dfa3.py

class SymbolException(Exception):
    pass

def make_dfa(symbols, states, start, finals, transitions):
    def dfa(input):
        cur_state = start
        for cur_input in input:
            if cur_input not in symbols:
                raise SymbolException
            cur_state = transitions[cur_state][cur_input]
        return cur_state in finals
    return dfa

def main():
    symbols = "01"
    states = set("q0 q1 q2 qc".split())
    start = "q0"
    finals = {"q0"}
    transitions = {
        "q0": {"0": "q0", "1": "q1"},
        "q1": {"0": "q2", "1": "qc"},
        "q2": {"0": "q1", "1": "q2"},
        "qc": {"0": "q0", "1": "q1"},
    }
    dfa = make_dfa(symbols, states, start, finals, transitions)

    string = raw_input("Enter a string to test >>> ")
    try:
        result = dfa(string.strip())
    except SymbolException:
        print "Bad input"
    else:
        print "Accepted!" if result else "Rejected =("

main()