csv.DictWriter.writeheader loses field-name representations for equal keys

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python csv documentation, DictWriter.writeheader(): “Write a row with the field names (as specified in the constructor) to the writer’s file object, formatted according to the current dialect.”

Expected: '1,True\r\n'

Actual: 'True,True\r\n'

import csv
import io

fields = [1, True]
formatting = {}
case = {"fieldnames": fields, "formatting": formatting}
try:
    for field in fields:
        hash(field)  # Valid dictionary keys; fieldnames is a finite sequence.
    actual, expected = io.StringIO(), io.StringIO()
    csv.DictWriter(actual, fieldnames=fields, **formatting).writeheader()
    csv.writer(expected, **formatting).writerow(fields)
    actual, expected = actual.getvalue(), expected.getvalue()
    if actual != expected:
        print("REFUTATION CONFIRMED:", case, "actual:", repr(actual),
              "expected:", repr(expected))
    else:
        print("REFUTATION REJECTED: actual matches documented expectation")
except Exception as exc:
    print("REFUTATION REJECTED:", type(exc).__name__, str(exc))

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

REFUTATION CONFIRMED: {'fieldnames': [1, True], 'formatting': {}} actual: 'True,True\r\n' expected: '1,True\r\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/csv-20261003-022404-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).