Testing assertions `assertContains` and `assertInHTML` should output the haystack on failure

[Thibaud Colas]

If I use the vanilal Python assertIn or assertRegex, when the assertion fails, it’s very simple to assess what went wrong from the test error only:

    self.assertIn("<b>hey</b>", "<p>Howdy!</p>")
AssertionError: '<b>hey</b>' not found in '<p>Howdy!</p>'

With Django’s assertContains and assertInHTML, this gets much harder:

    self.assertInHTML("<b>hey</b>", "<p>Howdy!</p>")
  File "/Users/thibaudcolas/Dev/django/html-testing-with-django/.venv/lib/python3.11/site-packages/django/test/testcases.py", line 1076, in assertInHTML
    self.assertTrue(
AssertionError: False is not true : Couldn't find '<b>
hey
</b>' in response

and:

    self.assertContains(res, "<b>hey</b>")
  File "/Users/thibaudcolas/Dev/django/html-testing-with-django/.venv/lib/python3.11/site-packages/django/test/testcases.py", line 660, in assertContains
    self.assertTrue(
AssertionError: False is not true : Couldn't find '<b>hey</b>' in response

---

In both cases, Django doesn’t display the haystack – so I have to waste a lot of time going back to my HTML templates to check what they might be outputting / or potentially load the same scenario in a browser / or manually add print statements to my test cases. This is all very time-consuming. Instead, it’d be much better if the haystack was just present.

Additionally for assertInHTML – it’s annoying that needle as displayed in the failure message is the parser’s output, which is therefore broken up over multiple lines. It’d be much nicer if the example above resulted in AssertionError: False is not true : Couldn't find '<b>hey</b>' in response.

---

Test suite I used to compare output for reference: ​https://github.com/thibaudcolas/html-testing-with-django/blob/39c82c410cce4bea71ac54be27be67057f4d8dd8/testing_tests/tests.py#L5

And link to Python’s assertIn / assertNotIn implementations: ​https://github.com/python/cpython/blob/101d5ec7d7fe122fa81a377c8ab8b562d1add9ee/Lib/unittest/case.py#L1147-L1159

[Natalia Bidart]

Tentatively accepting because this is related to the testing framework, though I'm not 100% sure on procedure whether this specific case needs a forum post since it's marked as a new feature. I would argue this is a cleanup/optimization :thinking:

[Sarah Boyce]

I can see the value of this. The react testing library we use has a setting DEBUG_PRINT_LIMIT (​https://testing-library.com/docs/dom-testing-library/api-debugging/#automatic-logging) with a default of 7000 characters but you can increase it if you need to see more printed. I recommend that we have a similar setting 👍

[Mariusz Felisiak]

Replying to Sarah Boyce:

I can see the value of this. The react testing library we use has a setting DEBUG_PRINT_LIMIT (​https://testing-library.com/docs/dom-testing-library/api-debugging/#automatic-logging) with a default of 7000 characters but you can increase it if you need to see more printed. I recommend that we have a similar setting 👍

Creating a new setting is always controversial (we already have many of them) and users can already control it with maxDiff.

[Thibaud Colas]

Sorry, I didn’t realise feature requests require a forum post! I’ll know for next time.

[Shamil Abdulaev]

I have fixed this bug and sent a PR ​https://github.com/django/django/pull/17077 for consideration!)

[Sarah Boyce]

Hi Shamil, we still need to review and merge in a PR before we mark a ticket as fixed. In general I would leave the status alone as mostly the fellows take care of this
I've written a couple of comments on the PR, once they're sorted we can update the ticket to go into the review queue

[Chinmoy]

Hey, is this still being worked upon? Can I assign myself to this ticket?

[Natalia Bidart]

Replying to Chinmoy:

Hey, is this still being worked upon? Can I assign myself to this ticket?

Hi! Thank you for your interest in contributing. I checked and both related PRs are closed so if you can work on this, it'll be certainly welcomed! Thanks.

[Natalia Bidart]

Resetting patch flags.

[Chinmoy]

Hey, sorry I missed this in my mail. Will submit a PR soon.

[Chinmoy]

Here is the PR ​https://github.com/django/django/pull/17204.

[Mariusz Felisiak <felisiak.mariusz@…>]

In e99c7d88:

Refs #34657 -- Made assertInHTML() use unparsed needle in error messages.

[GitHub <noreply@…>]

In 679212a:

Refs #34657 -- Made msg_prefix handling in assertURLEqual()/assertInHTML consistent with other assertions.

Co-authored-by: Chinmoy Chakraborty <chinmoy12c@…>

[Mariusz Felisiak <felisiak.mariusz@…>]

In 1dae65dc:

Fixed #34657 -- Made assert(Not)Contains/assertInHTML display haystacks in error messages.