ExtendedInterpolation drops get() vars overrides during recursive interpolation

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: RawConfigParser.get documentation in the module docstring: "Additional substitutions may be provided using the vars argument, which must be a dictionary whose contents override any pre-existing defaults."

Expected: override

Actual: original

import configparser

section, b, v = "s", "original", "override"
options = {"x": "${y}", "y": "${z}", "z": b}
overrides = {"z": v}
data = {"section": section, "b": b, "v": v, "vars": overrides}

p = configparser.ConfigParser(interpolation=configparser.ExtendedInterpolation())
if section == p.default_section or not isinstance(overrides, dict) or any(
    not isinstance(s, str) or "$" in s for s in (b, v)
):
    print("REFUTATION REJECTED:", "invalid input")
else:
    p.read_dict({section: options})
    actual = p.get(section, "x", vars=overrides)
    substitutions = dict(options, **overrides)
    expected = substitutions["x"]
    while expected.startswith("${") and expected.endswith("}"):
        expected = substitutions[expected[2:-1]]
    if actual != expected:
        print("REFUTATION CONFIRMED:", data, "actual =", repr(actual),
              "expected =", repr(expected))
    else:
        print("REFUTATION REJECTED:", "actual matches documented expectation")

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

REFUTATION CONFIRMED: {'section': 's', 'b': 'original', 'v': 'override', 'vars': {'z': 'override'}} actual = 'original' expected = 'override'

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/configparser-20261003-021945-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).