Suggestion: reset sys.last_traceback using `del` rather than setting to `None`

[mdickinson]

PR #3384 introduced code that resets sys.last_traceback and friends by assigning None to them:

pytest/src/_pytest/runner.py

Line 124 in 7e10c81

sys.last_type, sys.last_value, sys.last_traceback = (None, None, None)

The normal CPython behaviour here is that once the exception has been cleared, the attributes are deleted rather than set to None. Could I suggest updating this line of code to delete the attributes?

For context: the Nones caused test failures in another project when we attempted to switch from unittest discover to pytest as our test runner; we had a test for some complex code that was messing with various sys attributes including the sys.last_* attributes, and the test verified that those attributes were unchanged by the code under test.

[nicoddemus]

Hi @mdickinson,

Definitely sounds reasonable, thanks.

[nicoddemus]

@mdickinson would you like to open a PR? Should be a simple change and there's already a test in place which just needs to be updated, test_store_except_info_on_error.

[mdickinson]

@nicoddemus Done! #6256

[RonnyPfannschmidt]

i just checked the python docs, and as far as i can tell we would be more in line with the setup python expects if this is implemented