[3.15] gh-151770: Fix datetime.fromisoformat() on an out-of-range month w/ a 24:00 time (GH-151771) (#151809)

(cherry picked from commit 1fb874c)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
Co-authored-by: Stan Ulbrych <stan@python.org>

Author: 3 people

Lib/_pydatetime.py

@@ -55,7 +55,7 @@ def _days_before_year(year):
 
 def _days_in_month(year, month):
     "year, month -> number of days in that month in that year."
-    assert 1 <= month <= 12, month
+    assert 1 <= month <= 12, f"month must be in 1..12, not {month}"
     if month == 2 and _is_leap(year):
         return 29
     return _DAYS_IN_MONTH[month]
@@ -1987,7 +1987,7 @@ def fromisoformat(cls, date_string):
                 if became_next_day:
                     year, month, day = date_components
                     # Only wrap day/month when it was previously valid
-                    if month <= 12 and day <= (days_in_month := _days_in_month(year, month)):
+                    if 1 <= month <= 12 and day <= (days_in_month := _days_in_month(year, month)):
                         # Calculate midnight of the next day
                         day += 1
                         if day > days_in_month:

Lib/test/datetimetester.py

@@ -3773,6 +3773,7 @@ def test_fromisoformat_fails_datetime_valueerror(self):
             "2009-04-01T12:30:90",          # Second out of range
             "2009-04-01T12:90:45",          # Minute out of range
             "2009-04-01T25:30:45",          # Hour out of range
+            "2009-00-01T24:00:00",          # Month below range
             "2009-13-01T24:00:00",          # Month out of range
             "9999-12-31T24:00:00",          # Year out of range
         ]

Misc/NEWS.d/next/Library/2026-06-20-15-00-00.gh-issue-151770.dtiso0.rst

@@ -0,0 +1,3 @@
+Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
+instead of :exc:`ValueError` for an out-of-range month combined with a
+``24:00`` time.

Modules/_datetimemodule.c

@@ -6144,7 +6144,7 @@ datetime_datetime_fromisoformat_impl(PyTypeObject *type, PyObject *string)
         goto error;
     }
 
-    if ((hour == 24) && (month <= 12))  {
+    if ((hour == 24) && (month >= 1 && month <= 12))  {
         int d_in_month = days_in_month(year, month);
         if (day <= d_in_month) {
             if (minute == 0 && second == 0 && microsecond == 0) {