Python challenge results explanation

Question

Dear All,
I’m a new learner and have a results can’t understand by myself hope someone could help me out.
What is the output for this code:
def f(x=[]):
x+=[1]
return sum(x)
Print(f()+f()+f())
Result is 6, I can’t understand why.
Please help me.
Thanks

Answer 1

def f(x=[]):
x+=[1]
return x
print(f()) # [1]
print(f()) # [1, 1]
print(f()) # [1 ,1, 1]
The list "x" is originally an empty list. In the first iteration a 1 gets added to it.
For the second iteration, "x" is no longer empty, but is equal to [1] and the function returns [1, 1].
For the third iteration, "x" equals [1, 1] and the function returns [1, 1, 1].
Summing the values of the list in each iteration gives you (1) + (1+1) + (1+1+1) == 6.

Answer 2

Thank yo so much