Improve diffs when comparing classes with repr

[alexprengere]

What's the problem this feature will solve?

I am a long-time user of pytest-clarity, and I see that some of its ideas were incorporated into core pytest, but there is one lacking feature that has proven really useful to me.

I am maintaining multiple large libraries with lots of classes, and I have many tests that basically amount to dumping class instances using custom serialization code, then loading them again and compare with the original instance. Those classes have a repr, and pytest-clarity shows a nice diff of the repr when the comparison fail.

Here is the basic reproducer.

class AC:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

    def __repr__(self):
        return f"{self.__class__.__name__}({self.x}, {self.y})"


def test_class():
    a = AC(1, 1)
    b = AC(1, 2)
    assert a == b

Here is the visual output :

For comparison here is the output when pytest-clarity is enabled:

Mind that in a more realistic example, such classes can have dozens of attributes, which can themselves be classes with dozens of attributes, etc. The actual test may use libraries like hypothesis to find subtle bugs in the serialization/deserialization code.

So here would be my feature request: when pytest is showing both repr, highlighting the difference (akin to what is done to namedtuple, dataclasses, dicts) would be a great help!

[alexprengere]

With #14235 it would look a bit better:

[RonnyPfannschmidt]

i like the idea of integrating this deeper

as far as i can tell tis the key functionality of pytest-clarity

@darrenburns whats your opinion on moving this into pytest core

a key caveat for this is that my "plan" for using rich for pytest ux has been set back since quite a while and it would be nice not to have to fall back to the old way