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
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue20630>
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