I've only just realised that the test cases don't cover all eventualities: none of them test what happens with multiple spaces _between_ the letters, such as:

    '  a  b c '.split(maxsplit=1) == ['a', 'b c ']

Comparing that with:

    '  a  b c '.split(' ', maxsplit=1)

you see that passing None as the split character does not mean "any whitespace character". There's clearly a little more to it than that.