Repository: python/cpython
Pull request: gh-126476: Raise `IllegalMonthError` for calendar.formatmonth method when the input month is not corret
Fix: #126476


<!-- gh-issue-number: gh-126476 -->
* Issue: gh-126476
<!-- /gh-issue-number -->


Problem statement:
No proper validation for month in Calendar Module
# 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.

```python
>>> 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

<!-- gh-linked-prs -->
### Linked PRs
* gh-126484
* gh-126878
* gh-126879
<!-- /gh-linked-prs -->

Hints from issue discussion:
I think it'd make sense to raise `IllegalMonthError` in both cases. Would you like to send a PR?
@Zheaoli Also add the validation for arguments, right after get options. Because the same issue happening for cli. 

```python
    if options.month is not None and options.month not in range(1, 13):
        parser.error(IllegalMonthError(options.month).__str__())
        sys.exit(1)
```
> 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.

Commit message: Fix review idea

Patch to review:
diff --git a/Lib/calendar.py b/Lib/calendar.py
index 069dd5174112ae..8c1c646da46a98 100644
--- a/Lib/calendar.py
+++ b/Lib/calendar.py
@@ -27,7 +27,9 @@
 error = ValueError
 
 # Exceptions raised for bad input
-class IllegalMonthError(ValueError):
+# This is trick for backward compatibility. Since 3.13, we will raise IllegalMonthError instead of
+# IndexError for bad month number(out of 1-12). But we can't remove IndexError for backward compatibility.
+class IllegalMonthError(ValueError, IndexError):
     def __init__(self, month):
         self.month = month
     def __str__(self):
@@ -158,11 +160,14 @@ def weekday(year, month, day):
     return Day(datetime.date(year, month, day).weekday())
 
 
+def _validate_month(month):
+    if not 1 <= month <= 12:
+        raise IllegalMonthError(month)
+
 def monthrange(year, month):
     """Return weekday of first day of month (0-6 ~ Mon-Sun)
        and number of days (28-31) for year, month."""
-    if not 1 <= month <= 12:
-        raise IllegalMonthError(month)
+    _validate_month(month)
     day1 = weekday(year, month, 1)
     ndays = mdays[month] + (month == FEBRUARY and isleap(year))
     return day1, ndays
@@ -370,6 +375,8 @@ def formatmonthname(self, theyear, themonth, width, withyear=True):
         """
         Return a formatted month name.
         """
+        _validate_month(themonth)
+
         s = month_name[themonth]
         if withyear:
             s = "%s %r" % (s, theyear)
@@ -500,6 +507,7 @@ def formatmonthname(self, theyear, themonth, withyear=True):
         """
         Return a month name as a table row.
         """
+        _validate_month(themonth)
         if withyear:
             s = '%s %s' % (month_name[themonth], theyear)
         else:
@@ -786,6 +794,8 @@ def main(args=None):
         if options.month is None:
             optdict["c"] = options.spacing
             optdict["m"] = options.months
+        if options.month is not None:
+            _validate_month(options.month)
         if options.year is None:
             result = cal.formatyear(datetime.date.today().year, **optdict)
         elif options.month is None:
diff --git a/Lib/test/test_calendar.py b/Lib/test/test_calendar.py
index f119d89c0ec39a..073df310bb49eb 100644
--- a/Lib/test/test_calendar.py
+++ b/Lib/test/test_calendar.py
@@ -457,6 +457,11 @@ def test_formatmonth(self):
             calendar.TextCalendar().formatmonth(0, 2),
             result_0_02_text
         )
+    def test_formatmonth_with_invalid_month(self):
+        with self.assertRaises(calendar.IllegalMonthError):
+            calendar.TextCalendar().formatmonth(2017, 13)
+        with self.assertRaises(calendar.IllegalMonthError):
+            calendar.TextCalendar().formatmonth(2017, -1)
 
     def test_formatmonthname_with_year(self):
         self.assertEqual(
@@ -1121,7 +1126,7 @@ def test__all__(self):
         not_exported = {
             'mdays', 'January', 'February', 'EPOCH',
             'different_locale', 'c', 'prweek', 'week', 'format',
-            'formatstring', 'main', 'monthlen', 'prevmonth', 'nextmonth'}
+            'formatstring', 'main', 'monthlen', 'prevmonth', 'nextmonth', ""}
         support.check__all__(self, calendar, not_exported=not_exported)
 
 
@@ -1149,6 +1154,13 @@ def test_formatmonth(self):
         self.assertIn('class="text-center month"',
                       self.cal.formatmonth(2017, 5))
 
+    def test_formatmonth_with_invalid_month(self):
+        with self.assertRaises(calendar.IllegalMonthError):
+            self.cal.formatmonth(2017, 13)
+        with self.assertRaises(calendar.IllegalMonthError):
+            self.cal.formatmonth(2017, -1)
+
+
     def test_formatweek(self):
         weeks = self.cal.monthdays2calendar(2017, 5)
         self.assertIn('class="wed text-nowrap"', self.cal.formatweek(weeks[0]))
diff --git a/Misc/NEWS.d/next/Library/2024-11-06-18-30-50.gh-issue-126476.F1wh3c.rst b/Misc/NEWS.d/next/Library/2024-11-06-18-30-50.gh-issue-126476.F1wh3c.rst
new file mode 100644
index 00000000000000..0b816dca80693a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-11-06-18-30-50.gh-issue-126476.F1wh3c.rst
@@ -0,0 +1,2 @@
+Raise :class:`calendar.IllegalMonthError` for :func:`calendar.month`
+when the input month is not correct.


Code review comments:
diff_hunk: @@ -0,0 +1,2 @@
+Raise :class:`calendar.IllegalMonthError` for :func:`calendar.month`
path: Misc/NEWS.d/next/Library/2024-11-06-18-30-50.gh-issue-126476.F1wh3c.rst
text: @user1:
```suggestion
Raise :class:`calendar.IllegalMonthError` (now a subclass of :class:`IndexError`) for :func:`calendar.month`
```