Second solution for IndexError problem in QuerySet

[artemrizhov]

This is a fix for problem described in ticket #23555 https://code.djangoproject.com/ticket/23555

I've sent a pull request with a fix already #3274
But if somebody thinks the code becomes too complicated and prefers to use try/catch despite the problem then I propose this solution with special exception class. Now you can use this safely anywhere:

try:
    return qs[0]
except QuerySetIndexError:
    return None

[tchaumeny]

The test you're providing doesn't show a flaw in the current implementation, as you have to create that flaw in the test itself. See https://code.djangoproject.com/ticket/23555#comment:3 which provides examples of such IndexError in cPython itself.

[artemrizhov]

@tchaumeny The test that I've provided does show the problem. It demostrates that external errors are not handled right way. Please see my comment on the ticket for more explanation https://code.djangoproject.com/ticket/23555#comment:4, and also please read the comments (there is a lot of) on the tests code and on the model used for tests, and also please read the comments on first pull request #3274.
Also please note that this is alternate solution for the case the maintainers think such approach is better. Imho, first solution demonstrates the probalem much better (see #3274).

[timgraham]

The first solution seems simpler, although I'm not entirely sold on it. Someone more familiar with the ORM may provide more feedback.

[artemrizhov]

The first solution seems simpler

Yes, it does. However please note that it does not provide a way for safe use of try/except with queryset's IndexError. On the current stage this just should be avoided.