Christopher Barker writes:
For example, a common confusion is to identify None with NULL in DB, ... because most frameworks do not distinguish "Do not filter" and "Keep only rows where value is NULL",
Good example— wouldn’t it be nice if all the database interaction packages used the same Sentinel for this? In that case it would go in the DB api, but the idea holds.
The question is whether it's an *example* of a common need, or a *nearly unique use case* of a generally useful distinction that justifies a new (semi-)singleton. Tal says that at least in the stdlib and the other cases he's seen it's almost always an application-specific need. We need to see a *list* of generally useful sentinel classes that aren't served by Tal's "_local_sentinel = Sentinel()" idiom. Otherwise such sentinels can be added ad hoc to specific (though widely used) modules as module-specific APIs. Steve