Making the change suggested by nmknco will help but it is not sufficient. Here is a counter-example:

                ['a2', '1', '0']]```

Now imagine we want to find the shortest path from '0' to '2'. The solution will loop forever.
Dijkstra's shortest path algorithm provides a solution. One has to avoid placing nodes on the queue that have previously been popped off the queue.

You could also use collections.deque and call the popleft() method.
Anyway, good job writting this concise solution.