tail_call.py

class Deferred:
    def __init__(self, call, *args, **kwargs):
        self.call = getattr(call, 'func', call)
        self.args = args
        self.kwargs = kwargs

    def result(self):
        return self.call(*self.args, **self.kwargs)


class Context:
    def __init__(self):
        self.depth = 0

    def __bool__(self):
        return self.depth > 0

    def __enter__(self):
        depth = self.depth
        self.depth += 1
        return depth

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.depth -= 1


class TailCallDecorator:
    def __init__(self, func):
        self.func = func
        self.ctx = Context()

    def __call__(self, *args, **kwargs):
        if self.ctx:
            return Deferred(self.func, *args, **kwargs)

        with self.ctx:
            result = self.func(*args, **kwargs)
            while isinstance(result, Deferred):
                result = result.result()

        return result


def tco(f):
    return TailCallDecorator(f)


@tco
def fact(n, r=1):
    if n <= 1:
        return r
    else:
        return fact(n - 1, n * r)


print(fact(1000))