Support PEP-415's Exception.__suppress_context__

[jmoldow]

• Include a detailed description of the bug or suggestion

PEP-415 states that exception.__context__ should be suppressed in traceback outputs, if exception.__suppress_context__ is True.

If a raise exception from None is caught by pytest, pytest should not chain the context in the test report.

The current algorithm in _pytest._code.code.FormattedExcinfo is:

if e.__cause__ is not None:
    # Code to chain the cause.
elif e.__context__ is not None:
    # Code to chain the context.

which means that pytest always chains the exception, assuming that e.__context__ hasn't been set to None.

By comparison, the algorithm in traceback.TracebackException is:

if e.__cause__ is not None:
    # Code to chain the cause.
elif (e.__context__ is not None and not e.__suppress_context__):
    # Code to chain the context.

Exception.__suppress_context__ is available in all of the versions of Python 3 that are supported by pytest, so it is trivial to add support for this feature.

• Minimal example if possible

Here's a test that has a raise exception from None in it:

def test_raise_from_none():
    try:
        raise ValueError()
    except Exception:
        raise AttributeError() from None

This is the test output (with the chained exception traceback) that results when running against the pytest feature branch (commit 768edde):

_____________________________________________________ test_raise_from_none _____________________________________________________

    def test_raise_from_none():
        try:
>           raise ValueError()
E           ValueError

testing/code/test_excinfo.py:1257: ValueError

During handling of the above exception, another exception occurred:

    def test_raise_from_none():
        try:
            raise ValueError()
        except Exception:
>           raise AttributeError() from None
E           AttributeError

testing/code/test_excinfo.py:1259: AttributeError

But running the same code in a terminal produces this shorter, non-chained traceback:

AttributeError                            Traceback (most recent call last)
<ipython-input-51-e2e3809c49fb> in <module>()
----> 1 test_raise_from_none()

<ipython-input-50-d651befdf00e> in test_raise_from_none()
      3         raise ValueError()
      4     except Exception:
----> 5         raise AttributeError() from None
      6

AttributeError:

And this is what the pytest output should look like:

_____________________________________________________ test_raise_from_none _____________________________________________________

    def test_raise_from_none():
        try:
            raise ValueError()
        except Exception:
>           raise AttributeError() from None
E           AttributeError

testing/code/test_excinfo.py:1259: AttributeError

[nicoddemus]

Fixed by #2632