equivalent functools.partial instances should compare equal

[theller]

BPO	21130
Nosy	@theller, @rhettinger, @vstinner, @bitdancer, @serhiy-storchaka, @eryksun, @Vgr255, @JelleZijlstra
Files	issue21130.patch partial_richcompare_1_regenerated.patch partial_richcompare_2.patch issue_21130_2.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2014-04-02.07:57:52.117>
labels = ['easy', '3.8', '3.9', '3.10', 'type-feature', 'library']
title = 'equivalent functools.partial instances should compare equal'
updated_at = <Date 2021-03-19.05:13:32.636>
user = 'https://github.com/theller'

bugs.python.org fields:

activity = <Date 2021-03-19.05:13:32.636>
actor = 'eryksun'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2014-04-02.07:57:52.117>
creator = 'theller'
dependencies = []
files = ['41909', '43206', '43211', '43279']
hgrepos = []
issue_num = 21130
keywords = ['patch', 'easy']
message_count = 17.0
messages = ['215363', '215369', '215374', '215377', '215403', '215405', '215510', '215650', '260184', '267300', '267307', '267321', '267421', '267422', '267437', '267441', '267700']
nosy_count = 9.0
nosy_names = ['theller', 'rhettinger', 'vstinner', 'r.david.murray', 'serhiy.storchaka', 'eryksun', 'abarry', 'JelleZijlstra', 'shakur shams Mullick']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21130'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-65329: Equivalent functools.partial instances should compare equal #103255

[theller]

I think that 'equivalent' functools.partial objects should compare equal, so the following snippet SHOULD print True:

>>> import functools
>>> f = lambda x: x
>>> a = functools.partial(f, 42)
>>> b = functools.partial(f, 42)
>>> a == b
False
>>>

[rhettinger]

Even equivalent functions don't compare as equal:

   >>> (lambda x: x*x) == (lambda x: x*x)
   False

[theller]

Sure. I'm not sure 'equivalent' is the word I should have used.

I suggest (and I would have expected) that partial instances (a, b) should compare equal when

a.func == b.func \
and a.args == b.args \
and a.keywords == b.keywords

[eryksun]

Function equality is based on identity, as inherited from object. But there's a precedent for this idea in method_richcompare, which compares the method's __func__ and __self__:

http://hg.python.org/cpython/file/04f714765c13/Objects/classobject.c#l208

[bitdancer]

But since the two partial instances are (conceptually, I don't care how they are implemented) two separate functions, then reasoning by analogy from two identical functions not comparing equal, I would expect two partial instances to not compare equal.

[bitdancer]

Of course, as soon as I hit send, I had second thoughts :). I guess a partial is a binding, not a function.

[theller]

My usecase is: I create kind of bound methods with functools.partial.

Apologies for the confusion by using the word 'equivalent'; what I mean is that partial instances should (IMO) compare equal when they contain the same function and args/keywords which compare equal.

I guess functools.partialmethod has the same problem and I would suggest the same fix/enhancement but I have not used them yet because I have to write python2/3 compatible code.

Anyway; if this behaviour is not seen as a bug then it is probably python-ideas material, but I'm too tired to start a discussion there atm.

[rhettinger]

I guess a partial is a binding, not a function.

Viewed in that light, it makes sense to implement same logic as used for bound methods in method_richcompare.

[shakurshamsMullick]

I have submitted a patch for the issue. Please review and give feedback.

[JelleZijlstra]

I think the patch needs more tests, including:

• Comparing a partial against some other objects
• Comparing two partials with a different function
• Comparing two partials with different args

You also need to check each call to PyObject_RichCompareBool for error (it returns -1 on error). I believe the current implementation would silently ignore the error. This behavior should also be tested.

Rietveld is not picking up this diff, maybe because it is a git rather than an hg diff. Generating the diff with hg will make it easier to review.