csv.Sniffer.has_header raises csv.Error for valid CR-terminated CSV

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: Python csv documentation, Sniffer.has_header(sample): "Analyze the sample text (presumed to be in CSV format) and return True if the first row appears to be a series of column headers." The Dialect.lineterminator documentation also states: "The reader is hard-coded to recognise either '\r' or '\n' as end-of-line."

Expected: Return a bool for the valid sample (normally True).

Actual: Raises csv.Error: new-line character seen in unquoted field - do you need to open the file with newline=''?

import csv
import io

s = "name,age\r1,2\r3,4\r"
try:
    rows = list(csv.reader(io.StringIO(s, newline=""), delimiter=",", strict=True))
    valid = rows == [["name", "age"], ["1", "2"], ["3", "4"]]
except csv.Error:
    valid = False

if not valid:
    print("REFUTATION REJECTED:", "input failed independent CSV validation")
else:
    expected = bool
    try:
        value = csv.Sniffer().has_header(s)
        actual = ("result", value)
        broken = type(value) is not expected
    except csv.Error as e:
        actual = ("exception", type(e).__name__, str(e))
        broken = True
    except Exception as e:
        actual = ("exception", type(e).__name__, str(e))
        broken = False
    if broken:
        print("REFUTATION CONFIRMED:", repr(s), "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED:", "no violation of the claimed bool/csv.Error property;", actual)

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

REFUTATION CONFIRMED: 'name,age\r1,2\r3,4\r' actual: ('exception', 'Error', "new-line character seen in unquoted field - do you need to open the file with newline=''?") expected: <class 'bool'>

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-20261003-135109-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).