Function lazy

django/utils/functional.py:77–197

Turn any callable into a lazy evaluated callable. result classes or types is required -- at least one is needed so that the automatic forcing of the lazy evaluation code is triggered. Results are not memoized; the function is evaluated on every access.

(func, *resultclasses)

def lazy(func, *resultclasses):
    """
    Turn any callable into a lazy evaluated callable. result classes or types
    is required -- at least one is needed so that the automatic forcing of
    the lazy evaluation code is triggered. Results are not memoized; the
    function is evaluated on every access.
    """

    class __proxy__(Promise):
        """
        Encapsulate a function call and act as a proxy for methods that are
        called on the result of that function. The function is not evaluated
        until one of the methods on the result is called.
        """

        def __init__(self, args, kw):
            self._args = args
            self._kw = kw

        def __reduce__(self):
            return (
                _lazy_proxy_unpickle,
                (func, self._args, self._kw, *resultclasses),
            )

        def __deepcopy__(self, memo):
            # Instances of this class are effectively immutable. It's just a
            # collection of functions. So we don't need to do anything
            # complicated for copying.
            memo[id(self)] = self
            return self

        def __cast(self):
            return func(*self._args, **self._kw)

        # Explicitly wrap methods which are defined on object and hence would
        # not have been overloaded by the loop over resultclasses below.

        def __repr__(self):
            return repr(self.__cast())

        def __str__(self):
            return str(self.__cast())

        def __eq__(self, other):
            if isinstance(other, Promise):
                other = other.__cast()
            return self.__cast() == other

        def __ne__(self, other):
            if isinstance(other, Promise):
                other = other.__cast()
            return self.__cast() != other

        def __lt__(self, other):
            if isinstance(other, Promise):
                other = other.__cast()
            return self.__cast() < other