Django/Python assertRaises with message check

Question

I am relatively new to Python and want to use a assertRaises test to check for a ValidationError, which works ok. However, I have many ValidationErrors and I want to make sure the right one is returned. I figured I could pass something into assertRaises but it doesn't look like I can, so I figured I would just do an assertTrue and check the exception message. However, I don't know how to access it. Is this even a good way to approach this issue? thanks.

class DailyEntriesTests(TestCase):
def test_cant_have_ip_and_user(self):
    u = createUser(False)
    de = createDailyEntry(u, "1.1.1.1", 1)
    with self.assertRaises(ValidationError) as cm:
        de.full_clean()

    # this line bombs - message doesn't exist. I also tried "error_code" like I saw in the documentation, but that doesn't work
    print(cm.exception.message)

    self.assertTrue(cm.exception.message.contains("Both"))

Louis · Accepted Answer · 2014-10-30 22:54:48Z

51

You can just use assertRaisesRegexp.

with self.assertRaisesRegexp(ValidationError, "Both"):
    de.full_clean()

When you use it as a context manager the 2nd argument is a regular expression to search through the exception's string representation.

answered Oct 30, 2014 at 22:54

Louis

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1 Comment

Ron Over a year ago

I just feel sorry for myself that this post is from 2014 and I never knew about this until now 🙈😅

Kim · Accepted Answer · 2021-08-16 10:11:56Z

Since the question is related to Django, you could also use the assertRaisesMessage context manager when inheriting from django's TestCase.

from django.test import TestCase

class ExceptionTest(TestCase):

    def test_call_raises_exception_with_custom_message(self):
        with self.assertRaisesMessage(Exception, 'My custom message!'):
            call_that_causes_exception()

Note: The assertRaisesMessage manager does an in lookup on the exceptions message: Say your exception raises "My custom message!", asserting for "custom message" passes. Bear this in mind especially if you have multiple (custom) exceptions with similar messages.

(E.g. two different exceptions raising "My custom message! Further details..." and "My custom message! No details." would both pass an assert for "My custom message!").

Christian Long · Accepted Answer · 2016-05-16 19:42:42Z

17

Nowadays you can use assertRaises as a context manager. This way you can capture the exception and inspect it later.

with self.assertRaises(SomeException) as cm:
    do_something()

the_exception = cm.exception
self.assertEqual(the_exception.error_code, 3)

answered May 16, 2016 at 19:42

Christian Long

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3 Comments

ems Over a year ago

this gave me the following error: '_AssertRaisesContext' object has no attribute 'message'

Evandro Coan Over a year ago

I just tested and it is working fine with CPython 3.6.4

hlongmore Over a year ago

If you want to test the message, you can do something like: self.assertEqual('{}'.format(cm.exception), 'I expected this message')

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