datedelta 1.0.0

Behavior

There are two date arithmetic traps in the Gregorian calendar:

1. Leap years. Problems arise when adding years to a February 29th gives a result in a non-leap year.

2. Variable number of days in months. Problems arise when adding months to a 29th, 30th or 31th gives a result in a month where that day doesn’t exist.

In both cases, the result must be changed to the first day of the next month.

Provided periods are represented by [start date inclusive, end date exclusive), datedelta gives consistent results. (This representation of periods is akin to 0-based indexing, which is the convention Python uses.)

Operations are always performed on years, then months, then days. This order usually provides the expected behavior. It also minimizes loss of precision.

Limitations

Additions involving datedelta are neither associative not commutative in general. Here are two examples where adding a datedelta then subtracting it doesn’t return the original value:

>>> import datetime
>>> import datedelta

>>> datetime.date(2020, 2, 29) + datedelta.datedelta(years=1)
datetime.date(2021, 3, 1)

>>> datetime.date(2021, 3, 1) - datedelta.datedelta(years=1)
datetime.date(2020, 3, 1)

>>> datetime.date(2020, 1, 31) + datedelta.datedelta(months=1)
datetime.date(2020, 3, 1)

>>> datetime.date(2020, 3, 1) - datedelta.datedelta(months=1)
datetime.date(2020, 2, 1)

To avoid counter-intuitive results, datedelta only implements operations that have unambiguous semantics:

• Adding a datedelta to a date

• Subtracting a datedelta from a date

• Adding a datedelta to a datedelta when components have the same sign

• Subtracting a datedelta from a datedelta when components have opposite signs

(PEP 20 says: “In the face of ambiguity, refuse the temptation to guess.”)

Alternatives

datedelta.datedelta is smarter than datetime.timedelta because it knows about years and months in addition to days.

datedelta.datedelta provides a subset of the features found in dateutil.relativedelta. Not only does it only support dates, but:

• It omits the “replace” behavior which is very error-prone.

• It doesn’t allow explicit control of leapdays.

• It uses keyword-only arguments.

• It requires Python 3.

Handling leapdays automatically reduces the number of choices the programmer must make and thus the number of errors they can make.

If you’re stuck with Python 2, just copy the code, make datedelta inherit from object, and remove the * in the signature of __init__.

If you’re comfortable with dateutil and don’t mind its larger footprint, there’s little to gain by switching to datedelta.

Changelog

1.0

• Initial stable release.