Turing Machine Python

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class Tape(object):

blank_symbol = " "

def __init__(self,
             tape_string = ""):
    self.__tape = dict((enumerate(tape_string)))
    # last line is equivalent to the following three lines:
    #self.__tape = {}
    #for i in range(len(tape_string)):
    #    self.__tape[i] = input[i]

def __str__(self):
    s = ""
    min_used_index = min(self.__tape.keys()) 
    max_used_index = max(self.__tape.keys())
    for i in range(min_used_index, max_used_index):
        s += self.__tape[i]
    return s    

def __getitem__(self,index):
    if index in self.__tape:
        return self.__tape[index]
    else:
        return Tape.blank_symbol

def __setitem__(self, pos, char):
    self.__tape[pos] = char 

class TuringMachine(object):

def __init__(self, 
             tape = "", 
             blank_symbol = " ",
             initial_state = "",
             final_states = None,
             transition_function = None):
    self.__tape = Tape(tape)
    self.__head_position = 0
    self.__blank_symbol = blank_symbol
    self.__current_state = initial_state
    if transition_function == None:
        self.__transition_function = {}
    else:
        self.__transition_function = transition_function
    if final_states == None:
        self.__final_states = set()
    else:
        self.__final_states = set(final_states)

def get_tape(self): 
    return str(self.__tape)

def step(self):
    char_under_head = self.__tape[self.__head_position]
    x = (self.__current_state, char_under_head)
    if x in self.__transition_function:
        y = self.__transition_function[x]
        self.__tape[self.__head_position] = y[1]
        if y[2] == "R":
            self.__head_position += 1
        elif y[2] == "L":
            self.__head_position -= 1
        self.__current_state = y[0]

def final(self):
    if self.__current_state in self.__final_states:
        return True
    else:
        return False

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turing Machine