assertWarns has some restrictions

[therve]

First uses of assertWarns have shown some limits:

• it can't be used to check warnings emitted by class instantiation or implicit __call__ method
• it doesn't work with warnings filtered by default like PendingDeprecationWarning

[therve]

Ready to review in assertwarns-features-2703.

[jml]

What exactly do you mean by "it can't be used with class"?

[therve]

I've modified the description to reflect what I meant.

[exarkun]

Arg, I hate the stupid warnings module.

Noble effort, but the only actually correct way to get __warningsregistry__ is to go through the same dance as warnings.warn. Even if we duplicate it, there's no guarantee Python devs won't change it randomly for the next release.

I'm trying to remember why we don't do something like patch warnings.warn_explicit instead of trying to re-assemble the events after the fact. I think the reason we didn't patch warnings.warn was to support from warnings import warn, but I can't think of a similar reason not to patch warn_explicit.

(This is not a review, just some thoughts.)

[therve]

We dont't patch warn_explicit because we didn't implemented like that at the beginning :). I guess that could work though.

[therve]

I've tried it, and it's too good to be true :).

[exarkun]

Hmm, for me, some tests in twisted.trial.test.test_tests fail w/ this branch merged into trunk, mostly similar to this:

===============================================================================
[FAIL]: twisted.trial.test.test_tests.SuppressionTest.testSuppressModule

Traceback (most recent call last):
  File "/home/exarkun/Projects/Twisted/trunk/twisted/trial/test/test_tests.py", line 507, in testSuppressModule
    self.assertSubstring(suppression.METHOD_WARNING_MSG, self.getIO())
  File "/home/exarkun/Projects/Twisted/trunk/twisted/trial/unittest.py", line 307, in failUnlessSubstring
    return self.failUnlessIn(substring, astring, msg)
twisted.trial.unittest.FailTest: 'method warning message' not in ''
-------------------------------------------------------------------------------

[therve]

Damn, sorry, that was something stupid.

[exarkun]

No problem. Looks great, please merge :)

[therve]

(In [20610]) Merge assertwarns-features-2703

Author: therve Reviewer: exarkun Fixes #2703

Fix implementation of assertWarns to work with all callables and with all type of warnings.