How to properly use unit-testing's assertRaises() with NoneType objects

Question

I did a simple test case:

def setUp(self):

  self.testListNone = None

def testListSlicing(self):

  self.assertRaises(TypeError, self.testListNone[:1])

And I am expecting the test to pass, but I am getting an exception:

Traceback (most recent call last):

    self.assertRaises(TypeError, self.testListNone[:1])

TypeError: 'NoneType' object is unsubscriptable

I thought that assertRaises will pass since TypeError exception will be raised. What is the explanation?

https://ongspxm.github.io/blog/2016/11/assertraises-testing-for-errors-in-unittest/ You can refer to this document as he/she has explained why use a lambda/context manager to wrap the code — Sanjeev Siva, Apr 21 '20 at 11:33

score 292 · Accepted Answer · edited Jan 06 '23 at 21:56

292

If you are using Python 2.7 or above, you can use the ability of assertRaises to be used as a context manager and do:

with self.assertRaises(TypeError):
    self.testListNone[:1]

If you are using Python 2.6, another way beside the one given until now is to use unittest2 which is a back port of unittest's new features to Python 2.6, and you can make it work using the code above.

N.B: I'm a big fan of the new feature (SkipTest, test discovery, etc.) of unittest, so I intend to use unittest2 as much as I can. I advise to do the same, because there is a lot more than what unittest come with in Python 2.6 or lower.

edited Jan 06 '23 at 21:56

Peter Mortensen

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answered May 23 '11 at 23:00

mouad

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2.7 _or above_ .. are you hiding Python 2.8 from us? – Robin De Schepper Mar 28 '21 at 15:08
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The code you mention does not seem to help with the problem of ending the script before it comes to the assert-comparison. Still fails the test (using python 3.10) – Asriel Dec 21 '21 at 15:44

score 176 · Answer 2 · answered May 23 '11 at 22:53

176

The problem is the TypeError gets raised 'before' assertRaises gets called since the arguments to assertRaises need to be evaluated before the method can be called. You need to pass a lambda expression like:

self.assertRaises(TypeError, lambda: self.testListNone[:1])

answered May 23 '11 at 22:53

Bas Bossink

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Probably best answer – Christophe Roussy Jul 12 '16 at 13:18
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The answer from @unutbu shows how to call `assertRaises` (no lambda necessary) – scharfmn Apr 04 '18 at 17:45

score 109 · Answer 3 · answered May 23 '11 at 22:46

109

The usual way to use assertRaises is to call a function:

self.assertRaises(TypeError, test_function, args)

to test that the function call test_function(args) raises a TypeError.

The problem with self.testListNone[:1] is that Python evaluates the expression immediately, before the assertRaises method is called. The whole reason why test_function and args is passed as separate arguments to self.assertRaises is to allow assertRaises to call test_function(args) from within a try...except block, allowing assertRaises to catch the exception.

Since you've defined self.testListNone = None, and you need a function to call, you might use operator.itemgetter like this:

import operator
self.assertRaises(TypeError, operator.itemgetter, (self.testListNone,slice(None,1)))

since

operator.itemgetter(self.testListNone,slice(None,1))

is a long-winded way of saying self.testListNone[:1], but which separates the function (operator.itemgetter) from the arguments.

answered May 23 '11 at 22:46

unutbu

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For a single assertRaises, this looks like the best answer. For a large bank of exception based tests (assuming they all have the same exception type) the with self.assertRaises(...) would be a good choice. – user632657 May 01 '14 at 17:55
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Yeah, this should be the correct answer since it actually explains the problem. – Henry Gomersall Oct 03 '14 at 14:59
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This is definitely the right answer, which explains the problem. – Ismael Dec 11 '14 at 13:31
Can you please help me in a related question ? https://stackoverflow.com/questions/39909935/how-do-you-show-an-error-message-when-a-test-does-not-throw-an-expected-exceptio – Erran Morad Oct 07 '16 at 17:11
+1 It can be simplified to `self.assertRaises(TypeError, self.testListNone.__getitem__, slice(None,1))`, because the operator.itemgetter adds some small useless complexity. – hynekcer Apr 06 '21 at 12:31

score 35 · Answer 4 · edited Jan 06 '23 at 21:58

A complete snippet would look like the following. It expands mouad's answer to asserting on the error's message (or generally the string representation of its arguments), which may be useful.

from unittest import TestCase


class TestNoneTypeError(TestCase):

  def setUp(self):
    self.testListNone = None

  def testListSlicing(self):
    with self.assertRaises(TypeError) as ctx:
        self.testListNone[:1]
    self.assertEqual("'NoneType' object is not subscriptable", str(ctx.exception))

How to properly use unit-testing's assertRaises() with NoneType objects

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