DFA for music moods | Python Fiddle

class DFA:
    current_state = None;
    def __init__(self, states, alphabet, transition_function, start_state):
        self.states = states;
        self.alphabet = alphabet;
        self.transition_function = transition_function;
        self.start_state = start_state;
        self.current_state = start_state;
        return;
    
    # Move to a new state given an input
    def transition_to_state_with_input(self, input_value):
        # Do we have a rule to move from the current state to somewhere else based on the input_value?
        # if not, we should freak out
        if ((self.current_state, input_value) not in self.transition_function.keys()):
            self.current_state = None;
            return;
        
        # else, the current state becomes whatever is defined in the transition_function.
        self.current_state = self.transition_function[(self.current_state, input_value)];
        return;
        
    
    # Reset to the start state
    def go_to_initial_state(self):
        self.current_state = self.start_state;
        return;
    
    
    def __str__(self):
        "The current_state is: " + self.current_state; 
    
    # Homework :P
    # def run_with_input_list(self, input_list):
      # This one is for you to figure out, given a list of moods, go through all the necessary transitions.

states = {'Neutral', 'Happy', 'Sad', 'Focus'};
alphabet = {'Happy', 'Sad', 'Focus'};

transitions_map = dict();
transitions_map[('Neutral', 'Happy')] = 'Happy';
transitions_map[('Neutral', 'Sad')] = 'Sad';
transitions_map[('Neutral', 'Focus')] = 'Focus';
transitions_map[('Happy', 'Sad')] = 'Neutral';
transitions_map[('Sad', 'Happy')] = 'Neutral';

start_state = 'Neutral';

d = DFA(states, alphabet, transitions_map, start_state);
print d;

d.transition_to_state_with_input('Happy');
print d;

d.transition_to_state_with_input('Sad');
print d;

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