, args, kwargs))

    def __evaluated__(self):
        try:
            object.__getattribute__(self, '__thing')
        except AttributeError:
            return False
        return True

    def __maybe_evaluate__(self):
        return self._get_current_object()

    def __evaluate__(self,
                     _clean=('_Proxy__local',
                             '_Proxy__args',
                             '_Proxy__kwargs')):
        try:
            thing = Proxy._get_current_object(self)
        except Exception:
            raise
        else:
            object.__setattr__(self, '__thing', thing)
            for attr in _clean:
                try:
                    object.__delattr__(self, attr)
                except AttributeError:  # pragma: no cover
                    # May mask errors so ignore
                    pass
            try:
                pending = object.__getattribute__(self, '__pending__')
            except AttributeError:
                pass