Aggregate elements of a list containing relativedelta objects

Question

I have a list myList that contains relativedelta objects. I want to add all of these relativedeltas objects. I tried:

sum(myList)

but it gives TypeError.

TypeError: unsupported operand type(s) for +: 'int' and 'relativedelta'

Even though relativedelta objects can be added with + Opeartor

Code somehow:

from datetime import datetime
from dateutil.relativedelta import relativedelta

date1 = datetime(2018, 4, 9).date()
date2 = datetime(2019, 8, 18).date()
date3 = datetime(2014, 2, 1).date()
date4 = datetime(2018, 12, 30).date()

myList = []

rdelta1 = relativedelta(date2, date1)
myList.append(rdelta1)


rdelta2 = relativedelta(date4, date3)
myList.append(rdelta2)

print(sum(myList))

There seems to be an integer in your list of relativedeltas, without a reproducible example is all we can say — ScootCork, Oct 14 '21 at 07:12
You should provide a minimal reproducible example (i.e. a small list reproducing the issue) — mozway, Oct 14 '21 at 07:17
@ScootCork there is no integer in code. I updated the question. see the code section to reproduce. — Shaida Muhammad, Oct 14 '21 at 07:30

Abdul Niyas P M · Answer 1 · 2021-10-14T07:48:12.870

2

This is because sum uses start value as 0.

>>> help(sum)
Help on built-in function sum in module builtins:

sum(iterable, /, start=0)
    Return the sum of a 'start' value (default: 0) plus an iterable of numbers

    When the iterable is empty, return the start value.
    This function is intended specifically for use with numeric values and may    reject non-numeric types.

For example, sum([1, 2]) is 0 + 1 + 2

>>> sum([1, 2])
3

sum([1, 2], start=7) is 7 + 1 + 2

>>> sum([1, 2], start=7)
10

So to solve this use one item(relativedelta object) as start argument and keep everything else in the iterable.

from datetime import datetime
from dateutil.relativedelta import relativedelta

date1 = datetime(2018, 4, 9).date()
date2 = datetime(2019, 8, 18).date()
date3 = datetime(2014, 2, 1).date()
date4 = datetime(2018, 12, 30).date()

myList = []

rdelta1 = relativedelta(date2, date1)

rdelta2 = relativedelta(date4, date3)
myList.append(rdelta2)

print(sum(myList, rdelta1)) # so both start and items in `MyList` are `relativedelta` types.

edited Oct 14 '21 at 07:48

answered Oct 14 '21 at 07:20

Abdul Niyas P M

18,035
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1

Nicely spotted! – mozway Oct 14 '21 at 07:33
relativedate objects are added via a for loop and I don't want to use an extra variable for storing rdelta1. In your answer, I have to do something before for loop like date1 = dates[0] and then use the usual for loop. – Shaida Muhammad Oct 14 '21 at 07:48
1

@ShaidaMuhammad Agree!, My post explains why `sum` raises error when adding `relativedelta` objects(As stated in your question) and how to get rid of it(using `sum` itself), But that does not mean it's the *pythonic* solution :). I would also recommend to use the `reduce ` function. – Abdul Niyas P M Oct 14 '21 at 07:55
@AbduNIyas Gotcha – Shaida Muhammad Oct 14 '21 at 10:40

score 0 · Accepted Answer · answered Oct 14 '21 at 07:42

Use reduce

from datetime import datetime
from dateutil.relativedelta import relativedelta
from functools import reduce

date1 = datetime(2018, 4, 9).date()
date2 = datetime(2019, 8, 18).date()
date3 = datetime(2014, 2, 1).date()
date4 = datetime(2018, 12, 30).date()

myList = []

rdelta1 = relativedelta(date2, date1)
myList.append(rdelta1)


rdelta2 = relativedelta(date4, date3)
myList.append(rdelta2)

print(reduce(lambda x, y: x + y, myList))

Aggregate elements of a list containing relativedelta objects

2 Answers2