FA Machine to Dot Graphviz description

#!/usr/bin/env python
'''
This simple module allow you to generate DOT graph descriptions using the
fa_to_dot() function. So the function would return something like the following:

digraph fi {
    "x-" -> "y" [label="a"];
    "x-" -> "z+" [label="b"];
    "y" -> "x-" [label="a"];
    "y" -> "z+" [label="b"];
    "z+" -> "z+" [label="a"];
    "z+" -> "z+" [label="b"];
}

Please have a look at the source of the main() function of the module to see how
to use fa_to_dot().
'''

def check_state(state, start_state, final_states):
    '''
    Return the state as a begin (-) or end (+) state if the state is a begin
    or end state.
    '''
    result_state = state
    if state == start_state:
        result_state += '-'
    if state in final_states:
        result_state += '+'
    return result_state

def fa_to_dot(states, alphabet, transistions, start_state, final_states):
    '''
    Builds a DOT graph description using a FA Machine.

    TODO: Check that the states and alphabet that is in use by the transistions
    match the parameters.
    '''
    dot = 'digraph fa {\n'
    for transition in transistions:
        from_state, input_letter = transition
        to_state = transistions[transition]
        from_node = check_state(from_state, start_state, final_states)
        to_node = check_state(to_state, start_state, final_states)
        label = input_letter
        dot += '\t"' + from_node + '" -> "' + to_node + '" [label="' \
            + label + '"]\n'
    dot += '}'
    return dot

def main():
    # S - Set of the states
    states = set('xyz')
    # Σ (Sigma) - Alphabet of the input letters
    alphabet = set('ab')
    # T: SxΣ -> S - Set of transistions
    # Because we want it to resebmle a set of functions we use a dict:
    # T(S,Σ) = S is (S, Σ): S
    # Where (S, Σ) on the LHS is the key and S on the RHS is the value of
    # the dict item
    transistions = {('x', 'a'): 'y',
         ('x', 'b'): 'z',
         ('y', 'a'): 'x',
         ('y', 'b'): 'z',
         ('z', 'a'): 'z',
         ('z', 'b'): 'z'}
    # s0 - Start state
    start_state = 'x'
    # F - Set of Final States
    final_states = set('z')

    print fa_to_dot(states, alphabet, transistions, start_state, final_states)

if __name__ == '__main__':
    main()