[issue39725] unrelated `from None` exceptions lose prior exception information

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

Interesting problem.

Example which does not depend on os.environ:

import traceback
try:
    try:
        raise TypeError
    except:
        try:
            raise ValueError
        except:
            raise KeyError from None
except BaseException as exc:
    e = exc

print([e, e.__cause__, e.__context__, e.__suppress_context__])
traceback.print_exception(type(e), e, e.__traceback__)


"from None" sets __suppress_context__ to True. Currently, if __suppress_context__ is true, the output of __context__ will be suppressed in the traceback. But what if we suppress only a part __context__ and output the "original" exception. The problem is what is the "original" exception. Is it __context__.__context__? Or __context__.__cause__ if it is not None? Should we take __context__.__suppress_context__ into account? Maybe go down recursively?

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nosy: +benjamin.peterson

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39725>
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