GH-111429: Speed up pathlib.PurePath.[is_]relative_to()

[barneygale]

Avoid unnecessary calls to with_segments(). This makes both is_relative_to() and relative_to() faster when passed a PurePath object, and makes relative_to() faster when passed another kind of path-like object (like a str).

Also, use _from_parsed_parts() in relative_to() to return a pre-parsed path. Operations like str(p.relative_to(q)) are faster as a result.

• Issue: Speed up pathlib.PurePath.[is_]relative_to() #111429

[Jason-Y-Z]

Thanks for the change! Overall LGTM. A small suggestion would be - would you mind doing a quick profiling of these 2 functions before/after the change please, just so that we can better understand the effect of the change?
Something like timeit might be helpful for that.

[barneygale]

The improvement depends on the type of the argument, number of segments in each path, and in the case of relative_to(), how the result is used. So take with a pinch of salt - the important bit is that some things are faster and nothing is slower:

$ ./python -m timeit \
    -s 'from pathlib import Path; p0 = Path("foo/bar"); p1 = Path("foo")' \
    'str(p0.relative_to(p1))'
10000 loops, best of 5: 20.3 usec per loop  # before
50000 loops, best of 5: 9.13 usec per loop  # after

$ ./python -m timeit \
    -s 'from pathlib import Path; p0 = Path("foo/bar"); p1 = "foo"' \
    'str(p0.relative_to(p1))'
10000 loops, best of 5: 20.4 usec per loop  # before
20000 loops, best of 5: 14.5 usec per loop  # after

$ ./python -m timeit \
    -s 'from pathlib import Path; p0 = Path("foo/bar"); p1 = Path("foo")' \
    'p0.is_relative_to(p1)'
50000 loops, best of 5: 9.01 usec per loop  # before
50000 loops, best of 5: 4.15 usec per loop  # after

$ ./python -m timeit \
    -s 'from pathlib import Path; p0 = Path("foo/bar"); p1 = "foo"' \
    'p0.is_relative_to(p1)'
50000 loops, best of 5: 9.04 usec per loop  # before
50000 loops, best of 5: 8.91 usec per loop  # after

[Jason-Y-Z]

@pitrou Sorry for tagging, but based on contributor history, would you mind giving a quick review?

[barneygale]

Thanks for reviewing @Jason-Y-Z!