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

    if (repl is None):
        repl = ''
    def f(x):
        if (x[start:stop] == ''):
            local_stop = start
        else:
            local_stop = stop
        y = ''
        if (start is not None):
            y += x[:start]
        y += repl
        if (stop is not None):
            y += x[local_stop:]
        return y
    return _na_map(f, arr)

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

def str_slice_replace(arr, start=None, stop=None, repl=None):