String Tree implementation

string_tree.py

from abc import ABC, abstractmethod

class Node(ABC):
    length: int = 0

    @abstractmethod
    def char_at(self, index: int) -> str:
        raise IndexError()
    
    @abstractmethod
    def sub_string(self, start: int, end: int) -> str:
        raise IndexError()
    
    @abstractmethod
    def delete(self, index:int) -> None:
        raise IndexError()


class InternalNode(Node):
    def __init__(self, length=0, left=None, right=None):
        self.length = length
        self.left = left
        self.right = right

    def char_at(self, index: int) -> str:
        if index >= self.length:
            raise IndexError()
        if self.left and index < self.left.length:
            return self.left.char_at(index)
        if self.right:
            return self.right.char_at(index - self.left.length)
        raise IndexError()
    
    def sub_string(self, start: int, end: int) -> str:
        substr = []
        if self.left and start < self.left.length:
            substr.append(self.left.sub_string(start, min(end, self.left.length - 1)))
        if self.right and end >= self.left.length:
            substr.append(self.right.sub_string(0, end - self.left.length))
        return ''.join(substr)
    
    def delete(self, index:int) -> None:
        if index >= self.length:
            raise IndexError()
        if self.left and index < self.left.length:
            return self.left.delete(index)
        if self.right:
            return self.right.delete(index - self.left.length)
        

class Leaf(Node):
    def __init__(self, string: str='', length=0):
        self.string = string
        self.length = length or len(string)

    def char_at(self, index: int) -> str:
        if index >= self.length:
            raise IndexError()
        return self.string[index]
    
    def sub_string(self, start: int, end: int) -> str:
        return self.string[start:end + 1]
    
    def delete(self, index:int) -> None:
        if index >= self.length:
            raise IndexError()
        self.string = self.string[:index] + self.string[index + 1:]
        self.length -= 1

class StringTree:
    def __init__(self):
        self.root = InternalNode()

if __name__ == '__main__':
    leaf1 = Leaf(string='ABCDE')
    leaf2 = Leaf(string='FGHIJKLMNO')
    leaf3 = Leaf(string='PQRSTUVWXYZ')
    inode1 = InternalNode(21, left=leaf2, right=leaf3)
    root = InternalNode(26, left=leaf1, right=inode1)

    print(root.char_at(25))

    print(root.sub_string(1, 3))