How to create a recursive function that returns a copy of a single parameter (a list of strings), but with every character duplicated in each place?

Question

Basically, I need to write a recursive function that takes a list of strings and then returns a list of strings with each character in each string repeated/doubled.

For example, doubleStringList(['I','love','overflow']) would return ['II','lloovvee','oovveerrffllooww'].

I decided to split it up into two recursive functions. I already made one function that returns the strings with doubled characters:

def doubleCharacter(seq):
    if len(seq) == 0:
        return seq
    else:
        return (seq[0] * 2) + doubleCharacter(seq[1:])

For example, entering doubleCharacter("overflow") would return 'oovveerrffllooww'

I'm trying to create a second recursive function that uses the first except for a list of strings as the paremeter rather than a single string. For example:

def doubleStringList(sList):
    if len(sList) == 0:
        return sList
    else:
        return doubleCharacter(sList[0]) + doubleStringList(sList[1:])

Theoretically, if I entered doubleStringList(["overflow","is","great"]), it should return ["oovveerrffllooww", "iiss", "ggrreeaatt"]. If someone could please help me figure out this code, I would appreciate it greatly.

Answer 1

The error message gives a big clue:

TypeError: can only concatenate str (not "list") to str

So, the last line should be

return [doubleCharacter(sList[0])] + doubleStringList(sList[1:])