`is' operator returns False on classmethods

[mstefanro]

BPO	15773
Nosy	@ncoghlan
Files	is_on_classmethods.py

^{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 = <Date 2012-08-24.00:50:13.333>
created_at = <Date 2012-08-23.23:35:07.285>
labels = ['type-bug', 'invalid']
title = "`is' operator returns False on classmethods"
updated_at = <Date 2012-08-24.00:50:13.287>
user = 'https://bugs.python.org/mstefanro'

bugs.python.org fields:

activity = <Date 2012-08-24.00:50:13.287>
actor = 'ncoghlan'
assignee = 'none'
closed = True
closed_date = <Date 2012-08-24.00:50:13.333>
closer = 'ncoghlan'
components = []
creation = <Date 2012-08-23.23:35:07.285>
creator = 'mstefanro'
dependencies = []
files = ['26978']
hgrepos = []
issue_num = 15773
keywords = []
message_count = 2.0
messages = ['168967', '168968']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'mstefanro']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue15773'
versions = ['Python 3.3']

[mstefanro]

Here are a few counter-intuitive outputs:

    >>> dict.fromkeys is dict.fromkeys
    False

    >>> id(dict.fromkeys) == id(dict.fromkeys)
    True

    >>> x=dict.fromkeys; id(x) == id(x)
    True

    >>> x=dict.fromkeys; id(x) == id(dict.fromkeys)
    False
    
    >>> x=dict.fromkeys; y=dict.fromkeys; id(x),id(y),id(dict.fromkeys)
    (39888824, 39064632, 39065144)

    >>> a=id(dict.fromkeys); x=dict.fromkeys; b=id(dict.fromkeys); a,b
    (39888824, 39480568)

Attached is a failing test.

[ncoghlan]

Bound methods are created dynamically on lookup, while object ids may be reused after the original object is destroyed.

There's no bug here - just a combination of those two language behaviours that is frequently surprising to users that have just noticed it.

There's not a lot we can do about that - it's a genuinely surprising moment in people's understanding of the way methods (and descriptors in general) work.