No proper validation for month in Calendar Module

[gugupy]

Bug report

Bug description:

In the calendar module, IllegalMonthError is not handled properly. For months greater than 12, an IndexError is raised instead.

formatmonthname do not raise IndexError for negative integer because list support negative integer indexing.

>>> import calendar

>>> calendar.month(2024, -1)
Traceback (most recent call last):
  File "<python-input-12>", line 1, in <module>
    calendar.month(2024, -1)
    ~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 395, in formatmonth
    for week in self.monthdays2calendar(theyear, themonth):
                ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 284, in monthdays2calendar
    days = list(self.itermonthdays2(year, month))
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 240, in itermonthdays2
    for i, d in enumerate(self.itermonthdays(year, month), self.firstweekday):
                ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 228, in itermonthdays
    day1, ndays = monthrange(year, month)
                  ~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 165, in monthrange
    raise IllegalMonthError(month)
calendar.IllegalMonthError: bad month number -1; must be 1-12

>>> calendar.month(2024, 13)
Traceback (most recent call last):
  File "<python-input-13>", line 1, in <module>
    calendar.month(2024, 13)
    ~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 390, in formatmonth
    s = self.formatmonthname(theyear, themonth, 7 * (w + 1) - 1)
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 373, in formatmonthname
    s = month_name[themonth]
        ~~~~~~~~~~^^^^^^^^^^
  File "/Users/gugu/Code/cpython/Lib/calendar.py", line 103, in __getitem__
    funcs = self._months[i]
            ~~~~~~~~~~~~^^^
IndexError: list index out of range

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-126484

[tomasr8]

I think it'd make sense to raise IllegalMonthError in both cases. Would you like to send a PR?

[gugupy]

@Zheaoli Also add the validation for arguments, right after get options. Because the same issue happening for cli.

    if options.month is not None and options.month not in range(1, 13):
        parser.error(IllegalMonthError(options.month).__str__())
        sys.exit(1)

[gugupy]

I think it'd make sense to raise IllegalMonthError in both cases. Would you like to send a PR?

Yes, but @Zheaoli already did it.

[ZeroIntensity]

@Zheaoli For future reference, it's a good idea to let an issue author send a PR if asked, especially for first time contributors.