With how old pytest is, I assume that's where they got it from. Does it perform recursive value printing, or bespoke comparisons?

e.g. in pytest it won't just print out the values of "a" and "b", it will recursively document intermediate values until it's reached the toplevel expression:

    assert f() == g()
    assert 42 == 43
      where 42 = <function TestFailing.test_simple.<locals>.f at 0xdeadbeef0002>()
      and   43 = <function TestFailing.test_simple.<locals>.g at 0xdeadbeef0003>()

and it's possible to customise the report so you can report as a diff:

    assert "foo 1 bar" == "foo 2 bar"
      - foo 2 bar
      ?     ^
      + foo 1 bar
      ?     ^