From 8146fa4727314cb32af5eb53bb4f4a3fa8c6fe26 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Wed, 24 Jun 2026 16:10:54 +0200
Subject: [PATCH] [3.15] gh-152060: Fix `_pydatetime.fromisoformat()` raising
 `AssertionError` on invalid lengths (GH-152061) (#152081)
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(cherry picked from commit ff781d52d451db56e154aac35ae7f2c41b1695a4)

Co-authored-by: tonghuaroot (童话) <tonghuaroot@gmail.com>
---
 Lib/_pydatetime.py                                             | 3 ++-
 Lib/test/datetimetester.py                                     | 1 +
 .../Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst     | 3 +++
 3 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst

diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index c1448374402..db4ea8d30c7 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -358,7 +358,8 @@ def _find_isoformat_datetime_separator(dtstr):
 def _parse_isoformat_date(dtstr):
     # It is assumed that this is an ASCII-only string of lengths 7, 8 or 10,
     # see the comment on Modules/_datetimemodule.c:_find_isoformat_datetime_separator
-    assert len(dtstr) in (7, 8, 10)
+    if len(dtstr) not in (7, 8, 10):
+        raise ValueError("Invalid isoformat string")
     year = int(dtstr[0:4])
     has_sep = dtstr[4] == '-'
 
diff --git a/Lib/test/datetimetester.py b/Lib/test/datetimetester.py
index 1cbe78c1ecb..e29f5e3ecb5 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -3757,6 +3757,7 @@ def test_fromisoformat_fails_datetime(self):
             '2009-04-19T12:30:45+00:00:90', # Time zone field out from range
             '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
         ]
 
         for bad_str in bad_strs:
diff --git a/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst b/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst
new file mode 100644
index 00000000000..6088a6b8082
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-24-10-46-35.gh-issue-152060.f2asrt.rst
@@ -0,0 +1,3 @@
+Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
+instead of :exc:`ValueError` for some malformed strings in the pure-Python
+implementation, matching the C implementation.