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Replacing python-dateutil to remove six

Python dateutil six
Two panel woodcut. On the left, an almost stick figure carrying a Christmas tree through dark woodland. On the right, a Jugendstil calendar with highly stylised letters for December, the days in German and numbers.
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The dateutil library is a popular and powerful Python library for dealing with dates and times.

However, it still supports Python 2.7 by depending on the six compatibility shim, and I’d prefer not to install for Python 3.10 and higher.

Here’s how I replaced three uses of its relativedelta in a couple of CLIs that didn’t really need to use it.

One #

norwegianblue was using it to calculate six months from now:

import datetime as dt

from dateutil.relativedelta import relativedelta

now = dt.datetime.now(dt.timezone.utc)
# datetime.datetime(2025, 12, 29, 15, 59, 44, 518240, tzinfo=datetime.timezone.utc)
six_months_from_now = now + relativedelta(months=+6)
# datetime.datetime(2026, 6, 29, 15, 59, 44, 518240, tzinfo=datetime.timezone.utc)

But we don’t need to be so precise here, and 180 days is good enough, using the standard library’s datetime.timedelta:

import datetime as dt

now = dt.datetime.now(dt.timezone.utc)
# datetime.datetime(2025, 12, 29, 15, 59, 44, 518240, tzinfo=datetime.timezone.utc)
six_months_from_now = now + dt.timedelta(days=180)
# datetime.datetime(2026, 6, 27, 15, 59, 44, 518240, tzinfo=datetime.timezone.utc)

Two #

pypistats was using it get the last day of a month:

import datetime as dt

first = dt.date(year, month, 1)
# datetime.date(2025, 12, 1)
last = first + relativedelta(months=1) - relativedelta(days=1)
# datetime.date(2025, 12, 31)

Instead, we can use the stdlib’s calendar.monthrange:

import calendar
import datetime as dt

last_day = calendar.monthrange(year, month)[1]
# 31
last = dt.date(year, month, last_day)
# datetime.date(2025, 12, 31)

Three #

Finally, to get last month as a yyyy-mm string:

import datetime as dt

from dateutil.relativedelta import relativedelta

today = dt.date.today()
# datetime.date(2025, 12, 29)
d = today - relativedelta(months=1)
# datetime.date(2025, 11, 29)
d.isoformat()[:7]
# '2025-11'

Instead:

import datetime as dt

today = dt.date.today()
# datetime.date(2025, 12, 29)
if today.month == 1:
    year, month = today.year - 1, 12
else:
    year, month = today.year, today.month - 1
    # 2025, 11
f"{year}-{month:02d}"
# '2025-11'

Goodbye six, and we also get slightly quicker install, import and run times.

Bonus #

I recommend Adam Johnson’s tip to import datetime as dt to avoid the ambiguity of which datetime is the module and which is the class.

Header photo: Ver Sacrum calendar by Alfred Roller