import inspect


class n_partialable(object):

    @staticmethod
    def arity_evaluation_checker(function):
        function_info = inspect.getargspec(function)
        function_args = function_info.args
        if inspect.isclass(function):
            # This is to handle the fact that self will get passed in automatically.
            function_args = function_args[1:]
        def evaluation_checker(*args, **kwargs):
            acceptable_kwargs = function_args[len(args):]
            kwarg_keys = set(kwargs.keys())
            # Make sure that we didn't get an argument we can't handle.
            assert kwarg_keys.issubset(acceptable_kwargs)
            needed_args = function_args[len(args):]
            if function_info.defaults:
                needed_args = needed_args[:-len(function_info.defaults)]
            return not needed_args or kwarg_keys.issuperset(needed_args)
        return evaluation_checker

    def __init__(self, function, evaluation_checker=None, args=None, kwargs=None):
        self.function = function
        self.evaluation_checker = (evaluation_checker or
                                   self.arity_evaluation_checker(function))
        self.args = args or ()
        self.kwargs = kwargs or {}

    def __call__(self, *args, **kwargs):
        new_args = self.args + args
        new_kwargs = self.kwargs.copy()
        new_kwargs.update(kwargs)
        if self.evaluation_checker(*new_args, **new_kwargs):
            return self.function(*new_args, **new_kwargs)
        else:
            return type(self)(self.function, self.evaluation_checker,
                              new_args, new_kwargs)


n_partialable = n_partialable(n_partialable)