datedelta.datedelta
is datetime.timedelta
for date arithmetic.
It can add years, months, or days to dates while accounting for oddities of the Gregorian calendar. It can also subtract years, months, or days from dates.
Typically, it's useful to compute yearly or monthly subscriptions periods.
pip install datedelta
The most common operations are adding a datedelta
to a date
and
subtracting a datedelta
from a date
.
The YEAR
, MONTH
, and DAY
constants allow expressing common
calculations with little code.
>>> import datetime
>>> import datedelta
>>> datetime.date(2016, 1, 1) + datedelta.YEAR
datetime.date(2017, 1, 1)
>>> datetime.date(2017, 1, 1) - datedelta.YEAR
datetime.date(2016, 1, 1)
>>> datetime.date(2016, 2, 29) + datedelta.YEAR
datetime.date(2017, 3, 1)
>>> datetime.date(2017, 3, 1) - datedelta.YEAR
datetime.date(2016, 3, 1)
>>> datetime.date(2016, 1, 1) + datedelta.MONTH
datetime.date(2016, 2, 1)
>>> datetime.date(2016, 2, 1) - datedelta.MONTH
datetime.date(2016, 1, 1)
>>> datetime.date(2016, 1, 31) + datedelta.MONTH
datetime.date(2016, 3, 1)
>>> datetime.date(2016, 3, 1) - datedelta.MONTH
datetime.date(2016, 2, 1)
>>> datetime.date(2016, 1, 1) + datedelta.DAY
datetime.date(2016, 1, 2)
>>> datetime.date(2016, 1, 1) - datedelta.DAY
datetime.date(2015, 12, 31)
Note that datedelta.DAY
behaves exactly like datetime.timedelta(1)
.
It's only provided for consistency.
datedelta
objects provide support for arbitrary calculations.
>>> import datetime
>>> import datedelta
>>> datetime.date(2016, 3, 23) + datedelta.datedelta(years=1, months=1, days=-1)
datetime.date(2017, 4, 22)
>>> datetime.date(2016, 3, 23) - datedelta.datedelta(years=-1, months=-1, days=1)
datetime.date(2017, 4, 22)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=2)
datetime.date(2018, 3, 1)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=2)
datetime.date(2018, 3, 1)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=2, days=-1)
datetime.date(2018, 2, 28)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=2, days=1)
datetime.date(2018, 2, 28)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=2, months=6)
datetime.date(2018, 9, 1)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=2, months=-6)
datetime.date(2018, 9, 1)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=4)
datetime.date(2020, 2, 29)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=4)
datetime.date(2016, 2, 29)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=4, days=1)
datetime.date(2020, 3, 1)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=4, days=-1)
datetime.date(2016, 3, 1)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=4, months=6)
datetime.date(2020, 8, 29)
>>> datetime.date(2020, 2, 29) - datedelta.datedelta(years=4, months=-6)
datetime.date(2016, 8, 29)
These results may appear slightly surprising. However, they're consistent, for reasons explained in the "Behavior" section below.
datedelta
instances can be added, subtracted, and multiplied with an
integer. However there are some restrictions on addition and subtraction.
As demonstrated in the "Limitations" section below, adding then subtracting a
given datedelta to a date doesn't always return the original date. In order to
prevent bugs caused by this behavior, when the result of adding or subtracting
two datedelta
isn't well defined, that operation raises ValueError
.
>>> import datedelta
>>> datedelta.YEAR + datedelta.YEAR
datedelta.datedelta(years=2)
>>> 3 * datedelta.YEAR
datedelta.datedelta(years=3)
>>> datedelta.YEAR - datedelta.DAY
datedelta.datedelta(years=1, days=-1)
>>> datedelta.YEAR - datedelta.YEAR
Traceback (most recent call last):
...
ValueError: cannot subtract datedeltas with same signs
>>> datedelta.datedelta(months=6) + datedelta.datedelta(months=-3)
Traceback (most recent call last):
...
ValueError: cannot add datedeltas with opposite signs
There are two date arithmetic traps in the Gregorian calendar:
- Leap years. Problems arise when adding years to a February 29th gives a result in a non-leap year.
- Variable number of days in months. Problems arise when adding months to a 29th, 30th or 31st 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.
This method gives consistent results provided periods are represented by (start date inclusive, end date exclusive) — that's [start date, end date) if you prefer the mathematical notation. This representation of periods is akin to 0-based indexing, which is the convention Python uses.
For example, if someone subscribes for a year starting on 2016-02-29 inclusive, the end date must be 2017-03-01 exclusive. If it was 2016-02-28 exclusive, the subscription would be one day too short.
Operations are always performed on years, then months, then days. This order usually provides the expected behavior. It also minimizes loss of precision.
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)
Here are two examples where adding two datedelta
gives a different result
depending on the order of operations:
>>> import datetime
>>> import datedelta
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(months=6) + datedelta.datedelta(years=1)
datetime.date(2017, 8, 29)
>>> datetime.date(2016, 2, 29) + datedelta.datedelta(years=1) + datedelta.datedelta(months=6)
datetime.date(2017, 9, 1)
>>> datetime.date(2016, 1, 31) + datedelta.datedelta(months=2) + datedelta.datedelta(months=5)
datetime.date(2016, 8, 31)
>>> datetime.date(2016, 1, 31) + datedelta.datedelta(months=5) + datedelta.datedelta(months=2)
datetime.date(2016, 9, 1)
To avoid problems, you should always start from the same reference date and add
a single datedelta
. Don't chain additions or subtractions.
To minimize the risk of incorrect 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.")
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 leap days automatically reduces the number of choices the programmer must make and thus the number of errors they can make.
Note that datedelta.datedelta
adjusts non-existing days to the first day of
the next month while dateutil.relativedelta
adjusts them to the last day of
the current month.
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
.
- Add
YEAR
,MONTH
, andDAY
constants.
- Initial stable release.