json.loads ignores explicitly supplied false-valued parse_float callables

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: json.loads docstring: "parse_float, if specified, will be called with the string of every JSON float to be decoded."

Expected: Call hook('1.5') exactly once and return ''.

Actual: Returns 1.5 without calling the hook.

import json, math, re

class FalseBool:
    def __init__(self):
        self.calls = []
    def __bool__(self):
        return False
    def __call__(self, text):
        self.calls.append(text)
        return "<sentinel>"

document, hook = "1.5", FalseBool()
if not (re.fullmatch(r"-?(?:0|[1-9][0-9]*)\.[0-9]+", document)
        and math.isfinite(float(document)) and callable(hook) and not bool(hook)):
    print("REFUTATION REJECTED:", "invalid input or hook")
else:
    expected = {"result": "<sentinel>", "calls": [document]}
    try:
        result = json.loads(document, parse_float=hook)
        actual = {"result": result, "calls": hook.calls}
    except Exception as error:
        actual = {"exception": repr(error), "calls": hook.calls}
    if actual != expected:
        print("REFUTATION CONFIRMED:",
              {"document": document, "parse_float": "FalseBool"},
              "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED:", "actual matches documented expectation")

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library json:

REFUTATION CONFIRMED: {'document': '1.5', 'parse_float': 'FalseBool'} actual: {'result': 1.5, 'calls': []} expected: {'result': '<sentinel>', 'calls': ['1.5']}

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/json-20261003-051249-c4

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).