Python/Django: how to assert that unit test result contains a certain string?

Question

In a python unit test (actually Django), what is the correct assert statement that will tell me if my test result contains a string of my choosing?

self.assertContainsTheString(result, {"car" : ["toyota","honda"]})

I want to make sure that my result contains at least the json object (or string) that I specified as the second argument above

{"car" : ["toyota","honda"]}

would [https://docs.djangoproject.com/en/1.11/topics/testing/tools/#django.test.Response.json] help? — coya, Jun 07 '17 at 22:24

score 159 · Answer 1 · answered Mar 15 '16 at 20:07

159

To assert if a string is or is not a substring of another, you should use assertIn and assertNotIn:

# Passes
self.assertIn('bcd', 'abcde')

# AssertionError: 'bcd' unexpectedly found in 'abcde'
self.assertNotIn('bcd', 'abcde')

These are new since Python 2.7 and Python 3.1

answered Mar 15 '16 at 20:07

Ed I

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Also `assertIn` provides helpful messages on failure as you've illustrated. – jamesc Mar 25 '16 at 13:02

score 76 · Accepted Answer · edited Oct 10 '17 at 12:24

76

self.assertContains(result, "abcd")

You can modify it to work with json.

Use self.assertContains only for HttpResponse objects. For other objects, use self.assertIn.

edited Oct 10 '17 at 12:24

GabLeRoux

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answered Jul 09 '13 at 05:33

Akshar Raaj

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yes but due to the json structure, it could have some whitespace added which are not problematic in json but problematic if you want to compare with a python string. – francois Aug 13 '13 at 09:29
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assertContains is not intended to by used for other object than HttpReponse, prefer using assertTrue with "in" python keyword instead (see my answser below). – Pierre Criulanscy Mar 23 '15 at 08:07
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This is wrong. Assertconains is for http responses. – kagronick Apr 28 '16 at 15:23
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This answer should clarify that it is tailored for HttpResponse only – rtindru Aug 07 '17 at 07:05

score 30 · Answer 3 · answered Mar 23 '15 at 08:05

30

You can write assertion about expected part of string in another string with a simple assertTrue + in python keyword :

self.assertTrue("expected_part_of_string" in my_longer_string)

answered Mar 23 '15 at 08:05

Pierre Criulanscy

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The issue with this strategy is that is can give poor failure messages of the form "AssertionError: False is not true" – jamesc Mar 25 '16 at 12:58
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@jamesc agree with you, test should show the error details, any workaround to this issue if using assertTrue? – Luk Aron Nov 08 '19 at 04:05
@LukAron If you have to use assertTrue, then you could pass a message which you build beforehand to contain more details: assertTrue(expr, msg=message). If the message gets complicated, then you can extract the message building and assertTrue check into a separate assertion helper, which may have its own tests to ensure it behaves as expected. – jamesc Nov 10 '19 at 13:54

Vincent Audebert · Answer 4 · 2014-03-11T04:02:09.833

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Build a JSON object using json.dumps().

Then compare them using assertEqual(result, your_json_dict)

import json

expected_dict = {"car":["toyota", "honda"]}
expected_dict_json = json.dumps(expected_dict)

self.assertEqual(result, expected_dict_json)

edited Mar 11 '14 at 04:02

answered Jul 08 '13 at 22:20

Vincent Audebert

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Why using `assertTrue()` instead of [`assertEqual()`](http://docs.python.org/3/library/unittest.html#unittest.TestCase.assertEqual)? At least with `assertEqual()`, the module will print both the result and the expected values. – Denilson Sá Maia Mar 11 '14 at 03:40
You are right, assertEqual() is better. I can't find the link but I was pretty sure to have read somewhere to use assertTrue instead of assertEqual. Obviously, I was wrong :) I'll fix the example above. – Vincent Audebert Mar 11 '14 at 04:00
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Note that this will be unreliable when you have more than one key in any dictionary because the `dumps()` uses arbitrary ordering and we don't know the ordering of keys in `result`. You'd be much better off using `self.assertEqual(json.loads(result), expected_dict)`. – André Caron Sep 26 '14 at 13:19

score 10 · Answer 5 · edited May 23 '17 at 11:47

As mentioned by Ed I, assertIn is probably the simplest answer to finding one string in another. However, the question states:

I want to make sure that my result contains at least the json object (or string) that I specified as the second argument above,i.e., {"car" : ["toyota","honda"]}

Therefore I would use multiple assertions so that helpful messages are received on failure - tests will have to be understood and maintained in the future, potentially by someone that didn't write them originally. Therefore assuming we're inside a django.test.TestCase:

# Check that `car` is a key in `result`
self.assertIn('car', result)
# Compare the `car` to what's expected (assuming that order matters)
self.assertEqual(result['car'], ['toyota', 'honda'])

Which gives helpful messages as follows:

# If 'car' isn't in the result:
AssertionError: 'car' not found in {'context': ..., 'etc':... }
# If 'car' entry doesn't match:
AssertionError: Lists differ: ['toyota', 'honda'] != ['honda', 'volvo']

First differing element 0:
toyota
honda

- ['toyota', 'honda']
+ ['honda', 'volvo']

Python/Django: how to assert that unit test result contains a certain string?

5 Answers5