I'm suggesting this idea to consistency. Why an IllegalMonthError exists in calendar module if we don't raise this error when required?

What would you say if I asked you to "What is the month number coming after 156th month?" Would you say 157 or prefer to inform me that I'm doing something wrong?

>>> import calendar
>>> calendar.nextmonth(2018, 12)
(2019, 1)

If Python is smart enough to jump next year's first month and not say (2018, 13) blindly, it should also check if the given month is valid. 

But:
>>> calendar.nextmonth(2018, 157)
(2018, 158)

I think this is clearly a bug in the code.


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I'll wander away from the this issue but some of the functions in calendar module also not consistent with each other:

>>> calendar.monthcalendar(2018, 12) # Runs with no problem.
>>> calendar.monthcalendar(2018, 0)  # Raises IllegalMonthError.
>>> calendar.monthcalendar(2018, 13) # Raises IllegalMonthError.

>>> calendar.month(2018, 12) # Runs with no problem.
>>> calendar.month(2018, 0)  # Raises IllegalMonthError.
>>> calendar.month(2018, 13) # Raises IndexError???

Why? Wouldn't it be more reasonable if the last one also had raised IllegalMonthError? 

>>> calendar.monthrange(2018, 12) # Runs with no problem.
>>> calendar.monthrange(2018, 0)  # Raises IllegalMonthError.
>>> calendar.monthrange(2018, 13) # Raises IllegalMonthError.

>>> calendar.prmonth(2018, 12) # Runs with no problem.
>>> calendar.prmonth(2018, 0)  # Raises IllegalMonthError.
>>> calendar.prmonth(2018, 13) # Raises IndexError.