Help with plugin assertion rewriting

[JBKahn]

Hey, I was looking at the pytest-clarity package and it does something really nice, imo, which is it turns this:

into:

The downside is that the rewriting results in a loss of functionality:

a = [1, 2, 3]
b = [1, 2]

def test_a():
    assert len(a) == len(b)

Goes from

    def test_a():
>       assert len(a) == len(b)
E       assert 3 == 2
E        +  where 3 = len([1, 2, 3])
E        +  and   2 = len([1, 2])

to

    def test_a():
>       assert len(a) == len(b)
E       assert left == right failed.
E         Showing split diff:
E
E         left:  3
E         right: 2

I've been digging through the source and I can't quite find the right part to use to do this. It stems from use of pytest_assertrepr_compare but I can't find anything in pytest I can use to construct that 3 = len([1, 2, 3]). I'm looking for some pointers if anyone can help put me on the right track. I was barking down the AssertionRewriter which I think is what I'm looking for but I can't quite put my finger on how to modify these assertions in way that still utilizes that code.

[blueyed]

Might be good to get feedback in #5933 how the new method / alternatives would be useful for the plugin, since the plugin would benefit here from having the original explanation (or a way to say where it should be inserted (via special placeholder)).

[JBKahn]

I'll take a look tomorrow myself

[JBKahn]

Thanks for getting back so quickly!