Can anybody tell Me why output Code Is 7

Question

There is the code
def f(values,arr=[]):
for i in values:
if i%2==0:
arr.append(i)
print(arr)
return len(arr)
print(f(range(4))+f(range(5)))

Can anybody tell me, what i dont understand why output code is 7, because after all iterations we have array
arr=[0,2,0,2,4]
What i do wrong?
https://code.sololearn.com/cWJuOJUxGSA8/?ref=app

Answer 1

it will be
print(len([0,2])+len([0,2,0,2,4]))
i hope u will understand now

Answer 2

I had a similar question a while back and the following reply from John Wells was the best

https://docs.quantifiedcode.com/JUMP_LINK__&&__python__&&__JUMP_LINK-anti-patterns/correctness/mutable_default_value_as_argument.html

Answer 3

This happens because after you call f(range(4)), your array keeps the two values added. So it already has length 2. Now, when you run f(range(5)) three more values are added and now f(range(5)) will return 5.
So you end up with 7.
What you should do is maybe pass an empty array to f every time it is called