This has finally bugged me enough to file an issue, although I wouldn't be able to use it until Python 3.7.  There's a subtle but documented difference in str.split() when sep=None:

>>> help(''.split)
Help on built-in function split:

split(...) method of builtins.str instance
    S.split(sep=None, maxsplit=-1) -> list of strings
    
    Return a list of the words in S, using sep as the
    delimiter string.  If maxsplit is given, at most maxsplit
    splits are done. If sep is not specified or is None, any
    whitespace string is a separator and empty strings are
    removed from the result.

I.e., that empty strings are removed from the result.  This does not happen when sep is given, leading to this type of unfortunate code:

>>> 'foo,bar,baz'.split(',')
['foo', 'bar', 'baz']
>>> 'foo,bar,baz'.replace(',', ' ').split()
['foo', 'bar', 'baz']
>>> ''.split(',')
['']
>>> ''.replace(',', ' ').split()
[]

Specifically, code that wants to split on say commas, but has to handle the case where the source string is empty, shouldn't have to also filter out the single empty string item.

Obviously we can't change existing behavior, so I propose to add a keyword argument `prune` that would make these two bits of code identical:

>>> ''.split()
[]
>>> ''.split(' ', prune=True)
[]

and would handle the case of ''.split(',') without having to resort to creating an ephemeral intermediate string.

`prune` should be a keyword-only argument, defaulting to False.