bpo-44630: Fix assertion errors in csv module (GH-27127)

Fix incorrect handling of exceptions when interpreting dialect objects in
the csv module. Not clearing exceptions between calls to
PyObject_GetAttrString() causes assertion failures in pydebug mode (or with
assertions enabled).

Add a minimal test that would've caught this (passing None as dialect, or
any object that isn't a csv.Dialect subclass, which the csv module allows
and caters to, even though it is not documented.) In pydebug mode, the test
triggers the assertion failure in the old code.

Contributed-By: T. Wouters [Google]
(cherry picked from commit 0093876328afa330224c9d887c18dee0b3117852)

Co-authored-by: T. Wouters <thomas@python.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 2dab17d..89fec4b 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -442,9 +442,15 @@ class TestDialectRegistry(unittest.TestCase):
         class testUni(csv.excel):
             delimiter = "\u039B"
 
+        class unspecified():
+            # A class to pass as dialect but with no dialect attributes.
+            pass
+
         csv.register_dialect('testC', testC)
         try:
             self.compare_dialect_123("1,2,3\r\n")
+            self.compare_dialect_123("1,2,3\r\n", dialect=None)
+            self.compare_dialect_123("1,2,3\r\n", dialect=unspecified)
             self.compare_dialect_123("1\t2\t3\r\n", testA)
             self.compare_dialect_123("1:2:3\r\n", dialect=testB())
             self.compare_dialect_123("1|2|3\r\n", dialect='testC')