Pytest: __tracebackhide__ does not work with chained exceptions

[The-Compiler]

On Python 3, with this code:

def bar():
    __tracebackhide__ = True
    raise ValueError

def assert_foo():
    __tracebackhide__ = True
    bar()

def test_foo():
    assert_foo()

I get a nice cleaned up traceback as I'd expect:

_____________________ test_foo ______________________

    def test_foo():
>       assert_foo()
E       ValueError

tbh.py:10: ValueError

However, if I re-raise the exception:

def bar():
    __tracebackhide__ = True
    raise ValueError

def assert_foo():
    __tracebackhide__ = True
    try:
        bar()
    except ValueError:
        raise TypeError

def test_foo():
    assert_foo()

The traceback gets quite verbose:

_____________________ test_foo ______________________

    def assert_foo():
        __tracebackhide__ = True
        try:
>           bar()

tbh.py:8: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def bar():
        __tracebackhide__ = True
>       raise ValueError
E       ValueError

tbh.py:3: ValueError

During handling of the above exception, another exception occurred:

    def test_foo():
>       assert_foo()
E       TypeError

tbh.py:13: TypeError

cc @roolebo

[roolebo]

Thanks for the report. I'll look at it.

[lumbduck]

I'm running into this too, including on AssertionError, which is very problematic for test reporting.

I.e., this will also show the full stack trace

def bar():
    __tracebackhide__ = True
    raise ValueError

def assert_foo():
    __tracebackhide__ = True
    try:
        bar()
    except ValueError:
        assert False

def test_foo():
    assert_foo()

[wimglenn]

I would consider this a feature not a bug. It's good that pytest gives you all the context by default. To opt-out of the extra-context:

def assert_foo():
    __tracebackhide__ = True
    try:
        bar()
    except ValueError:
        raise TypeError from None

cross-compat version of the same, if you need to support 2.7 still:

from future.utils import raise_from

def assert_foo():
    __tracebackhide__ = True
    try:
        bar()
    except ValueError:
        raise_from(TypeError, None)

Similarly, assert statements can just be replaced with

raise AssertionError from None    # assert False
raise AssertionError("your message") from None    # assert False, "your message"

So, what is the bug exactly? Are you suggesting to check if __tracebackhide__ is set in the frame, and always disable exception chaining in that case?

[The-Compiler]

@wimglenn Yes - the whole idea of __tracebackhide__ is to suppress a verbose traceback, I don't see why that should change when exception chaining comes into play.

[blueyed]

For reference, here is a failing test:

def test_tbh_chained(testdir):
    p = testdir.makepyfile(
        """
        import pytest

        def f1():
            __tracebackhide__ = True
            try:
                return f1.meh
            except AttributeError:
                pytest.fail("fail")

        @pytest.fixture
        def fix():
            f1()


        def test(fix):
            pass
        """
    )
    result = testdir.runpytest(str(p))
    assert "'function' object has no attribute 'meh'" not in result.stdout.str()
    assert result.ret == 0

Output:

test_tbh_chained.py E                                                    [100%]

==================================== ERRORS ====================================
____________________________ ERROR at setup of test ____________________________

    def f1():
        __tracebackhide__ = True
        try:
>           return f1.meh
E           AttributeError: 'function' object has no attribute 'meh'

test_tbh_chained.py:6: AttributeError

During handling of the above exception, another exception occurred:

    @pytest.fixture
    def fix():
>       f1()
E       Failed: fail

test_tbh_chained.py:12: Failed
=========================== 1 error in 0.05 seconds ============================
=========================== short test summary info ============================
FAILED testing/test_tbhide_chained.py::test_tbh_chained

I guess in this case the whole chain should be removed?