main.py

from collections import defaultdict


class Transition:
    def __init__(self, label, ins, outs):
        self.label = label
        self.inputs = frozenset(ins)
        self.outputs = frozenset(outs)

    def __repr__(self):
        return f'Transition({repr(self.label)}, {list(self.inputs)}, {list(self.outputs)})'


transitions = []
transitions.append(Transition('1', ['a', 'b'], ['c', 'd']))
transitions.append(Transition('2', ['c'], ['a']))
transitions.append(Transition('3', ['d'], ['b']))

marking =  frozenset(['a', 'b'])
visited = defaultdict(dict)
unfinished_markings = set([marking])
while True:
    enabled = [t for t in transitions if  marking.intersection(t.inputs) == t.inputs]
    enabled = [t for t in enabled if t.label not in visited[marking]]
    #add detection of 1 safety violation here
    if len(enabled):
        e = enabled[0]
        new_marking = marking.difference(e.inputs).union(e.outputs)
        visited[marking][e.label] = new_marking
        marking = new_marking
        unfinished_markings.add(marking)
    else:
        unfinished_markings.remove(marking)
        if len(unfinished_markings):
            marking = next(iter(unfinished_markings))
        else:
            break

for i, v in visited.items():
    for i2, v2 in v.items():
        m1 = sorted(list(i))
        tr = i2
        m2 = sorted(list(v2))
        print(f"{m1} ---{tr}--> {m2}")