A Python stack implementation (Linked list)

stack.py

# This stack was programmed by Jackson Lohman for part of a CS2420 assignment

class Stack:
    def __init__(self):
        self.head = None
    
    def push(self, item):
        """ Push an item onto the stack. Size increases by 1 """
        if self.head is None:
            self.head = self.Node(value=item)
        else:
            after = self.head
            self.head = self.Node(value=item)
            self.head.next = after
        return self.head.value

    def pop(self):
        """ Remove the top item from the stack and return it. Raise an IndexError if the stack is empty """
        if self.head is None:
            return None
        else:
            popped_value = self.head.value
            self.head = self.head.next
            return popped_value

    def top(self):
        """ Return the item on top of the stack without removing it. Raise an IndexError if the stack is empty """
        if self.head is None:
            return None
        else:
            return self.head.value

    def size(self):
        """ Return the number of items on the stack """
        count = 0
        current = self.head
        while current is not None:
            count += 1
            current = current.next
        return count

    def clear(self):
        """ Empty the stack """
        self.head = None

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

# Quick test code
def main_ja():
    stack = Stack()
    print(stack.size())
    print(stack.push('j'))
    print(stack.push('a'))
    print(stack.top())
    print(stack.push('x'))
    print(stack.size())
    print(stack.pop())
    print(stack.pop())
    print(stack.size())
    # Only j
    print(stack.top())
    print(stack.push('a'))
    print(stack.push('x'))
    print(stack.clear())
    print(stack.size())


if __name__=="__main__":
    main_ja()