Question

I’m writing some unit tests for a Python library and would like certain warnings to be raised as exceptions, which I can easily do with the simplefilter function. However, for one test I’d like to disable the warning, run the test, then re-enable the warning.

I’m using Python 2.6, so I’m supposed to be able to do that with the catch_warnings context manager, but it doesn’t seem to work for me. Even failing that, I should also be able to call resetwarnings and then re-set my filter.

Here’s a simple example which illustrates the problem:

>>> import warnings
>>> warnings.simplefilter("error", UserWarning)
>>> 
>>> def f():
...     warnings.warn("Boo!", UserWarning)
... 
>>> 
>>> f() # raises UserWarning as an exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
UserWarning: Boo!
>>> 
>>> f() # still raises the exception
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f
UserWarning: Boo!
>>> 
>>> with warnings.catch_warnings():
...     warnings.simplefilter("ignore")
...     f()     # no warning is raised or printed
... 
>>> 
>>> f() # this should raise the warning as an exception, but doesn't
>>> 
>>> warnings.resetwarnings()
>>> warnings.simplefilter("error", UserWarning)
>>> 
>>> f() # even after resetting, I'm still getting nothing
>>> 

Can someone explain how I can accomplish this?

EDIT: Apparently this is a known bug: http://bugs.python.org/issue4180

Answer 1

Reading through the docs and few times and poking around the source and shell I think I’ve figured it out. The docs could probably improve to make clearer what the behavior is.

The warnings module keeps a registry at __warningsregistry__ to keep track of which warnings have been shown. If a warning (message) is not listed in the registry before the ‘error’ filter is set, any calls to warn() will not result in the message being added to the registry. Also, the warning registry does not appear to be created until the first call to warn:

>>> import warnings
>>> __warningregistry__
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
NameError: name '__warningregistry__' is not defined

>>> warnings.simplefilter('error')
>>> __warningregistry__
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
NameError: name '__warningregistry__' is not defined

>>> warnings.warn('asdf')
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
UserWarning: asdf

>>> __warningregistry__
{}

Now if we ignore warnings, they will get added to the warnings registry:

>>> warnings.simplefilter("ignore")
>>> warnings.warn('asdf')
>>> __warningregistry__
{('asdf', <type 'exceptions.UserWarning'>, 1): True}
>>> warnings.simplefilter("error")
>>> warnings.warn('asdf')
>>> warnings.warn('qwerty')
------------------------------------------------------------
Traceback (most recent call last):
  File "<ipython console>", line 1, in <module>
UserWarning: qwerty

So the error filter will only apply to warnings that aren’t already in the warnings registry. To make your code work you’ll need to clear the appropriate entries out of the warnings registry when you’re done with the context manager (or in general any time after you’ve used the ignore filter and want a prev. used message to be picked up the error filter). Seems a bit unintuitive…