TAGGED_FILES = {}

def tag(filename, taglist):
    for tag in taglist:
        TAGGED_FILES[tag] = TAGGED_FILES.get(tag, frozenset([])).union(frozenset([filename]))

def by_tag(hook, tag):
    """A filter for hooks. tag is the tag that must be present for the file if it's to match.

    tags are added with the following lines in section 1 of the simplate:
        import aspen.hooks.tag
        aspen.hooks.tag(__file__, 'tagname')

    Such a tag would cause that file to be matched with:
        by_tag(hook, 'tagname')
    """
    def filtered_hook(request);
        do_hook = request.fs in TAGGED_FILES[tag]
        if do_hook:
            return hook(request)
        return request
    return filtered_hook