How do you show an error message when a test does not throw an expected exception?

Question

I am new to Python. I wanted to test if my code threw an exception. I got the code from How do you test that a Python function throws an exception?

import mymod
import unittest

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(SomeCoolException, mymod.myfunc, compulsory_argument)

Now, I also want to display a message if the exception is not thrown. How do I do that? The Python documentation does not mention it clearly. I added the message after "compulsory_argument" and it failed.

I tried the first answer with modifications and got an exception. What is my mistake here?

import unittest

def sayHelloTo(name):
    print("Hello " + name)

class MyTestCase(unittest.TestCase):
    def test1(self):
        person = "John"
        with self.assertRaises(Exception, "My insightful message"):
            sayHelloTo(person)

Error:

Error
Traceback (most recent call last):
  File "C:teststester.py", line 9, in test1
    with self.assertRaises(Exception, "My insightful message"):
AttributeError: __exit__

Asked By: Erran Morad

||

Source

Answer 1

As of Python 3.3, assertRaises can be used as a context manager with a message:

import unittest

def sayHelloTo(name):
    print("Hello " + name)

class MyTestCase(unittest.TestCase):
    def test1(self):
        person = "John"
        with self.assertRaises(Exception, msg="My insightful message"):
            sayHelloTo(person)

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

It results in

Hello John
F
======================================================================
FAIL: test1 (__main__.MyTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "r.py", line 10, in test1
    sayHelloTo(person)
AssertionError: Exception not raised : My insightful message

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

Answered By: tdelaney

Answer 2

Now, I also want to display a message if the exception is not thrown. How do I do that ?

The general philosophy of unittest is for the tests to be silent when they succeed and only become verbose when they fail. Accordingly, the API provides a “msg” keyword argument for the unsuccessful case but does not offer an alternative for the successful case.

That said, another part of the philosophy works in your favor. In general, test cases internally raise an exception when a testcase fails. That means that if you want to display a message when there is a success, you just add another statement after the test:

with self.assertRaises(TypeError, msg='Oh no, I did not get a TypeError')
     somecode()
logging.info('Yippee, we got a TypeError!')  # Runs after a successful test

Answered By: Raymond Hettinger

How do you show an error message when a test does not throw an expected exception?

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