test_ok1/testing/plugin/test_pytest_assertion.py

def test_functional(testdir):
    testdir.makepyfile("""
        def test_hello():
            x = 3
            assert x == 4
    """)
    result = testdir.runpytest()
    assert "3 == 4" in result.stdout.str()
    result = testdir.runpytest("--no-assert")
    assert "3 == 4" not in result.stdout.str()

def test_traceback_failure(testdir):
    p1 = testdir.makepyfile("""
        def g():
            return 2
        def f(x):
            assert x == g()
        def test_onefails():
            f(3)
    """)
    result = testdir.runpytest(p1)
    result.stdout.fnmatch_lines([
        "*test_traceback_failure.py F",
        "====* FAILURES *====",
        "____*____",
        "",
        "    def test_onefails():",
        ">       f(3)",
        "",
        "*test_*.py:6: ",
        "_ _ _ *",
        #"",
        "    def f(x):",
        ">       assert x == g()",
        "E       assert 3 == 2",
        "E        +  where 2 = g()",
        "",
        "*test_traceback_failure.py:4: AssertionError"
    ])