gh-156241: Add a no-escape fast path to the _json str escapers

[eendebakpt]

ascii_escape_unicode() and escape_unicode() now return the input bulk-copied with surrounding quotes when nothing needs escaping, via a shared quote_unescaped_unicode() helper, like the writer-based write_escaped_ascii()/write_escaped_unicode() already do.

Benchmark	main	PR
dumps ascii 10	342 ns	349 ns: ~same
dumps ascii 100	508 ns	442 ns: 1.15x faster
dumps ascii 1 kB	1.73 µs	1.19 µs: 1.45x faster
dumps ascii 16 kB	21.4 µs	13.2 µs: 1.63x faster
dumps unicode 1 kB, ensure_ascii=False	3.04 µs	1.80 µs: 1.69x faster
dump list of 100 strings	23.2 µs	21.1 µs: 1.10x faster
dumps escapes 100	531 ns	520 ns: ~same
dumps list of 3 short strings	1.30 µs	1.29 µs: ~same

Benchmark script

"""Benchmark for the _json str-escaper no-escape fast path.

json.dumps(str) uses encode_basestring[_ascii] directly; json.dump() uses
it for every encoded string; dumps() of a container uses the writer-based
escapers (unchanged).
"""
import functools
import io
import json

import pyperf

CASES = {
    "ascii 10": "callsign-7",
    "ascii 100": "The quick brown fox jumps over the lazy dog. " * 2 + "0123456789",
    "ascii 1k": "abcdefghij" * 100,
    "ascii 16k": "abcdefghij" * 1600,
    "escapes 100": ('line\n"quoted"\ttab\\ ' * 5)[:100],
    "unicode 1k": "héllo wörld unicode … " * 47,
}


def dumps_obj(o, **kw):
    json.dumps(o, **kw)


if __name__ == "__main__":
    runner = pyperf.Runner()
    for name, s in CASES.items():
        runner.bench_func("dumps %s" % name, dumps_obj, s)
    runner.bench_func("dumps unicode 1k noascii",
                      functools.partial(dumps_obj, CASES["unicode 1k"],
                                        ensure_ascii=False))
    runner.bench_func("dumps list of 3 short strings", dumps_obj,
                      ["alpha-01", "beta-002", "gamma-03"])

    def dump_obj(o):
        json.dump(o, io.StringIO())
    runner.bench_func("dump list of 3 short strings", dump_obj,
                      ["alpha-01", "beta-002", "gamma-03"])
    runner.bench_func("dump list of 100 strings", dump_obj,
                      ["string number %04d of the benchmark payload" % i
                       for i in range(100)])

Run with python bench_escape.py --fast -o out.json per build; compare with pyperf compare_to base.json branch.json --table.

• Issue: Improve performance of json.dumps and json.dump for strings #156241