(improvement) Skip tuple(partial(...)) construction for empty callbacks/errbacks (190ns saving per call - 63%)

[mykaul]

Summary

In _set_final_result and _set_final_exception, skip building the tuple(partial(fn, ...)) when the callbacks/errbacks list is empty. Most queries use the synchronous result() path and never register callbacks or errbacks.

Motivation

Every query completion (_set_final_result / _set_final_exception) previously constructed a tuple(partial(fn, response, *args, **kwargs) for ...) from the callbacks/errbacks list, even when that list is empty. This creates an empty generator and an empty tuple on every query. The synchronous result() API (the overwhelmingly common usage pattern) never registers callbacks, so this work is wasted.

Benchmark (CPython 3.14, per-call)

Callback state	Original	Optimized	Savings per call
Empty (synchronous result())	304ns	114ns	190ns (63%)
1 callback (async path)	562ns	560ns	2ns (no regression)

The synchronous result() API is the common path — this saves 190ns per query completion.

Changes

• cassandra/cluster.py:
  • _set_final_result: Check if callbacks: before building to_call; set to_call = None otherwise
  • _set_final_exception: Same pattern for errbacks
  • Guard the application loop with if to_call: check

Testing

Unit tests pass (58/58 in test_response_future.py and test_cluster.py). The lock semantics are preserved — the check and tuple construction still happen inside _callback_lock.

[mykaul]

Benchmark results (CPython 3.14, 500k iterations)

Callback state	Original	Optimized	Δ per call
Empty (synchronous result())	304ns	114ns	-190ns
1 callback (async path)	562ns	560ns	-2ns (no regression)

The synchronous result() API is the overwhelmingly common usage pattern and never registers callbacks. This skips the tuple(partial(fn, ...) for ...) generator + empty tuple construction on every query completion.