python
@@ -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] == '-'
 

@@ -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
@@ -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.