gh-150816: Speed up inspect.signature() for Python functions

[eendebakpt]

For a Python function, _signature_from_function() builds every Parameter through the public constructor, which validates the kind, the default and the name. This PR adds a private Parameter._from_code() that fills the slots directly and checks only the name.

This picks up #150823 and addresses its review comments:

• The fast path is used only when Signature._parameter_cls is Parameter itself. (subclasses tested in test_signature_parameter_cls_subclass).
• Parameter names are checked for every object, and function-like objects keep using the regular constructor, so invalid names still raise (new test test_signature_functionlike_invalid_names).

Benchmark	main	PR	Speedup
function, 7 params	22.95 µs	14.19 µs	1.62x
function, 1 param	8.18 µs	7.23 µs	1.13x
bound method	25.03 µs	21.03 µs	1.19x
671 stdlib functions	9.73 ms	7.36 ms	1.32x

Benchmark script

Windows x64 release build. Base and PR Lib/inspect.py were swapped via PYTHONPATH on the same interpreter. Each figure is the best of 3 interleaved runs, each the minimum of 15 repeats.

import inspect
import timeit


def f7(a, axis=None, dtype=None, out=None, keepdims=False, *, initial=None, where=True):
    pass


def f1(x):
    pass


class C:
    def method(self, a, b=1):
        pass


def stdlib_functions():
    import argparse, configparser, email.message, http.client, json, logging, subprocess
    funcs = []
    for mod in (argparse, configparser, email.message, http.client, json, logging, subprocess):
        for obj in vars(mod).values():
            if inspect.isfunction(obj):
                funcs.append(obj)
            elif inspect.isclass(obj):
                funcs.extend(v for v in vars(obj).values() if inspect.isfunction(v))
    return funcs


funcs = stdlib_functions()
bound = C().method
cases = {
    "function, 7 params": (lambda: inspect.signature(f7), 20_000),
    "function, 1 param": (lambda: inspect.signature(f1), 20_000),
    "bound method": (lambda: inspect.signature(bound), 20_000),
    f"{len(funcs)} stdlib functions": (lambda: [inspect.signature(f) for f in funcs], 50),
}
for name, (stmt, number) in cases.items():
    t = min(timeit.repeat(stmt, number=number, repeat=15)) / number
    unit, scale = ("ms", 1e3) if t > 1e-3 else ("us", 1e6)
    print(f"{name:28s} {t * scale:8.2f} {unit}")

Generated with Claude Code

• Issue: Speed up inspect.signature() for Python functions #150816