[issue20630] Add sorting helpers for collections containing None values

[Nick Coghlan]

New submission from Nick Coghlan:

Currently, it's a bit annoying to sort collections containing "None" values in Python 3. While the default behaviour isn't going to change, it would be good to offer standard "none_first" and "none_last" helps (inspired by the SQL NULL FIRST and NULL LAST ordering control).

Suggested home: functools (since that is where the total_ordering class decorator already lives), but collections would also be a reasonable choice (as this feature mostly relates to sorting containers)

The minimal option (suggested by Peter Otten):

    def none_first(v):
        return v is not None, v

    def none_last(v):
        return v is None, v

A more complex alternative would be to provide general purpose SortsFirst and SortsLast singletons:

    @functools.total_ordering
    class _AlwaysLesser:
        def __eq__(self, other):
            return isinstance(other, _AlwaysLesser):
        def __lt__(self, other):
            return not isinstance(other, _AlwaysLesser):

    @functools.total_ordering
    class _AlwaysGreater:
        def __eq__(self, other):
            return isinstance(other, _AlwaysGreater):
        def __gt__(self, other):
            return not isinstance(other, _AlwaysGreater):

    SortsFirst = _AlwaysLesser()
    SortsLast = _AlwaysGreater()

    def none_first(v):
        return SortsFirst if v is None else v
    def none_last(v):
        return SortsLast if v is None else v

The advantage of the latter more complex approach is that you can embed the SortsFirst and SortsLast values inside a tuple as part of a more complex key, whereas the simple solution only handles the case where the entire value is None.

(Inspired by Chris Withers's python-dev thread: https://mail.python.org/pipermail/python-dev/2014-February/132332.html)

----------
messages: 211251
nosy: ncoghlan
priority: normal
severity: normal
status: open
title: Add sorting helpers for collections containing None values
type: enhancement
versions: Python 3.5

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20630>
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