#17572: Avoid chained exceptions while passing bad directives to...

#17572: Avoid chained exceptions while passing bad directives to time.strptime().  Initial patch by Claudiu Popa.

Lib/_strptime.py

@@ -326,7 +326,7 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
                     bad_directive = "%"
                 del err
                 raise ValueError("'%s' is a bad directive in format '%s'" %
-                                    (bad_directive, format))
+                                    (bad_directive, format)) from None
             # IndexError only occurs when the format string is "%"
             except IndexError:
                 raise ValueError("stray %% in format '%s'" % format)

Lib/test/test_strptime.py

@@ -218,6 +218,12 @@ class StrptimeTests(unittest.TestCase):
             else:
                 self.fail("'%s' did not raise ValueError" % bad_format)
 
+    def test_strptime_exception_context(self):
+        # check that this doesn't chain exceptions needlessly (see #17572)
+        with self.assertRaises(ValueError) as e:
+            _strptime._strptime_time('', '%D')
+        self.assertIs(e.exception.__suppress_context__, True)
+
     def test_unconverteddata(self):
         # Check ValueError is raised when there is unconverted data
         self.assertRaises(ValueError, _strptime._strptime_time, "10 12", "%m")

Lib/test/test_time.py

@@ -193,6 +193,12 @@ class TimeTestCase(unittest.TestCase):
         self.assertRaises(TypeError, time.strptime, b'2009', "%Y")
         self.assertRaises(TypeError, time.strptime, '2009', b'%Y')
 
+    def test_strptime_exception_context(self):
+        # check that this doesn't chain exceptions needlessly (see #17572)
+        with self.assertRaises(ValueError) as e:
+            time.strptime('', '%D')
+        self.assertIs(e.exception.__suppress_context__, True)
+
     def test_asctime(self):
         time.asctime(time.gmtime(self.t))
 

Misc/NEWS

@@ -21,6 +21,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #17572: Avoid chained exceptions while passing bad directives to
+  time.strptime().  Initial patch by Claudiu Popa.
+
 - Issue #14254: IDLE now handles readline correctly across shell restarts.
 
 - Issue #17614: IDLE no longer raises exception when quickly closing a file.