The time it takes to add two ~20 letter strings should not be an important consideration.
But good point for the list creation. It also looks cleaner without the brackets!

this is painfully unreadable

List creation and string concatenation is unnecessary computation in the function

Cool use of short circuiting

You shouldn't use default values which are mutable. Google why. Quick example:

def blah(a=1, b=[]):
    b.append(a)
    print(a,b)

>>> blah(2)
2 [2]
>>> blah(3)
3 [2,3]