Add a hook to modify final error text

[Rizhiy]

What's the problem this feature will solve?

I have a test where I need to compare long strings.
A common failure is for spaces to be changed to tabs or other invisible characters.
I need a way to visualise invisible characters.

Describe the solution you'd like

Being able to modify the output string would enable me to replace invisible characters with visible substitutes and make debugging easier.

Concise example:

def test_tabs_spaces():
    pad = "foobar\n" * 10
    first_str = pad + "	" + pad  # Tab in the middle
    second_str = pad + "    " + pad  # Spaces in the middle
    assert first_str == second_str

This produces:

E       AssertionError: assert 'foobar\nfoob...bar\nfoobar\n' == 'foobar\nfoob...bar\nfoobar\n'
E         Skipping 60 identical leading characters in diff, use -v to show
E           ar
E           foobar
E         - 	foobar
E         +     foobar
E           foobar
E           foobar...
E
E         ...Full output truncated (8 lines hidden), use '-vv' to show

With two lines indistinguishable because my terminal doesn't show tabs as visible characters.

Alternative Solutions

I have tried pytest_assertrepr_compare, but then nice formatting is gone and it is impossible to find the error in a long string.

I guess, I can set my terminal to display tabs visibly, but that is not very user-friendly since that is not the default for the vast majority of people.

Additional context

TBH, might be a good idea to invisible characters using visible substitutes, e.g. "▸" for tab and "·" for space.

[RonnyPfannschmidt]

https://docs.pytest.org/en/7.1.x/reference/reference.html#pytest.hookspec.pytest_assertrepr_compare

[Rizhiy]

@RonnyPfannschmidt Already explained.

[RonnyPfannschmidt]

as for nice formatting, the builtin variant does quite some extra work

i recommend taking a look

[Rizhiy]

Why not allow us to modify the final string?

Alternatively the "extra work" can be exposed as a function to more easily implement in the current hook.

[RonnyPfannschmidt]

Hookwappers may force/alter outcomes

[Rizhiy]

Not really sure what you mean.

[RonnyPfannschmidt]

# untested 
@pytest.hookimpl(hookwapper=true) 
def pytest_assertrepr_compare(...):
  outcome = yield 
  Stuff.do_with(outcome)