Below is my attempt at a doubly-linked list in Python (3):

class LinkedListNode:
    def __init__(self, value):
        self.value = value
        self.left  = None
        self.right = None


class LinkedList:
    def __init__(self):
        self.head = self.tail = None
        self.size = 0

    def get_first(self):
        if self.head:
            return self.head.value
        raise Exception('Reading from an empty list.')

    def get_last(self):
        if self.tail:
            return self.tail.value
        raise Exception('Reading from an empty list.')

    def add_first(self, value):
        node = LinkedListNode(value)
        if self.head:
            node.right = self.head
            self.head.left = node
            self.size += 1
        else:
            self.head = self.tail = LinkedListNode(value)
            self.size = 1

    def add_last(self, value):
        node = LinkedListNode(value)
        if self.tail:
            node.left = self.tail
            self.tail.right = node
            self.size += 1
        else:
            self.head = self.tail = node
            self.size = 1

    def remove_first(self):
        if self.head:
            ret = self.head.value
            self.head = self.head.right
            self.head.left = None
            self.size -= 1
            return ret
        raise Exception('Removing from an empty list.')

    def remove_last(self):
        if self.tail:
            ret = self.tail.value
            self.tail = self.tail.left
            self.tail.right = None
            self.size -= 1
            return ret
        raise Exception('Removing from an empty list.')

    def __repr__(self):
        str = '['
        if self.size > 0:
            str += self.head.value
            node = self.head.right
            while node:
                str += ', ' + node.value
                node = node.right
        return str + ']'


def main():
    list = LinkedList()

    while True:
        cmd_line = input()
        if cmd_line == 'quit':
            break
        command_tokens = cmd_line.split()
        command = command_tokens[0]
        try:
            if command == "get_first":
                print(list.get_first())
            elif command == "get_last":
                print(list.get_last())
            elif command == "add_first":
                list.add_first(command_tokens[1])
            elif command == "add_last":
                list.add_last(command_tokens[1])
            elif command == "remove_first":
                list.remove_first()
            elif command == "remove_last":
                list.remove_last()
            elif command == "print":
                print(list)
        except Exception:
            print("Unknown command")


main()

How would I rewrite it in a professional manner?