gh-126476: Raise IllegalMonthError for calendar.formatmonth method when the input month is not corret

[Zheaoli]

Fix: #126476

• Issue: No proper validation for month in Calendar Module #126476

[ericvsmith]

I'm not sure this is a good idea. If someone was catching IndexError previously, then the change to IllegalMonthError will break their code.

[ZeroIntensity]

Yeah, Eric is right. I would be ok with this if IllegalMonthError also inherited from IndexError.

[Zheaoli]

I have update this PR,PTAL @ericvsmith @ZeroIntensity

[ZeroIntensity]

I'm not sure how to feel about this. It works, and the implementation looks fine, but it's also a little hacky so I'm hesitant to approve. I'll wait for Eric's input.

[ericvsmith]

Yeah, I'm not crazy about it. It seems like there must be other places where we've done something similar, though. Does anyone feel like searching the stdlib for some place else we've done this?

[tomasr8]

I didn't find any examples of errors inheriting from IndexError, but there is precedent for changing IndexError to something else. Here are two examples that I found:

https://github.com/python/cpython/pull/8184/files#diff-654faea1665f8db34a06e155e3f168b467e2071967404ea9692f41190fcbb5d4

https://github.com/python/cpython/pull/123613/files#diff-f6ee3a81bee278bb507d0cba22bb56acb204667c1271fc5d3fc41aa702248c35

[ericvsmith]

I was thinking more along the lines of using multiple inheritance for exceptions. But those are good examples of changing the type of an exception.

[tomasr8]

I was thinking more along the lines of using multiple inheritance for exceptions.

Ah sorry, I misunderstood. I checked again and found two examples of multiple inheritance:

class MultipartConversionError(MessageError, TypeError):
    """Conversion to a multipart is prohibited."""


class UnsupportedOperation(OSError, ValueError):
        pass

[ethanfurman]

Multiple inheritance for exceptions is a common technique (at least in my own code 😄 ). Looks good to me.

[hauntsaninja]

Thanks Tomas for finding examples! I'd vote that it's probably okay to change the exception type. Adding IndexError as a base can break code too if someone has an except IndexError for some other reason and calls monthrange.

(side note: the UnsupportedOperation Tomas mentions has finicky history)