Why do I need both condition branches for the rreverse function?

Question

I had an exercise and tried to use parts of code I found here on another person's question, but I found that I needed a part of code that I have no idea why I do.

The full code I was using for my function is this:

def rreverse(s):
    if s == "":
        return s
    else:
        return rreverse(s[1:]) + s[0]

But I only used the else as a statement, and I didn't get the result I was hoping for.

def recur_reverse(x):
    if x != "":
        return recur_reverse(x[1:]) + x[0]

I got TypeError saying "unsupported operand type(s) for +: 'NoneType' and 'str'."

What is the logic behind this first example working fine and the second one throwing an error when the difference is this if statement? and why is my version incorrect?

Thank you!

Answer 1

the problem with the second construct is that if s is the empty string, the function returns None which isn't a string, and the end-caller expects a string, even empty, so returning None will make the caller code break (which leads to questions like Why does my function return None?)

You could write it with a ternary expression to make sure that it returns something whatever the value of x

def recur_reverse(x):
    return recur_reverse(x[1:]) + x[0] if x else ""