human
I wrote the below code

    def step(ctxt, value):
        if (value is not None):
            ctxt.append(value)
    def finalize(ctxt):
        if (not ctxt):
            return None
        return sep.join(ctxt)
    return ([], step, finalize)

What's a good function header?

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

def Concatenate(sep=u','):