custom dict that allows delete during iteration

Question

UPDATED based on Lennart Regebro\'s answer

Suppose you iterate through a dictionary, and sometimes need to delete an element. The following is very efficient:

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Answer 1

1. You can make a copy of the list of keys (you don't need to copy te values) at the beginning of the iteration, and iterate over those (checking that the key is there). This is inefficient if there ar a lot of keys.
2. You can arrange embed your first example code inside a class. __iter__ and __delitem__ and other special methods need to collaborate to keep a list of items to be removed while an iteration happens. When there are no current iterations, __delitem__ can just delete an item, but when at least one iteration is happening, it should just add the key to be deleted into a list. When the last active iteration finishes, it should actually delete things. This somewhat inefficient if there's a lot of keys to remove, and will, of course, blow up if there's always at least one iteration going on.