Improve read_partial() with callables?

[jstorrs]

I've been using read_partial() a bit lately in order to implement some processing and have run into some frustrations with limitations on how the processing can be customized.

Basically I've been trying to use specific_tags and stop_when and I like stop_when but specific_tags gets a bit frustrating (particularly with private tags). It just feels like stop_when should have a callable counterpart that inspects tags and serves the same purpose as testing for values in specific_tags, but dynamically. Then more complex extraction logic could be put into that callable.

One simple example would be a callable that keeps track of private reservations and flags private tags to load correctly even if the private group has been moved to an unanticipated reservation. And also to only export the tag if the correct private group reservation is even present. That sort of thing.

Another idea is handling of items within specific sequences. This is another topic that I think a callable approach could work well with but I haven't looked at read_partial() sequences yet (I will need to soon when I start working with enhanced MR dataset). I'm not entirely sure how the specific_tags works (or even is intended to work) with sequences (DCMTK has flags for recursive vs non-recursive sequence processing and that may also be useful).

Any opinions about this approach? I'm happy to work on this, but just wanting to see what opinions or ideas others might have. Some variations of this would be a function that returns some sort of state "load", "skip", "terminate", "enter" (for sequences). Or a class that has methods for each of the "questions".