From: tonghuaroot (童话) <tonghuaroot@gmail.com>
Date: Wed, 1 Jul 2026 18:47:57 +0000 (+0800)
Subject: [3.13] gh-152060: Fix `_pydatetime.fromisoformat()` raising `AssertionError` on inval... 
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=f05bfc3685a803e41dafcdfe1cdf0f33136c6db2;p=thirdparty%2FPython%2Fcpython.git

[3.13] gh-152060: Fix `_pydatetime.fromisoformat()` raising `AssertionError` on invalid lengths (GH-152061) (#152787)

Co-authored-by: Stan Ulbrych <stan@python.org>
---

diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index 38e1f764f008..1135c1d1a1bc 100644
--- a/Lib/_pydatetime.py
+++ b/Lib/_pydatetime.py
@@ -335,7 +335,8 @@ 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 3e7aebe8ecf9..4c5d03700e15 100644
--- a/Lib/test/datetimetester.py
+++ b/Lib/test/datetimetester.py
@@ -3395,6 +3395,7 @@ class TestDateTime(TestDate):
             '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)
+            '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 000000000000..6088a6b80829
--- /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.