def implements(interface):
    import sys

    if not sys.flags.debug:
        return lambda func: func

    import inspect

    def get_filtered_members(item):
        return dict([(key, value) for key, value in inspect.getmembers(item)
                if not key.startswith('_')])

    def build_contract(item):
        argspec = inspect.getargspec(item)

        if argspec.defaults:
            num_keywords = len(argspec.defaults)
            args = ['_'] * (len(argspec.args) - num_keywords)
            args.extend(argspec.args[num_keywords-1:])
        else:
            args = ['_'] * len(argspec.args)

        if argspec.varargs:
            args.append('*args')

        if argspec.keywords:
            args.append('**kwargs')

        return ', '.join(args)

    def tester(klass):
        interface_elements = get_filtered_members(interface)
        class_elements = get_filtered_members(klass)

        for key, value in interface_elements.items():
            try:
                element = class_elements[key]
            except KeyError:
                raise AssertionError("{0!r} is required but missing.".format(key))

            if inspect.isfunction(value):
                assert build_contract(value) == build_contract(kspec), \
                        "{0!r} doesn't conform to interface.".format(key)

        return klass

    return tester