python-dateutil 2.9.0.post0: Parse an ambiguous date string in Python without silently getting the wrong day

{"case":{"caseId":"case:sha256:7a794c6818facb6f50920f4df838a9f76f5e53b2294fb1cc9dec5a52208b6a93","constraints":{"runtime":"python"},"contract":["parse an ISO timestamp and an ambiguous numeric date","assert dayfirst decides between 03/04 readings","assert an unparseable string raises rather than guessing","assert relativedelta adds calendar months, not 30 days"],"goal":"Parse an ambiguous date string in Python without silently getting the wrong day","kind":"FIX","packages":["pkg:pypi/python-dateutil@2.9.0.post0"],"schemaVersion":1,"symbols":["dateutil.parser.parse","relativedelta"]},"contractCommand":["python","test/contract.py"],"environment":{"arch":"x64","ecosystem":"pypi","executionContext":"python","language":"python","os":"linux","packageManager":"pip","runtime":"python","runtimeVersion":"3.12","schemaVersion":1},"license":"MIT-0","packages":["pkg:pypi/python-dateutil@2.9.0.post0"],"schemaVersion":1,"symbols":["dateutil.parser.parse","relativedelta"],"verifierAdapter":"python@1"}

python-dateutil==2.9.0.post0
six==1.17.0

"""Date parsing with dateutil, including the ambiguity nobody expects."""

from dateutil import parser
from dateutil.relativedelta import relativedelta


def parse(text, dayfirst=False):
    """Parse a date string.

    "03/04/2026" is March 4th in the US reading and April 3rd elsewhere, and
    dateutil picks the US one unless told otherwise. It is not a bug and it
    will not warn — pass dayfirst explicitly whenever the input is not ISO.
    """
    return parser.parse(text, dayfirst=dayfirst)


def add_months(dt, months):
    """Add calendar months. timedelta(days=30*n) drifts; relativedelta does not."""
    return dt + relativedelta(months=months)

import os
import sys

sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
from src.dates import add_months, parse

iso = parse("2026-08-13T09:05:00")
assert (iso.year, iso.month, iso.day, iso.hour) == (2026, 8, 13, 9)

# The ambiguity: same input, two readings, decided by dayfirst.
us = parse("03/04/2026")
eu = parse("03/04/2026", dayfirst=True)
assert (us.month, us.day) == (3, 4)
assert (eu.month, eu.day) == (4, 3)

try:
    parse("definitely not a date")
except Exception:
    pass
else:
    raise AssertionError("unparseable input must raise, not guess")

# Calendar months: Jan 31 + 1 month is Feb 28, not March 2.
end_of_jan = parse("2026-01-31")
assert add_months(end_of_jan, 1).strftime("%Y-%m-%d") == "2026-02-28"
assert add_months(parse("2026-08-30"), 6).strftime("%Y-%m-%d") == "2027-02-28"

print("CONTRACT PASS: dateutil resolved the ambiguous date and month arithmetic")