a proxy class example

simple_proxy.py

def _default_cls_attr(name, type_, cls_value):
    # Proxy uses properties to forward the standard
    # class attributes __module__, __name__ and __doc__ to the real
    # object, but these needs to be a string when accessed from
    # the Proxy class directly.  This is a hack to make that work.
    # -- See Issue #1087.

    def __new__(cls, getter):
        instance = type_.__new__(cls, cls_value)
        instance.__getter = getter
        return instance

    def __get__(self, obj, cls=None):
        return self.__getter(obj) if obj is not None else self

    return type(name, (type_, ), {
        '__new__': __new__, '__get__': __get__,
    })


class Proxy(object):
    """Proxy to another object."""

    # Code stolen from werkzeug.local.Proxy.
    __slots__ = ('__local', '__args', '__kwargs', '__dict__')

    def __init__(self, local,
                 args=None, kwargs=None, name=None, __doc__=None):
        object.__setattr__(self, '_Proxy__local', local)
        object.__setattr__(self, '_Proxy__args', args or ())
        object.__setattr__(self, '_Proxy__kwargs', kwargs or {})
        if name is not None:
            object.__setattr__(self, '__custom_name__', name)
        if __doc__ is not None:
            object.__setattr__(self, '__doc__', __doc__)

    @_default_cls_attr('name', str, __name__)
    def __name__(self):
        try:
            return self.__custom_name__
        except AttributeError:
            return self._get_current_object().__name__

    @_default_cls_attr('module', str, __module__)
    def __module__(self):
        return self._get_current_object().__module__

    @_default_cls_attr('doc', str, __doc__)
    def __doc__(self):
        return self._get_current_object().__doc__

    def _get_class(self):
        return self._get_current_object().__class__

    @property
    def __class__(self):
        return self._get_class()

    def _get_current_object(self):
        """Return the current object.  This is useful if you want the real
        object behind the proxy at a time for performance reasons or because
        you want to pass the object into a different context.
        """
        loc = object.__getattribute__(self, '_Proxy__local')
        if not hasattr(loc, '__release_local__'):
            return loc(*self.__args, **self.__kwargs)
        try:
            return getattr(loc, self.__name__)
        except AttributeError:
            raise RuntimeError('no object bound to {0.__name__}'.format(self))

    @property
    def __dict__(self):
        try:
            return self._get_current_object().__dict__
        except RuntimeError:  # pragma: no cover
            raise AttributeError('__dict__')

    def __repr__(self):
        try:
            obj = self._get_current_object()
        except RuntimeError:  # pragma: no cover
            return '<{0} unbound>'.format(self.__class__.__name__)
        return repr(obj)

    def __bool__(self):
        try:
            return bool(self._get_current_object())
        except RuntimeError:  # pragma: no cover
            return False
    __nonzero__ = __bool__  # Py2

    def __unicode__(self):
        try:
            return string(self._get_current_object())
        except RuntimeError:  # pragma: no cover
            return repr(self)

    def __dir__(self):
        try:
            return dir(self._get_current_object())
        except RuntimeError:  # pragma: no cover
            return []

    def __getattr__(self, name):
        if name == '__members__':
            return dir(self._get_current_object())
        return getattr(self._get_current_object(), name)

    def __setitem__(self, key, value):
        self._get_current_object()[key] = value

    def __delitem__(self, key):
        del self._get_current_object()[key]

    def __setslice__(self, i, j, seq):
        self._get_current_object()[i:j] = seq

    def __delslice__(self, i, j):
        del self._get_current_object()[i:j]

    __setattr__ = lambda x, n, v: setattr(x._get_current_object(), n, v)
    __delattr__ = lambda x, n: delattr(x._get_current_object(), n)
    __str__ = lambda x: str(x._get_current_object())
    __lt__ = lambda x, o: x._get_current_object() < o
    __le__ = lambda x, o: x._get_current_object() <= o
    __eq__ = lambda x, o: x._get_current_object() == o
    __ne__ = lambda x, o: x._get_current_object() != o
    __gt__ = lambda x, o: x._get_current_object() > o
    __ge__ = lambda x, o: x._get_current_object() >= o
    __hash__ = lambda x: hash(x._get_current_object())
    __call__ = lambda x, *a, **kw: x._get_current_object()(*a, **kw)
    __len__ = lambda x: len(x._get_current_object())
    __getitem__ = lambda x, i: x._get_current_object()[i]
    __iter__ = lambda x: iter(x._get_current_object())
    __contains__ = lambda x, i: i in x._get_current_object()
    __getslice__ = lambda x, i, j: x._get_current_object()[i:j]
    __add__ = lambda x, o: x._get_current_object() + o
    __sub__ = lambda x, o: x._get_current_object() - o
    __mul__ = lambda x, o: x._get_current_object() * o
    __floordiv__ = lambda x, o: x._get_current_object() // o
    __mod__ = lambda x, o: x._get_current_object() % o
    __divmod__ = lambda x, o: x._get_current_object().__divmod__(o)
    __pow__ = lambda x, o: x._get_current_object() ** o
    __lshift__ = lambda x, o: x._get_current_object() << o
    __rshift__ = lambda x, o: x._get_current_object() >> o
    __and__ = lambda x, o: x._get_current_object() & o
    __xor__ = lambda x, o: x._get_current_object() ^ o
    __or__ = lambda x, o: x._get_current_object() | o
    __div__ = lambda x, o: x._get_current_object().__div__(o)
    __truediv__ = lambda x, o: x._get_current_object().__truediv__(o)
    __neg__ = lambda x: -(x._get_current_object())
    __pos__ = lambda x: +(x._get_current_object())
    __abs__ = lambda x: abs(x._get_current_object())
    __invert__ = lambda x: ~(x._get_current_object())
    __complex__ = lambda x: complex(x._get_current_object())
    __int__ = lambda x: int(x._get_current_object())
    __float__ = lambda x: float(x._get_current_object())
    __oct__ = lambda x: oct(x._get_current_object())
    __hex__ = lambda x: hex(x._get_current_object())
    __index__ = lambda x: x._get_current_object().__index__()
    __coerce__ = lambda x, o: x._get_current_object().__coerce__(o)
    __enter__ = lambda x: x._get_current_object().__enter__()
    __exit__ = lambda x, *a, **kw: x._get_current_object().__exit__(*a, **kw)
    __reduce__ = lambda x: x._get_current_object().__reduce__()

    if not PY3:
        __cmp__ = lambda x, o: cmp(x._get_current_object(), o)  # noqa
        __long__ = lambda x: long(x._get_current_object())      # noqa


class PromiseProxy(Proxy):
    """This is a proxy to an object that has not yet been evaulated.

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

    """

    __slots__ = ('__pending__', )

    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:
            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
            else:
                try:
                    while pending:
                        fun, args, kwargs = pending.popleft()
                        fun(*args, **kwargs)
                finally:
                    try:
                        object.__delattr__(self, '__pending__')
                    except AttributeError:
                        pass
            return thing