It's trying to access a non-existing property:

return len(self._connections)

Please double-check the logic but maybe it should sum up the new properties like this:

return len(self._connections_free) + len(self._connections_used)

Referring to this line: https://github.com/mariadb-corporation/mariadb-connector-python/blob/1.1/mariadb/connectionpool.py#L292

Maybe also check if those lists are not None (because they are set to None in the "close" method).