[issue18173] Add MixedTypeKey to reprlib

[Mark Lawrence]

Change by Mark Lawrence :


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[issue18173] Add MixedTypeKey to reprlib

[Ethan Furman]

Changes by Ethan Furman et...@stoneleaf.us:


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[issue18173] Add MixedTypeKey to reprlib

[Berker Peksag]

Changes by Berker Peksag berker.pek...@gmail.com:


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[issue18173] Add MixedTypeKey to reprlib

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

See also issue22721.

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[issue18173] Add MixedTypeKey to reprlib

[Mark Lawrence]

Mark Lawrence added the comment:

Just a gentle reminder guys.

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versions: +Python 3.5 -Python 3.4

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[issue18173] Add MixedTypeKey to reprlib

[Ethan Furman]

Changes by Ethan Furman et...@stoneleaf.us:


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[issue18173] Add MixedTypeKey to reprlib

[Nick Coghlan]

New submission from Nick Coghlan:

Armin Ronacher pointed out that one downside of the removal of implicit 
cross-type comparisons in Python 3 is that it makes it harder to produce a 
stable repr for mixed-type containers.

This seems like a valid point to me, so I propose adding a suitable 
MixedTypeKey definition to reprlib. Passing key=reprlib.MixedTypeKey would 
then reproduce the old Python 2. sorting behaviour.

We can actually improve on the Python 2 approach by basing the fallback 
comparison on fully qualified type names rather than on the type id.

Proposed implementation (see https://gist.github.com/ncoghlan/5743523):

 class MixedTypeKey:
... Sort key for mixed type lists in Python 3
... 
...  sorted([None, ..., 1, 1j, , (), {}, []])
... Traceback (most recent call last):
...   File stdin, line 1, in module
... TypeError: unorderable types: ellipsis()  NoneType()
...  sorted([None, ..., 1, 1j, , (), {}, []], key=MixedTypeKey)
... [None, Ellipsis, 1, 1j, '', (), {}, []]
... 
... def __init__(self, k):
... self._sort_key = k
... self._type_name = self._get_fully_qualified_name(k)
... def _get_fully_qualified_name(self, k):
... k_type = type(k)
... # Use __qualname__ if available, __name__ otherwise
... try:
... k_name = k_type.__qualname__
... except AttributeError:
... k_name = k_type.__name__
... return k_type.__module__ + . + k_name
... def __lt__(self, other):
... # Try standard sorting first
... sort_key = self._sort_key
... try:
... other_sort_key = other._sort_key
... except AttributeError:
... other_sort_key = other
... try:
... return sort_key  other_sort_key
... except TypeError:
... pass
... # If that fails, sort by the fully qualified type names
... try:
... other_type_name = other._type_name
... except AttributeError:
... other_type_name = self._get_fully_qualified_name(other)
... return self._type_name  other_type_name
... 
 sorted([None, ..., 1, 1j, , (), {}, []])
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: unorderable types: ellipsis()  NoneType()
 sorted([None, ..., 1, 1j, , (), {}, []], key=MixedTypeKey)
[None, 1j, {}, Ellipsis, 1, [], '', ()]
 MixedTypeKey(None)  ...
True

The stdlib version could omit the fallback to __name__ (since it doesn't need 
to cope with old versions of Python)

Support for other comparisons could theoretically be added, but I advise 
against it without a solid use case (sorting only needs __lt__).

--
components: Library (Lib)
messages: 190855
nosy: ncoghlan
priority: normal
severity: normal
stage: needs patch
status: open
title: Add MixedTypeKey to reprlib
type: enhancement
versions: Python 3.4

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