Fix a sorting order in parameter-shift terms #5583
Merged
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Context:
Currently,
process_shifts
sorts shifts according to the absolute value of the shift.It uses
kind="stable"
, which guarantees to keep inputs in their input order if they have same absolute value.While this is stable against repeated calls, the stable order is somewhat arbitrarily depending on how a user/PennyLane
arrived at the input shifts.
This is visible in
tests/gradients/core/test_general_shift_rules.py
, where the expected output values sometimes have positive before negative shifts with same magnitude, and sometimes the other way around.Description of the Change:
This PR uses
lexsort
, which is likeargsort
but allows us to use the sign of the shift as a secondary sorting key, producing not only stable but also well-defined sorting results.Benefits:
Predictable sorting results beyond the absolute value alone.
Possible Drawbacks:
Related GitHub Issues:
[sc-45043]