Looking through the Wiki documentation on Data Types, I noticed that the SQL_GUID type is being converted to/from a string, instead of using the native UUID type provided by Python. The SQL_GUID type applies to the uniqueidentifer column type, which pyodbc appears to be testing against.

These tests don't appear to have changed since 2008 (added in c3f6b46), and there is a comment for the test that alludes to Python not having a UUID type. This was probably true back in 2008, but Python has come a long way since then and now it does have a UUID type.