PyTest compare numbers

def double(n):
    #return 2*n
    return 2+n

def test_string_equal():
    assert double(2) == 4
    assert double(21) == 42

    $ pytest test_number_equal.py

    def test_string_equal():
        assert double(2) == 4
>       assert double(21) == 42
E       assert 23 == 42
E        +  where 23 = double(21)