How to test with Python's unittest that a warning has been thrown?

Question

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"     else:         warning = Warning("the input is 0!")          print warning     return i 

Answer 1

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

with self.assertWarns(Warning):     do_something() 

Answer 2

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.     warnings.simplefilter("always")     # Trigger a warning.     fxn()     # Verify some things     assert len(w) == 1     assert issubclass(w[-1].category, DeprecationWarning)     assert "deprecated" in str(w[-1].message)