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In the review of my issue #27830 "Add _PyObject_FastCallKeywords(): pass keyword arguments as (key, value) pairs", Antoine Pitrou wrote about _PyStack_AsDict():
"It's a pity that you're doing two dict lookups per keyword argument.
Another strategy would be to check that `PyDict_Size() == nk` at the end, and if not, try to find the duplicate keyword."
https://bugs.python.org/review/27830/diff/18203/Objects/abstract.c#newcode2326_Objects/abstract.c:2326
In fact, I simply copied code from update_keyword_args(), helper function used by do_call() and ext_do_call() in Python/ceval.c.
This function is quite old, the last major change was made in 2001:
---
changeset: 15711:2481b5fe86d6
branch: legacy-trunk
user: Jeremy Hylton <jeremy@alum.mit.edu>
date: Wed Jan 03 23:52:36 2001 +0000
files: Python/ceval.c
description:
Revised implementation of CALL_FUNCTION and friends.
More revision still needed.
(...)
---
But this change just moved code. It looks like the real code was in fact added by this change made in 2000:
---
changeset: 12082:603db0d0aed1
branch: legacy-trunk
user: Jeremy Hylton <jeremy@alum.mit.edu>
date: Tue Mar 28 23:49:17 2000 +0000
files: Grammar/Grammar Include/opcode.h Lib/dis.py Misc/ACKS Python/ceval.c Python/compile.c Python/graminit.c
description:
slightly modified version of Greg Ewing's extended call syntax patch
executive summary:
Instead of typing 'apply(f, args, kwargs)' you can type 'f(*arg, **kwargs)'.
Some file-by-file details follow.
(...)
---

See also the issue #27358: "BUILD_MAP_UNPACK_WITH_CALL is slow.

_PyStack_AsDict() doesn't check anymore if keys are unique.
BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL opcodes now use PyDict_Update() to combine mappings and _PyDictView_Intersect() to check if keys are unique.
I don't know if there is still something to optimize. The opcode changed the method to C and Python functions (FASTCALL) changed a lot recently ;-)
I suggest to close the issue.