[3.13] gh-152060: Fix `_pydatetime.fromisoformat()` raising `AssertionError` on inval...

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

diff --git a/Lib/_pydatetime.py b/Lib/_pydatetime.py
index 38e1f764f008a9087c14d40c7787173ae7a025f1..1135c1d1a1bc52fc2c5653252c96d124b4b4ae9e 100644 (file)
--- 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 3e7aebe8ecf9bfd19a859d22304de2c1c9e48329..4c5d03700e15b03e427f4ee0e1916525f9f856d8 100644 (file)
--- 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 (file)
index 0000000..6088a6b
--- /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.