human
Here's a good function header given the following:
I wrote the below code

    def they_are(t):
        return (isinstance(dest, t) and isinstance(source, t))
    if ((source is None) and (dest is None)):
        return None
    elif (source is None):
        return None
    if they_are(list):
        dest[:] = (source + [x for x in dest if (x not in source)])
        return dest
    elif they_are(dict):
        for (sk, sv) in iteritems(source):
            if (override(sk) or (sk not in dest)):
                dest[sk] = copy.copy(sv)
            else:
                dest[sk] = _merge_yaml(dest[sk], source[sk])
        return dest
    else:
        return copy.copy(source)

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

def _merge_yaml(dest, source):