gh-118218: Reuse return tuple in itertools.pairwise

[hauntsaninja]

As per the comment, this is shamelessly based off of enumobject.c

With this change:

b1(1)            min=152.8us mean=155.5us ± 3.8us (25 repeats, 1000 loops)
b2(1)            min=149.0us mean=159.1us ± 8.8us (25 repeats, 1000 loops)
b3(1)            min=232.6us mean=242.5us ± 11.7us (25 repeats, 1000 loops)
b1(10)           min=279.2us mean=296.9us ± 16.6us (25 repeats, 1000 loops)
b2(10)           min=249.5us mean=259.2us ± 12.2us (25 repeats, 1000 loops)
b3(10)           min=386.6us mean=398.8us ± 10.1us (25 repeats, 1000 loops)
b1(1000)         min=20.3ms mean=20.7ms ± 0.5ms (25 repeats, 1000 loops)
b2(1000)         min=16.7ms mean=17.1ms ± 0.2ms (25 repeats, 1000 loops)
b3(1000)         min=26.0ms mean=26.2ms ± 0.3ms (25 repeats, 1000 loops)

Without this change:

b1(1)            min=142.2us mean=143.0us ± 0.9us (25 repeats, 1000 loops)
b2(1)            min=142.7us mean=143.3us ± 1.0us (25 repeats, 1000 loops)
b3(1)            min=219.8us mean=227.2us ± 4.4us (25 repeats, 1000 loops)
b1(10)           min=314.2us mean=323.8us ± 4.1us (25 repeats, 1000 loops)
b2(10)           min=335.4us mean=341.8us ± 5.1us (25 repeats, 1000 loops)
b3(10)           min=362.0us mean=386.2us ± 14.9us (25 repeats, 1000 loops)
b1(1000)         min=26.5ms mean=27.3ms ± 0.3ms (25 repeats, 1000 loops)
b2(1000)         min=29.8ms mean=30.2ms ± 0.2ms (25 repeats, 1000 loops)
b3(1000)         min=26.0ms mean=26.5ms ± 0.4ms (25 repeats, 1000 loops)

Benchmarking script:

Details

import statistics
import time
import timeit
from itertools import pairwise

def b1(n):
    for x in pairwise(range(n)):
        pass

def b2(n):
    for a, b in pairwise(range(n)):
        pass

def b3(n):
    list(pairwise(range(n)))


def format_time(t):
    if t >= 1e9:
        return f'{t/1e9:.1f}s'
    if t >= 1e6:
        return f'{t/1e6:.1f}ms'
    if t >= 1e3:
        return f'{t/1e3:.1f}us'
    return f'{t:.1f}ns'


def format_mean_stdev(ts):
    mean = statistics.mean(ts)
    stdev = statistics.stdev(ts)
    if mean >= 1e9:
        return f'{mean/1e9:.1f}s ± {stdev/1e9:.1f}s'
    if mean >= 1e6:
        return f'{mean/1e6:.1f}ms ± {stdev/1e6:.1f}ms'
    if mean >= 1e3:
        return f'{mean/1e3:.1f}us ± {stdev/1e3:.1f}us'
    return f'{mean:.1f}ns ± {stdev:.1f}ns'


def bench(stmt):
    repeat = 25
    number = 1000
    timings = timeit.repeat(
        stmt, globals=globals(), repeat=repeat, number=number, timer=time.perf_counter_ns
    )
    print(
        f'{stmt:16} '
        f'min={format_time(min(timings))} '
        f'mean={format_mean_stdev(timings)} '
        f'({repeat} repeats, {number} loops)'
    )

bench('b1(1)')
bench('b2(1)')
bench('b3(1)')

bench('b1(10)')
bench('b2(10)')
bench('b3(10)')

bench('b1(1000)')
bench('b2(1000)')
bench('b3(1000)')

• Issue: Speed up itertools.pairwise #118218