gh-152157: Reject empty fractions in _datetime.{date}time.fromisoformat (GH-152161)

(cherry picked from commit 112c69a)

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

Author: tonghuaroot

Lib/test/datetimetester.py

@@ -3581,6 +3581,10 @@ def test_fromisoformat_fails_datetime(self):
             '2009-04-19T12:30:45.400 +02:30',  # Space between ms and timezone (gh-130959)
             '2009-04-19T12:30:45.400 ',        # Trailing space (gh-130959)
             '2009-04-19T12:30:45. 400',        # Space before fraction (gh-130959)
+            '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:
@@ -4837,6 +4841,10 @@ def test_fromisoformat_fails(self):
             '12:30:45.400 +02:30',      # Space between ms and timezone (gh-130959)
             '12:30:45.400 ',            # Trailing space (gh-130959)
             '12:30:45. 400',            # Space before fraction (gh-130959)
+            '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 +1,3 @@
+The C implementations of :meth:`~datetime.datetime.fromisoformat` and :meth:`~datetime.time.fromisoformat`
+now reject a decimal separator that is not followed by any
+fractional digit before a timezone designator.

Modules/_datetimemodule.c

@@ -1031,7 +1031,16 @@ 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 == ':')) {
@@ -1040,14 +1049,10 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
             }
             continue;
         }
-        else if (c == '.' || c == ',') {
-            if (i < 2) {
-                return -3; // Decimal mark on hour or minute
-            }
-            break;
-        } else if (!has_separator) {
+        else if (!has_separator) {
             --p;
-        } else {
+        }
+        else {
             return -4;  // Malformed time separator
         }
     }