human
I wrote the below code

    if s:
        keys.setdefault(u'%', u'%')
        def resolve(match):
            key = match.expand(expand)
            try:
                resolver = keys[key]
            except KeyError:
                raise ValueError(UNKNOWN_SIMPLE_FORMAT_KEY.format(key, s))
            if isinstance(resolver, Callable):
                return resolver()
            return resolver
        return pattern.sub(resolve, s)
    return s

What's a good function header?

=====
Here's a good function header?
assistant
#Here's a good function header.

def simple_format(s, keys, pattern=RE_FORMAT, expand=u'\\1'):