Return Statement from a method/function

Question

I feel a little bit silly asking this, but I’m practicing some DSA. I’m watching a Youtube video, and I find odd that in some methods of the class the return method is left without returning a value. I know that it’ll return None but I don’t understand how the code works.

Here’s the code snippet:

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def to_list(self):
        out = []
        node = self.head
        while node:
            out.append(node.value)
            node = node.next
        return out

### In the following method is my doubt ###
def prepend(self, value):
    """ Prepend a value to the beginning of the list. """
    if self.head is None:
        self.head = Node(value)
        return # Why is this return value left alone without returning anything?
    
    new_head = Node(value)
    new_head.next = self.head
    self.head = new_head
    return # Why is this return value left alone without returning anything?

LinkedList.prepend = prepend

Can someone please explain to me, what is this code doing, because as I understand the prepend method is working with two None values from the 2 empty returns.

Asked By: David Torres

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Source

Answer 1

A return is used when you want to stop a function from executing further. The first one is necessary in order to not have the statements after the if self.head is None block be executed. The second one is unnecessary, as it’s at the end of the function, so the function will return regardless. It’s just a more explicit way of saying "I am returning here, but am not returning anything". That’s the default behavior of any function that you don’t explicitly put a return into.

It’s very common in programming languages to have explicit, blank returns. For instance in C, any function whose return type is void has to have any returns be empty. In Python there’s tons of built-in functions that don’t need to return anything, let alone all the custom ones one might create. For instance, list.append() returns None; which is what any function in Python returns by default when there’s not anything explicitly being returned.

Answered By: Random Davis

Answer 2

The first return in prepend is an early return and this is the only way of returning from this point in the function as it is currently written.

The second return is entirely redundant and can be removed.

Answered By: quamrana

Answer 3

It is just a matter of style, whether one just lets the function-body end returning None implicitly, writes a naked return statement or even writes return None explicitly. These are all functions which return None:

def a():
    pass

def b():
    return

def c():
    return None

However the first method does not allow you to return early. This is why an explicit return is needed in the if branch of the method. Otherwise control flow would continue and prepend the same value twice.

The second return statement at the end is redundant.

Answered By: user19685524

Return Statement from a method/function

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