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When bytes(x), bytes_new checks if x is integer via PyNumber_AsSize_t(x).
It cause TypeError internally.
When x is not an integer, especially bytearray or memoryview, the internal
exception cause significant overhead.
# HEAD
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.696 usec per loop
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.699 usec per loop
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.701 usec per loop
# this patch
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.265 usec per loop
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.265 usec per loop
$ ./python -m timeit -s 'data=bytearray(b"xyz")' 'bytes(data)'
1000000 loops, best of 3: 0.267 usec per loop

the patch seems to be fine.

bytearray suffers from the same issue. It would be nice to optimize it too.

Hi Serhiy
Thank you for your feedback.
Stephane

Thanks for comments.

LGTM.

New changeset 789a42401009 by Serhiy Storchaka in branch 'default':
Issue #27704: Optimized creating bytes and bytearray from byte-like objects
https://hg.python.org/cpython/rev/789a42401009 

Thank you for your contribution Naoki.