I have a following function in Python and I want to test with unittest that if the function gets 0 as argument, it throws a warning. I already tried assertRaises, but since I don’t raise the warning, that doesn’t work.

def isZero(i):
    if i != 0:
        print "OK"
        warning = Warning("the input is 0!") 
        print warning
    return i

Starting with Python 3.2, you can simply use assertWarns() method.

with self.assertWarns(Warning):

You can use the catch_warnings context manager. Essentially this allows you to mock the warnings handler, so that you can verify details of the warning. See the official docs for a fuller explanation and sample test code.

import warnings

def fxn():
    warnings.warn("deprecated", DeprecationWarning)

with warnings.catch_warnings(record=True) as w:
    # Cause all warnings to always be triggered.
    # Trigger a warning.
    # Verify some things
    assert len(w) == 1
    assert issubclass(w[-1].category, DeprecationWarning)
    assert "deprecated" in str(w[-1].message)

You can write your own assertWarns function to incapsulate catch_warnings context. I’ve just implemented it the following way, with a mixin:

class WarningTestMixin(object):
    'A test which checks if the specified warning was raised'

    def assertWarns(self, warning, callable, *args, **kwds):
        with warnings.catch_warnings(record=True) as warning_list:

            result = callable(*args, **kwds)

            self.assertTrue(any(item.category == warning for item in warning_list))

A usage example:

class SomeTest(WarningTestMixin, TestCase):
    'Your testcase'

    def test_something(self):
            5, 10, 'john', # args
            foo='bar'      # kwargs

The test will pass if at least one of the warnings issued by your_function is of type UserWarning.

@ire_and_curses’ answer is quite useful and, I think, canonical. Here is another way to do the same thing. This one requires Michael Foord’s excellent Mock library.

import unittest, warnings
from mock import patch_object

def isZero( i):
   if i != 0:
     print "OK"
     warnings.warn( "the input is 0!")
   return i

class Foo(unittest.TestCase):
    @patch_object(warnings, 'warn')
    def test_is_zero_raises_warning(self, mock_warn):

if __name__ == '__main__':

The nifty patch_object lets you mock out the warn method.

One problem with the warnings.catch_warnings approach is that warnings produced in different tests can interact in strange ways through global state kept in __warningregistry__ attributes.

To address this, we should clear the __warningregistry__ attribute of every module before every test that checks warnings.

class MyTest(unittest.TestCase):

  def setUp(self):
    # The __warningregistry__'s need to be in a pristine state for tests
    # to work properly.
    for v in sys.modules.values():
      if getattr(v, '__warningregistry__', None):
        v.__warningregistry__ = {}

  def test_something(self):
    with warnings.catch_warnings(record=True) as w:
      warnings.simplefilter("always", MySpecialWarning)
      self.assertEqual(len(w), 1)
      self.assertIsInstance(w[0].message, MySpecialWarning)

This is how Python 3’s assertWarns() method is implemented.

Building off the answer from @ire_and_curses,

class AssertWarns(warnings.catch_warnings):
    """A Python 2 compatible version of `unittest.TestCase.assertWarns`."""
    def __init__(self, test_case, warning_type):
        self.test_case = test_case
        self.warning_type = warning_type
        self.log = None
        super(AssertWarns, self).__init__(record=True, module=None)

    def __enter__(self):
        self.log = super(AssertWarns, self).__enter__()
        return self.log

    def __exit__(self, *exc_info):
        super(AssertWarns, self).__exit__(*exc_info)
        self.test_case.assertEqual(type(self.log[0]), self.warning_type)

This can be called similarly to unittest.TestCase.assertWarns:

with AssertWarns(self, warnings.WarningMessage):
    warnings.warn('test warning!') 

where self is a unittest.TestCase.