The csv module has some incorrect exception handling when dealing with dialect objects that are not csv.Dialect subclasses (or that otherwise raise errors when accessing the dialect attributes):

>>> csv.reader([], dialect=None)
python: ../../cpython/Objects/typeobject.c:3820: _PyType_Lookup: Assertion `!PyErr_Occurred()' failed.
Aborted

The problem is Modules/_csv.c tries to cater to dialects that lack the attributes it wants to access, but does so by leaving exceptions set between calls to PyObject_SetAttrString(). Since 3.7, that causes assertion failures. (I have a PR with a fix.)

New changeset 0093876328afa330224c9d887c18dee0b3117852 by T. Wouters in branch 'main':
bpo-44630: Fix assertion errors in csv module (GH-27127)
https://github.com/python/cpython/commit/0093876328afa330224c9d887c18dee0b3117852

New changeset bb260c2a212aee2a7d7a32bf5208fd554bf72713 by Miss Islington (bot) in branch '3.10':
[3.10] bpo-44630: Fix assertion errors in csv module (GH-27127) (GH-27129)
https://github.com/python/cpython/commit/bb260c2a212aee2a7d7a32bf5208fd554bf72713

New changeset fe73509c5ba3844b45a2253967522531ab80c849 by Miss Islington (bot) in branch '3.9':
bpo-44630: Fix assertion errors in csv module (GH-27127)
https://github.com/python/cpython/commit/fe73509c5ba3844b45a2253967522531ab80c849

Thanks!

Things like this both internal to CPython and in CPython API misuse in all sorts of third party extension modules are why i think we as a project really should to encourage more CI systems to have -UNDEBUG builds of CPython available and in use by default when people request Python...

To answer anyone's "why no news entry?" questions... This only shows up in -UNDEBUG (and thus pydebug) builds of CPython.  Which basically nobody uses.

We, Google, keep C assertions enabled by undefining NDEBUG in our default non-release builds that everything by default runs tests against as a way to help improve code quality.