[ Python 常見問題 ] How do you test that a Python function throws an exception?

Source From Here 
Question 
How does one write a unittest that fails only if a function doesn't throw an expected exception? 

How-To 
Use TestCase.assertRaises (or TestCase.failUnlessRaises) from the unittest module, for example: 
- TestExp.py 

view plaincopy to clipboardprint?

1. #!/usr/bin/env python3  
2. import unittest  
3.   
4. def my_fun(val):  
5.     if val % 2 == 0:  
6.         raise Exception('Val is even')  
7.     else:  
8.         return "Val is odd"  
9.   
10. class MyTestCase(unittest.TestCase):  
11.     def test_01(self):  
12.         self.assertEquals("Val is odd", my_fun(1))  
13.   
14.     def test_02(self):  
15.         # Even number will raise Exception  
16.         self.assertRaises(Exception, my_fun, 2)  
17.   
18.         # You can leverage with this way  
19.         with self.assertRaises(Exception) as context:  
20.             my_fun(4)  
21.   
22.         self.assertTrue('Val is even' in str(context.exception))  

Running example: 

# pytest -s TestExp.py 
... 
platform linux -- Python 3.5.0, pytest-3.2.1, py-1.4.34, pluggy-0.4.0 
rootdir: /tmp, inifile: 
plugins: celery-4.1.0 
collected 2 items 
... 
===== 2 passed in 0.02 seconds =====