Confusing error message from warnings.warn

[gerritholl]

BPO	11494
Nosy	@brettcannon, @gerritholl

^{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 2011-03-20.20:12:32.368>
created_at = <Date 2011-03-14.10:47:51.891>
labels = ['interpreter-core', 'type-bug', 'library']
title = 'Confusing error message from warnings.warn'
updated_at = <Date 2011-03-20.20:12:32.367>
user = 'https://github.com/gerritholl'

bugs.python.org fields:

activity = <Date 2011-03-20.20:12:32.367>
actor = 'brett.cannon'
assignee = 'none'
closed = True
closed_date = <Date 2011-03-20.20:12:32.368>
closer = 'brett.cannon'
components = ['Interpreter Core', 'Library (Lib)']
creation = <Date 2011-03-14.10:47:51.891>
creator = 'Gerrit.Holl'
dependencies = []
files = []
hgrepos = []
issue_num = 11494
keywords = []
message_count = 2.0
messages = ['130800', '131535']
nosy_count = 2.0
nosy_names = ['brett.cannon', 'Gerrit.Holl']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11494'
versions = ['Python 3.1', 'Python 2.7', 'Python 3.2', 'Python 3.3']

[gerritholl]

When accidentally passing a string to warnings.warn where one should pass a Warning-class, the error message is rather confusing:

 $ ./python 
 Python 2.7.1+ (release27-maint:88766, Mar  8 2011, 16:51:59) 
 [GCC 4.4.5] on linux2
 Type "help", "copyright", "credits" or "license" for more information.
 >>> import warnings
 >>> warnings.warn("aaa", "bbb")
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
 TypeError: issubclass() arg 1 must be a class

It would be better for the error message to say TypeError: warnings must be classes inherited from ... or something like that.

[brettcannon]

I'm not crazy about the idea of adding an inspect.isclass() check simply to clear up this rare error message. It's still decipherable what the problem is from the issubclass() message.

Closing as "won't fix".