celery/celery/local.py

# -*- coding: utf-8 -*-
"""
    celery.local
    ~~~~~~~~~~~~

    This module contains critical utilities that
    needs to be loaded as soon as possible, and that
    shall not load any third party modules.

    Parts of this module is Copyright by Werkzeug Team.

"""
from __future__ import absolute_import

import importlib
import sys

from .five import string

__all__ = ['Proxy', 'PromiseProxy', 'try_import', 'maybe_evaluate']

__module__ = __name__  # used by Proxy class body

PY3 = sys.version_info[0] == 3


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__,
    })


def try_import(module, default=None):
    """Try to import and return module, or return
    None if the module does not exist."""
    try:
        return importlib.import_module(module)
    except ImportError:
        return default


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


def maybe_evaluate(obj):
    try:
        return obj.__maybe_evaluate__()
    except AttributeError:
        return obj