assertRaises in python unit-test not catching the exception [duplicate]

Question

Can somebody tell me why the following unit-test is failing on the ValueError in test_bad, rather than catching it with assertRaises and succeeding? I think I'm using the correct procedure and syntax, but the ValueError is not getting caught.

I'm using Python 2.7.5 on a linux box.

Here is the code …

import unittest  class IsOne(object):     def __init__(self):         pass     def is_one(self, i):         if (i != 1):             raise ValueError  class IsOne_test(unittest.TestCase):      def setUp(self):         self.isone = IsOne()      def test_good(self):         self.isone.is_one(1)         self.assertTrue(True)      def test_bad(self):         self.assertRaises(ValueError, self.isone.is_one(2))  if __name__ == "__main__":     unittest.main() 

and here is the output of the unit-test:

====================================================================== ERROR: test_bad (__main__.IsOne_test) ---------------------------------------------------------------------- Traceback (most recent call last):   File "test/raises.py", line 20, in test_bad     self.assertRaises(ValueError, self.isone.is_one(2))   File "test/raises.py", line 8, in is_one     raise ValueError ValueError  ---------------------------------------------------------------------- Ran 2 tests in 0.008s  FAILED (errors=1) 

Answer 1

Unittest's assertRaises takes a callable and arguments, so in your case, you'd call it like:

self.assertRaises(ValueError, self.isone.is_one, 2) 

If you prefer, as of Python2.7, you could also use it as a context manager like:

with self.assertRaises(ValueError):     self.isone.is_one(2)