_renderers = {}

def getrenderer(typ):
    for t in typ.__mro__:
        if t in _renderers:
            return _renderers[t]
    else:
        raise LookupError, "no renderer for %r"%(typ,)

def setrenderer(typ, func):
    assert callable(func)
    _renderers[typ] = func


def renderer(*typs):
    def _(func):
        for t in typs:
            setrenderer(t, func)
        return func
    return _

def render(ob):
    while not isinstance(ob, str):
        ob = getrenderer(type(ob))(ob)
    return ob

@renderer(list, tuple)
def renderSequence(ob):
    return ''.join(map(render, ob))

@renderer(str)
def renderString(ob):
    return ob

@renderer(int, long, float)
def renderStr(ob):
    return str(ob)

@renderer(unicode)
def renderUnicode(ob):
    return ob.encode('utf-8')

from nevow import tags as T

@renderer(T.Tag)
def renderTag(ob):
    return ob.children


if __name__ == '__main__':
    print render(['a'])