gh-152157: Reject empty fraction before timezone in C fromisoformat

The C accelerator for datetime.fromisoformat() and time.fromisoformat()
accepted a decimal separator (. or ,) with no following digit when a
timezone designator came next, e.g. '12:34:56.+05:00', while the
pure-Python implementation correctly raised ValueError. Handle the
decimal-separator case before the generic end-of-substring check so an
empty fraction is rejected.

Author: tonghuaroot

Lib/test/datetimetester.py

@@ -3758,6 +3758,10 @@ def test_fromisoformat_fails_datetime(self):
             '2009-04-19T12:30:45-00:90:00', # Time zone field out from range
             '2009-04-19T12:30:45-00:00:90', # Time zone field out from range
             '2020-2020',                    # Ambiguous 9-char date portion
+            '2009-04-19T12:30:45.+05:00',   # Empty fraction before offset
+            '2009-04-19T12:30:45.-05:00',   # Empty fraction before offset
+            '2009-04-19T12:30:45.Z',        # Empty fraction before Z
+            '2009-04-19T12:30:45,+05:00',   # Empty fraction (comma) before offset
         ]
 
         for bad_str in bad_strs:
@@ -5003,6 +5007,10 @@ def test_fromisoformat_fails(self):
             '24:01:00.000000',          # Has non-zero minutes on 24:00
             '12:30:45+00:90:00',        # Time zone field out from range
             '12:30:45+00:00:90',        # Time zone field out from range
+            '12:30:45.+05:00',          # Empty fraction before offset
+            '12:30:45.-05:00',          # Empty fraction before offset
+            '12:30:45.Z',               # Empty fraction before Z
+            '12:30:45,+05:00',          # Empty fraction (comma) before offset
         ]
 
         for bad_str in bad_strs:

Misc/NEWS.d/next/Library/2026-06-25-07-08-17.gh-issue-152157.dt5Ef0.rst

@@ -0,0 +1,5 @@
+:meth:`~datetime.datetime.fromisoformat` and :meth:`~datetime.time.fromisoformat`
+now reject a decimal separator (``.`` or ``,``) that is not followed by any
+fractional digit before a timezone designator, such as ``'12:34:56.+05:00'``.
+The C implementation previously accepted such strings while the pure-Python
+implementation correctly raised :exc:`ValueError`.

Modules/_datetimemodule.c

@@ -1034,20 +1034,23 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
             has_separator = (c == ':');
         }
 
-        if (p >= p_end) {
+        if (c == '.' || c == ',') {
+            if (i < 2) {
+                return -3; // Decimal mark on hour or minute
+            }
+            if (p >= p_end) {
+                return -3;  // Decimal mark not followed by any digit
+            }
+            break;
+        }
+        else if (p >= p_end) {
             return c != '\0';
         }
         else if (has_separator && (c == ':')) {
             if (i == 2) {
                 return -4;  // Malformed microsecond separator
             }
             continue;
-        }
-        else if (c == '.' || c == ',') {
-            if (i < 2) {
-                return -3; // Decimal mark on hour or minute
-            }
-            break;
         } else if (!has_separator) {
             --p;
         } else {