python unittest handle exception handling

Question

I want to unittest an exception handling in my unittest and I'm using assertRaises to check if exception is thrown.

class Add():
    def __init__(self, x, y):
        if type(x) != int or type(y) != int:
            raise Exception('Not a valid number')


Add(None, None)            

import unittest
from test2 import Add
class TestAdd(unittest.TestCase):
    def test_check_for_valid_nos(self):
        self.assertRaises(Exception, Add(None,None))

if __name__ == '__main__':
    unittest.main()

When I run this piece of code it still raises exception. I expect the test to pass saying that the exception has been raised.

You can't call Add in the argument list, this will execute Add before calling assertRaises. — Paulo Scardine
– Paulo Scardine, Commented Dec 8, 2017 at 16:49

ely · Accepted Answer · 2017-12-08 16:50:32Z

3

You should not directly execute your Add function when passing it to assertRaises, rather pass the positional arguments afterwards.

assertRaises(Exception, Add, None, None)

This allows the internal implementation of assertRaises to control executing your function and checking for the right exception. If you invoke the function directly, such as assertRaises(Exception, Add(None, None)), then assertRaises must fully evaluate Add(None, None) before its own function body can execute, because it has to consume the result of that function call as an argument for its input. This is an unprotected call to Add with arguments that will trigger an exception, so the Exception is raised before the actual function body of assertRaises is ever used.

edited Dec 8, 2017 at 16:50

answered Dec 8, 2017 at 16:48

ely

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5 Comments

user1050619 Over a year ago

still same issue...I tried this - self.assertRaises(Exception, Add,None,None)

ely Over a year ago

Are you calling Add(None, None) somewhere in either of the two files? You have it in your source code just above the import statement. Where is that? Maybe it is throwing the exception when it tries to import the test2 module because Add(None, None) is in that file?

user1050619 Over a year ago

Thats right..I had a instance in the source code..thanks

ely Over a year ago

Note that you still would have needed the original change too .. rather than calling Add(None, None) as an argument to assertRaises. Either one of these invocations of Add(None, None) would cause it to fail.

Tomasz Gandor Over a year ago

Wow, assertRaises accepts arguments... I thought I would need a lambda to wrap my code.

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