Unhelpful error message in classmethoddescr_call()

[methane]

BPO	36433
Nosy	@methane, @serhiy-storchaka, @jdemeyer, @miss-islington
PRs	bpo-36433: fix confusing error messages in classmethoddescr_call #12556 [3.7] bpo-36433: fix confusing error messages in classmethoddescr_call (GH-12556) #12557

^{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 2019-03-26.09:50:45.347>
created_at = <Date 2019-03-26.08:11:55.738>
labels = ['interpreter-core', '3.7', '3.8']
title = 'Unhelpful error message in classmethoddescr_call()'
updated_at = <Date 2019-03-27.09:55:02.365>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2019-03-27.09:55:02.365>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-26.09:50:45.347>
closer = 'methane'
components = ['Interpreter Core']
creation = <Date 2019-03-26.08:11:55.738>
creator = 'methane'
dependencies = []
files = []
hgrepos = []
issue_num = 36433
keywords = ['patch']
message_count = 6.0
messages = ['338860', '338861', '338866', '338869', '338947', '338948']
nosy_count = 4.0
nosy_names = ['methane', 'serhiy.storchaka', 'jdemeyer', 'miss-islington']
pr_nums = ['12556', '12557']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue36433'
versions = ['Python 3.7', 'Python 3.8']

[methane]

>>> desc = dict.__dict__['fromkeys']
>>> desc(int, [])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor 'fromkeys' requires a subtype of 'dict' but received 'type

'type should be 'int'.

[serhiy-storchaka]

More confusing error message:

>>> desc(1, [])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor 'fromkeys' requires a type but received a 'dict'

[miss-islington]

New changeset 871309c by Miss Islington (bot) (Inada Naoki) in branch 'master':
bpo-36433: fix confusing error messages in classmethoddescr_call (GH-12556)
871309c

[miss-islington]

New changeset 0344085 by Miss Islington (bot) in branch '3.7':
bpo-36433: fix confusing error messages in classmethoddescr_call (GH-12556)
0344085

[jdemeyer]

I am curious, how did you find out about this bug? Do you have a concrete use case for directly calling an instance of classmethod_descriptor? Typically, one would write dict.fromkeys(...) instead of dict.__dict__['fromkeys'](dict, ...).

[methane]

See #11930

I found this while I wrote tests about error messages.