gh-158500: Reject trailing characters in date.fromisoformat()

Lib/_pydatetime.py

@@ -396,6 +396,10 @@ def _parse_isoformat_date(dtstr):
             pos += has_sep
 
             dayno = _read_isoformat_component(dtstr[pos:pos + 1], 1)
+            pos += 1
+
+        if pos != len(dtstr):
+            raise ValueError("Invalid isoformat string")
 
         return list(_isoweek_to_gregorian(year, weekno, dayno))
     else:
@@ -406,6 +410,10 @@ def _parse_isoformat_date(dtstr):
 
         pos += has_sep
         day = _read_isoformat_component(dtstr[pos:pos + 2], 2)
+        pos += 2
+
+        if pos != len(dtstr):
+            raise ValueError("Invalid isoformat string")
 
         return [year, month, day]
 
@@ -1984,7 +1992,7 @@ def fromisoformat(cls, date_string):
             raise ValueError(
                 f'Invalid isoformat string: {date_string!r}') from None
 
-        if tstr:
+        if separator_location < len(date_string):
             try:
                 (time_components,
                  became_next_day,

Lib/test/datetimetester.py

@@ -2236,6 +2236,10 @@ def test_fromisoformat_fails(self):
             '2020-W 5',         # space in the week number
             '2020061',          # 7 chars: day slice reads a 1-character tail
             '2020-W2',          # 1-digit week number
+            # gh-158500: trailing characters must not be ignored
+            '2020010112',       # YYYYMMDD followed by two characters
+            '2020W011xx',       # YYYYWwwD followed by two characters
+            '20200101\u00e9',   # 9 characters, but 10 bytes in UTF-8
             '٢025-03-09',       # Unicode characters
             '2009\ud80002\ud80028',     # Separators are surrogate codepoints
         ]
@@ -3977,6 +3981,11 @@ 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
+            # gh-158500: a separator must be followed by a time
+            '2009-04-19T',                  # Separator without a time
+            '20090419T',                    # Separator without a time
+            '2009-W16-7T',                  # Separator without a time
+            '2009W167T',                    # Separator without a time
         ]
 
         for bad_str in bad_strs:

Misc/NEWS.d/next/Library/2026-10-26-21-37-42.gh-issue-158500.zxyuq5.rst

@@ -0,0 +1,5 @@
+:meth:`datetime.date.fromisoformat` now raises :exc:`ValueError` for a
+basic-format date followed by extra characters, such as ``'2020010112'``,
+instead of silently ignoring them. The pure-Python implementation of
+:meth:`datetime.datetime.fromisoformat` now also rejects a date followed by a
+single character, such as ``'2020-01-01T'``, matching the C implementation.

Modules/_datetimemodule.c

@@ -977,6 +977,10 @@ parse_isoformat_date(const char *dtstr, const size_t len, int *year, int *month,
             iso_day = 1;
         }
 
+        if ((size_t)(p - dtstr) != len) {
+            return -1;
+        }
+
         int rv = iso_to_ymd(*year, iso_week, iso_day, year, month, day);
         if (rv) {
             return -3 + rv;
@@ -997,6 +1001,9 @@ parse_isoformat_date(const char *dtstr, const size_t len, int *year, int *month,
     if (p == NULL) {
         return -1;
     }
+    if ((size_t)(p - dtstr) != len) {
+        return -1;
+    }
     return 0;
 }