[WIP/RFC] pdb: set sys.last_{type,value,traceback} in _enter_pdb

[blueyed]

This affects pytest_exception_interact (--pdb) currently only, but
makes sense for all calls to post_mortem probably, or even more in
general.

Ref: https://github.com/python/cpython/blame/9f316bd9684d27b7e21fbf43ca86dc5e65dac4af/Doc/library/sys.rst#L891-L904

[blueyed]

Initially I wanted this to use Pdb.user_exception instead, but that would require some adjustments of (mocking) tests.
user_exception would set __exception__ into locals (but only type and value currently (https://bugs.python.org/issue36663)).

[asottile]

This seems fine / useful -- just needs a changelog entry

[blueyed]

From the description it sounds like this should be set in pytest core tiself maybe even:

These three variables are not always defined; they are set when an exception is
not handled and the interpreter prints an error message and a stack traceback.

(via https://github.com/python/cpython/blame/9f316bd9684d27b7e21fbf43ca86dc5e65dac4af/Doc/library/sys.rst#L891-L904)

[nicoddemus]

That's nice indeed, just needing a new changelog. Suggestion:

``sys.excinfo`` is now set with the causing exception when entering the debugger with ``--pdb``.

[blueyed]

I still think it should be more generic, and not via the debugging plugin probably, i.e. everytime Python "creates" an exception (failing test, error) it should make this info available for inspection.
But maybe having it via the debugging plugin is the easiest to achieve this - but it should certainly be in a more central place there then.

[blueyed]

Found it being done here already: https://github.com/blueyed/pytest/blob/73b74c74c9674947c8383f657c3a78b1d73ffe1b/src/_pytest/runner.py#L118-L131

[blueyed]

Reworked in blueyed#16, in case you are interested.