The operations for sum([[3], [5]], [])

Question

This is a very simple expression, but I have no idea how it operates.

print(sum([[3], [5]], []))

# returns [3,5] 

why on earth does sum([[3], [5]], []) return [3,5]? My knowledge on sum() is that it takes in an iterable and sums the elements in the iterable. So usually the elements are integers. For example, I'm familiar with something like:

sum([1,2,3])

# returns 6 

But sum([[3], [5]], []) has two inputs, one a list of lists, and another an empty list. I thought this would return an error, but surprisingly it returned the above result. I haven't seen something like this before, and I would appreciate it if anyone could tell me the operations going on for this example.

Answer 1

sum() takes two parameters :

• the iterable (here [[3],[5]])
• the start, which has a default value of 0 (an int)

When you use

sum([[3],[5]])

start has the default value 0, so the function tries to add [3] and [5] to 0, an int, so you have TypeError.

On the other hand, when you use

sum([[3],[5]],[])

with [] as start value, [] + [3] + [5], it first instantiates an empty list before adding list-type elements to it.

This sum is correct with output [3,5]

Answer 2

This is equivalent to:

s = []
for lst in [[3], [5]]:
    s += lst

You need to know that the list can be connected through the operator +?even if it contains an empty list, it will get a copy of another list.

About why sum ([[3], [5]]) raise error, see the signature of sum function:

sum(iterable, start)

If you don't specify start, the default value seems to be 0, which of course cannot be added to the list.