ISO週番号計算ツール

from datetime import date

def iso_week_number(d: date) -> tuple[int, int, int]:
    """Return (ISO year, ISO week number, ISO weekday) for a given date.
    ISO week 1 is the week containing the year's first Thursday.
    Monday = 1, Sunday = 7
    """
    iso_cal = d.isocalendar()
    return iso_cal.year, iso_cal.week, iso_cal.weekday

def dates_in_iso_week(year: int, week: int) -> list[date]:
    """Return all 7 dates in a given ISO week."""
    # ISO week 1, Monday
    jan4 = date(year, 1, 4)  # Jan 4 is always in week 1
    start_of_week1 = jan4 - timedelta(days=jan4.isocalendar().weekday - 1)
    from datetime import timedelta
    start = start_of_week1 + timedelta(weeks=week - 1)
    return [start + timedelta(days=i) for i in range(7)]

# Example
from datetime import timedelta
today = date.today()
y, w, wd = iso_week_number(today)
print(f"Today {today}: ISO week {w} of {y}, weekday {wd} (1=Mon)")

# All years have 52 or 53 ISO weeks
for yr in [2020, 2021, 2022, 2023, 2024]:
    last_week = date(yr, 12, 28).isocalendar().week  # Dec 28 is always in last week
    print(f"  {yr}: {last_week} ISO weeks")