Question

My professor posted the function below. I do not fully understand how it’s working. Could someone explain it?

def rev(a):
    if a == []:
        return []
    else:
        return rev(a[1:]) + [a[0]]

Answer 1

What this does is recursively reverses a list. The easiest way to see how it works is to follow through the execution.

The function takes the string and solves it by returning the reversed version of all but the first item (a[1:]) with the first item appended to the end.

Note that this is a bad way to do this in a real situation (I am presuming your professor is just showing the idea of recursion), as Python isn’t optimized for recursion. Instead, use the reversed() builtin.

Also, it isn’t particularly Pythonic code. If one had to have a recursive solution, instead of using the efficient, effective, well-tested, easy-to-use built-in, consider:

def rev(seq):
    return rev(seq[1:]) + [seq[0]] if seq else []

• We use the ternary operator to condense the if/else.
• Replacing a with seq makes the function clearer – Python doesn’t have strict typing, so using names that give a clue to what the function takes (in this case, a sequence), makes it clearer.
• We also replace the a == [] by simply checking seq. As lists evaluate to False when empty, there is no need to compare to an empty list.