examples/pytest/test_number_less_than.py
def get_number():
    return 23

def test_string_equal():
    assert get_number() < 0 
$ pytest test_number_less_than.py

    def test_string_equal():
>       assert get_number() < 0
E       assert 23 < 0
E        +  where 23 = get_number()