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As issue #17741 shows in detail, there's a number of implementation problems in the current ET that make a fully non-blocking parser more difficult to implement than it should be.
The main problem is that XMLParser._setevents relies on internal implementation details of the default TreeBuilder to do some of its tasks.
This has to be decoupled from implementation details completely. An XMLParser already has a non-blocking input side (feed/close) and a callback "push" API to the target. The API has to be fully expanded to include start-ns/end-ns and any other events that are required to allow custom tree builders as targets. This would make it possible to layer XMLPullParser on top of the stock XMLParser coupled with a special target that collects "events" from the callback calls.