GH-112361: Speed up pathlib by removing some temporary objects.

[barneygale]

Construct only one new list object (using list.copy()) when creating a new path object with a modified tail. This slightly speeds up with_name(), with_suffix(), _make_child_relpath() (used in walking and globbing), and glob().

• Issue: Speed up pathlib by removing a few temporary objects #112361

[barneygale]

Timings:

$ ./python -m timeit -n 1000000 -s "from pathlib import Path; p = Path('foo')" "p.with_name('bar')"
1000000 loops, best of 5: 1.44 usec per loop  # before
1000000 loops, best of 5: 1.31 usec per loop  # after

$ ./python -m timeit -n 1000000 -s "from pathlib import Path; p = Path('foo.txt')" "p.with_suffix('.zip')"
1000000 loops, best of 5: 2.02 usec per loop  # before
1000000 loops, best of 5: 1.83 usec per loop  # after

$ ./python -m timeit -n 1000000 -s "from pathlib import Path; p = Path('foo.txt')" "p.with_suffix('')"
1000000 loops, best of 5: 1.88 usec per loop  # before
1000000 loops, best of 5: 1.68 usec per loop  # after

$ ./python -m timeit -n 1000000 -s "from pathlib import Path; p = Path('foo')" "p._make_child_relpath('bar')"
1000000 loops, best of 5: 1.14 usec per loop  # before
1000000 loops, best of 5: 1.11 usec per loop  # after

[AlexWaygood]

Overall this looks really nice -- great work! I can reproduce the big speedups from the first three benchmarks in #112362 (comment).

I can't repro the reported speedup for _make_child_relpath, though (the fourth benchmark from #112362 (comment)) -- if anything, it looks like this PR makes it slightly slower for me locally? (I'm on Windows with a fresh PGO-optimised build from main.)

[barneygale]

I can't repro the reported speedup for _make_child_relpath, though (the fourth benchmark from #112362 (comment)) -- if anything, it looks like this PR makes it slightly slower for me locally? (I'm on Windows with a fresh PGO-optimised build from main.)

Hm. With a fresh PGO rebuild on Linux I still see an improvement, but it's really marginal - only 2% faster. I'll take the change out of the PR.

[AlexWaygood]

Looks great, thanks!

[barneygale]

Thank you for the review Alex!