From 897999de04bdeea77a1f32b7d195ad9b1a0a5392 Mon Sep 17 00:00:00 2001
From: Rupayon Haldar <80724680+rupayon123@users.noreply.github.com>
Date: Thu, 1 Oct 2026 05:24:58 -0400
Subject: [PATCH 1/2] gh-158500: Reject trailing date characters

---
 Lib/_pydatetime.py                                         | 6 ++++++
 Lib/test/datetimetester.py                                 | 2 ++
 .../Library/2026-10-01-04-30-00.gh-issue-158500.a1b2c3.rst | 2 ++
 Modules/_datetimemodule.c                                  | 7 +++++++
 4 files changed, 17 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-01-04-30-00.gh-issue-158500.a1b2c3.rst

diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index 50126d039e830a2..68f6f5b76de6f95 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -389,6 +389,9 @@ 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:
@@ -399,6 +402,9 @@ 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]
 
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 716c662ad453f4a..bbd9e4f47db1f9a 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -2114,6 +2114,8 @@ 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
+            '2020010112',       # Trailing characters after a basic-format date
+            '2020W011xx',       # Trailing characters after a basic-format week date
             '٢025-03-09',       # Unicode characters
             '2009\ud80002\ud80028',     # Separators are surrogate codepoints
         ]
diff --git a/Misc/NEWS.d/next/Library/2026-10-01-04-30-00.gh-issue-158500.a1b2c3.rst b/Misc/NEWS.d/next/Library/2026-10-01-04-30-00.gh-issue-158500.a1b2c3.rst
new file mode 100644
index 000000000000000..cbf103e699124b5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-01-04-30-00.gh-issue-158500.a1b2c3.rst
@@ -0,0 +1,2 @@
+Fix :meth:`datetime.date.fromisoformat` accepting basic-format date strings
+with trailing characters.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index bd76b3bd81cce40..9a78a27f462cf36 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -991,6 +991,10 @@ 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;
@@ -1011,6 +1015,9 @@ 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;
 }
 

From 57801f4600f1fd890874bc2e852e81fe9851e4cb Mon Sep 17 00:00:00 2001
From: Rupayon Haldar <80724680+rupayon123@users.noreply.github.com>
Date: Thu, 1 Oct 2026 08:18:26 -0400
Subject: [PATCH 2/2] gh-158500: Cover multibyte trailing date data

---
 Lib/test/datetimetester.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index bbd9e4f47db1f9a..d76a1f7a4a6e38f 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -2119,6 +2119,9 @@ def test_fromisoformat_fails(self):
             '٢025-03-09',       # Unicode characters
             '2009\ud80002\ud80028',     # Separators are surrogate codepoints
         ]
+        if not issubclass(self.theclass, datetime):
+            # 9 characters, 10 UTF-8 bytes; exercise the C parser's 10-byte path.
+            bad_strs.append('20200101\u00e9')
 
         for bad_str in bad_strs:
             with self.assertRaises(ValueError):