Hi!

I was going through script explore1.py from chapter 22, in particular in the init method and I wrote the following code instead of the code in the script:

self.__data = dict(mapping)
for key, val in self.__data.items():
    if keyword.iskeyword(key):
        newkey = key + '_'
        self[newkey] = val
        self.__data.pop(key)

Of course the above code raised the following exception:

RuntimeError: dictionary keys changed during iteration

Which googling a bit lead me to the conclusion that modifying containers while looping is something that never should be done and I should use a shallow copy instead if necessary :)

I'm creating this issue as a suggestion for perhaps a small warning/caution for a future edition. Doing a quick search in stackoverflow of the above issue showed me that this is something recurrent, even Alex Martelli did a comment about this 14 years ago here. Perhaps we can avoid some future Python programmers to stumble across this issue by warning them! 😃

Btw, thank you very much for the outstanding book! Looking forward for that next edition 😄

Best,

Andy