Description

EventSerializer._serialize_node_impl's Mapping branch copied the object in full (dict(obj.items())) before trimming it down to MAX_DATABAG_BREADTH (10 by default) items. That copy is O(len(obj)), even though at most MAX_DATABAG_BREADTH pairs ever get serialized.

This mostly doesn't matter for normal extra/tags payloads, but it's a real problem for local-variable capture (include_local_variables, on by default): frame locals are whatever a function happens to hold, and it's common for self in an instance method to reference a large in-memory structure (cache, lookup table, etc). We have big caches like this in production, and capturing one as a frame local cost hundreds of milliseconds to close to a second - with a burst of such exceptions, our whole process stalled for around 30 seconds.

The fix bounds the copy to remaining_breadth + 1 pairs via itertools.islice, falling back to a full copy only when breadth is unbounded (None/float("inf"), since islice doesn't accept a float stop value). Behavior and output are unchanged - only the cost of the copy changes.

Issues

• resolves: serialize() copies large Mapping locals in full before trimming to MAX_DATABAG_BREADTH #6773
• resolves: PY-2600