gh-152060: Fix _pydatetime.fromisoformat() raising AssertionError on invalid lengths (#152061)

tonghuaroot · web-flow · commit ff781d52d451 · 2026-06-24T14:40:46.000+01:00
diff --git 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
@@ -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.

Original file line number	Diff line number	Diff line change
`@@ -3757,6 +3757,7 @@ def test_fromisoformat_fails_datetime(self):`
`3757`	`3757`	`'2009-04-19T12:30:45+00:00:90', # Time zone field out from range`
`3758`	`3758`	`'2009-04-19T12:30:45-00:90:00', # Time zone field out from range`
`3759`	`3759`	`'2009-04-19T12:30:45-00:00:90', # Time zone field out from range`
	`3760`	`+ '2020-2020', # Ambiguous 9-char date portion`
`3760`	`3761`	`]`
`3761`	`3762`
`3762`	`3763`	`for bad_str in bad_strs:`
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	+Fix :meth:`datetime.datetime.fromisoformat` raising :exc:`AssertionError`
	`2`	+instead of :exc:`ValueError` for some malformed strings in the pure-Python
	`3`	`+implementation, matching the C implementation.`