GeezerVison

This alias is only used in _Database.__setitem__() as far as I can see (unless the definition is imported elsewhere).

value is parsed using s#, so I think this should be ReadOnlyBuffer:

https://github.com/python/cpython/blob/c1c3be0f9dc414bfae9a5718451ca217751ac687/Modules/_dbmmodule.c#L196

Alternatively, if we want to use the same type for all database types (which is reasonable), I think we should define _ValueType centrally and reuse it.

It's a bit more complicated than that. The C code you point to is for dbm.ndbm. The class at issue in this file, dbm._Database, doesn't exist at runtime. We use it as the return type for dbm.open, which can return an instance of any of the database classes in the dbm package. I think the intent is that dbm._Database reflects the common interface of all three. Maybe we should use a Union return type instead?

I think for now I'll switch it to str | bytes, which are the types that all three dbm implementations support. (dbm.dumb is str | bytes | bytearray, the other two are str | ReadOnlyBuffer).