from collections import defaultdict

tags_db = defaultdict(set)

def tags(*_tags):
    tags = set()
    def add_tag(tag):
        if isinstance(tag, (list, tuple)):
            map(add_tag, tag)
        elif isinstance(tag, (str, unicode)):
            for word in tag.split(" "):
                tags.add(word)
            tags.add(tag)
        else:
            raise TypeError("Tags should be strings or lists/tuples of strings. Got: %s, of type %s" % (tag, type(tag)))
    map(add_tag, _tags)
    tags = tuple(sorted(tags))
    def decorator(function):
        function.tags = tags
        function.__doc__ += "\n\nTags: %s" % ", ".join(tags)
        for tag in tags:
            tags_db[tag].add(function)
        return function
    return decorator

def search(*tags):
    return reduce(set.__and__, [tags_db[tag] for tag in tags])


if __name__ == '__main__':
    common_tags = ['c', 'd']

    @tags(common_tags, 'a', 'b', 'long tag')
    def f(a,b):
        ''' function f returns a+b '''
        return a+b
    
    @tags(common_tags, 'x')
    def g(a,b):
        ''' function f returns a-b '''
        return a-b
    
    @tags('c', 'x', 'y', 'z')
    def h(a,b):
        ''' function f returns a*b '''
        return a*b



    print f.__doc__
    print [x.__name__ for x in search('c', 'd')]
    print [x.__name__ for x in search('x')]