Mutating list while iterating

https://github.com/PrefectHQ/prefect/blob/8c886b64df8e264791ec1d0e401cbbd6c4719af5/src/prefect/agent/local/agent.py#L75-L77

It’s a no-no to mutate the list while iterating. Indeed, in my tests, a loop like this fails to examine the next item in the list, so it’s actually not working as intended. For instance:

x = [0,1,2,3,4,5,6,7,8]
for idx, y in enumerate(x):
    print(idx, y)
    if y == 3:
        z = x.pop(idx)
    print(idx, y)

yields this output; note that the 4 element in the list is never examined.

0 0
0 0
1 1
1 1
2 2
2 2
3 3
3 3
4 5
4 5
5 6
5 6
6 7
6 7
7 8
7 8

To be fair this is somewhat harmless, because the process will likely be examined in the next heartbeat call, or in the worst case after all of the processes earlier in the list are completed.

[joshmeek]

Haha so many different ways you could do it! Here’s my suggestion:

for idx, process in enumerate(list(self.processes)): 
     if process.poll() is not None: 
         self.processes.pop(idx) 

Make a new list() with self.processes 😉

[trkohler]

Hi! I’d like to help with this! May I? Do you have any solution in mind?