gh-155175: Reject fractional seconds without a decimal mark in C fromisoformat

Author: SEPURI-SAI-KRISHNA

Lib/test/datetimetester.py

@@ -3762,6 +3762,11 @@ def test_fromisoformat_fails_datetime(self):
             '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
+            '2009-04-19T12304578',          # Fraction without a decimal mark
+            '2009-04-19T123045789',         # Fraction without a decimal mark
+            '2009-04-19T123045123456789',   # Fraction without a decimal mark
+            '2009-04-19T123045+00000000',   # Offset fraction without decimal mark
+            '2009-04-19T12:30:45+00000000', # Offset fraction without decimal mark
         ]
 
         for bad_str in bad_strs:
@@ -5042,6 +5047,11 @@ def test_fromisoformat_fails(self):
             '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
+            '12304578',                 # Fraction without a decimal mark
+            '123045789',                # Fraction without a decimal mark
+            '123045123456789',          # Fraction without a decimal mark
+            '123045+00000000',          # Offset fraction without decimal mark
+            '12:30:45+00000000',        # Offset fraction without decimal mark
         ]
 
         for bad_str in bad_strs:

Misc/NEWS.d/next/Library/2026-08-04-19-40-00.gh-issue-155175.Kf3xQ1.rst

@@ -0,0 +1,5 @@
+Fix :meth:`datetime.time.fromisoformat` and
+:meth:`datetime.datetime.fromisoformat` in the C implementation accepting a
+fractional seconds component that is not introduced by a decimal mark, such
+as ``'12345678'`` (previously parsed as ``12:34:56.780000``).  Such strings
+are now rejected, matching the pure-Python implementation.

Modules/_datetimemodule.c

@@ -1021,6 +1021,7 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
     int *vals[3] = {hour, minute, second};
     // This is initialized to satisfy an erroneous compiler warning.
     unsigned char has_separator = 1;
+    unsigned char has_fraction = 0;
 
     // Parse [HH[:?MM[:?SS]]]
     for (size_t i = 0; i < 3; ++i) {
@@ -1041,6 +1042,7 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
             if (p >= p_end) {
                 return -3;  // Decimal mark not followed by any digit
             }
+            has_fraction = 1;
             break;
         }
         else if (p >= p_end) {
@@ -1060,6 +1062,14 @@ parse_hh_mm_ss_ff(const char *tstr, const char *tstr_end, int *hour,
         }
     }
 
+    // A fractional component must be introduced by a decimal mark.  Falling
+    // out of the loop above without having seen one means the basic format
+    // left unconsumed characters after SS (e.g. "12345678"), which would
+    // otherwise be silently parsed as a fraction.
+    if (!has_fraction) {
+        return -4;  // Malformed microsecond separator
+    }
+
     // Parse fractional components
     size_t len_remains = p_end - p;
     size_t to_parse = len_remains;