Issue #19545: Avoid chained exceptions while passing stray % to

time.strptime().  Initial patch by Claudiu Popa.

Lib/_strptime.py

@@ -329,7 +329,7 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
                                     (bad_directive, format)) from None
             # IndexError only occurs when the format string is "%"
             except IndexError:
-                raise ValueError("stray %% in format '%s'" % format)
+                raise ValueError("stray %% in format '%s'" % format) from None
             _regex_cache[format] = format_regex
     found = format_regex.match(data_string)
     if not found:

Lib/test/test_strptime.py

@@ -223,6 +223,10 @@ class StrptimeTests(unittest.TestCase):
         with self.assertRaises(ValueError) as e:
             _strptime._strptime_time('', '%D')
         self.assertIs(e.exception.__suppress_context__, True)
+        # additional check for IndexError branch (issue #19545)
+        with self.assertRaises(ValueError) as e:
+            _strptime._strptime_time('19', '%Y %')
+        self.assertIs(e.exception.__suppress_context__, True)
 
     def test_unconverteddata(self):
         # Check ValueError is raised when there is unconverted data

Lib/test/test_time.py

@@ -198,6 +198,10 @@ class TimeTestCase(unittest.TestCase):
         with self.assertRaises(ValueError) as e:
             time.strptime('', '%D')
         self.assertIs(e.exception.__suppress_context__, True)
+        # additional check for IndexError branch (issue #19545)
+        with self.assertRaises(ValueError) as e:
+            time.strptime('19', '%Y %')
+        self.assertIs(e.exception.__suppress_context__, True)
 
     def test_asctime(self):
         time.asctime(time.gmtime(self.t))

Misc/NEWS

@@ -68,6 +68,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #19545: Avoid chained exceptions while passing stray % to
+  time.strptime().  Initial patch by Claudiu Popa.
+
 - Issue #3158: doctest can now find doctests in functions and methods
   written in C.