human
I wrote the below code

    def step(ctxt, ndx, value):
        if (value is not None):
            ctxt[ndx] = value
    def finalize(ctxt):
        if (len(ctxt) == 0):
            return None
        return sep.join(map(ctxt.get, sorted(ctxt.iterkeys())))
    return ({}, step, finalize)

What's a good function header?

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

def SortedConcatenate(sep=u','):