difflib ignores falsey callable junk predicates

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: The Python library documentation for difflib.ndiff describes linejunk as: "A function that accepts a single string argument, and returns true if the string is junk, or false if not."

Expected: Both calls return ['- A\n', ' B\n', '+ A\n'].

Actual: The falsey predicate returns ['+ B\n', ' A\n', '- B\n']; its lambda wrapper returns the expected result.

import difflib

class Predicate:
    def __call__(self, line):
        return line == "A\n"

    def __bool__(self):
        return False

a, b, p = ["A\n", "B\n"], ["B\n", "A\n"], Predicate()
data = {"a": a, "b": b,
        "p": {"junk_membership": ["A\n"], "__bool__": False}}

if not (callable(p) and all(
    isinstance(s, str) and type(p(s)) is bool and p(s) == p(s)
    for s in a + b
)):
    print("REFUTATION REJECTED: input violates the documented predicate contract")
else:
    actual = list(difflib.ndiff(a, b, linejunk=p))
    # Independent alignment: B is the sole nonjunk anchor; A moves past it.
    expected = ["- " + a[0], "  " + a[1], "+ " + b[1]]
    wrapped = list(difflib.ndiff(a, b, linejunk=lambda s: p(s)))
    if actual != expected and wrapped == expected:
        print("REFUTATION CONFIRMED:", data,
              "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED: no independently verified wrapper discrepancy",
              data, "actual:", actual, "expected:", expected, "wrapped:", wrapped)

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

REFUTATION CONFIRMED: {'a': ['A\n', 'B\n'], 'b': ['B\n', 'A\n'], 'p': {'junk_membership': ['A\n'], '__bool__': False}} actual: ['+ B\n', '  A\n', '- B\n'] expected: ['- A\n', '  B\n', '+ A\n']

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/difflib-20261003-011733-c2

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).