classmethod abuse --> SystemError

[mwhudson]

BPO	753451
Nosy	@mwhudson, @brettcannon, @rhettinger

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2003-06-18.01:13:56.000>
created_at = <Date 2003-06-12.18:09:50.000>
labels = ['interpreter-core']
title = 'classmethod abuse --> SystemError'
updated_at = <Date 2003-06-18.01:13:56.000>
user = 'https://github.com/mwhudson'

bugs.python.org fields:

activity = <Date 2003-06-18.01:13:56.000>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2003-06-12.18:09:50.000>
creator = 'mwh'
dependencies = []
files = []
hgrepos = []
issue_num = 753451
keywords = []
message_count = 3.0
messages = ['16347', '16348', '16349']
nosy_count = 3.0
nosy_names = ['mwh', 'brett.cannon', 'rhettinger']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue753451'
versions = []

[mwhudson]

This is obviously a silly thing to do:

>>> classmethod(1).__get__(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
SystemError: ../Objects/classobject.c:2102: bad
argument to internal function

but I think that's still a bug.

[brettcannon]

Logged In: YES
user_id=357491

Well, couldn't we check if the argument is a non-data descriptor
at least? Since classmethod only works with new-style we know
the method will be a non-data descriptor. That should prevent
allowing arguments like 1 being allowed through.

[rhettinger]

Logged In: YES
user_id=80475

Added a callability check.
See Objects/funcobject.c 1.195