Add to a deque being iterated in Python?

Question

I have a deque in Python that I'm iterating over. Sometimes the deque changes while I'm interating which produces a RuntimeError: deque mutated during iteration.

If this were a Python list instead of a deque, I would just iterate over a copy of the list (via a slice like my_list[:], but since slice operations can't be used on deques, I wonder what the most pythonic way of handling this is?

My solution is to import the copy module and then iterate over a copy, like for item in copy(my_deque): which is fine, but since I searched high and low for this topic I figured I'd post here to ask?

Answer 1

You can "freeze" it by creating a list. There's no necessity to copy it to a new deque. A list is certainly good enough, since you only need it for iterating.

for elem in list(my_deque):
    ...

list(x) creates a list from any iterable x, including deque, and in most cases is the most pythonic way to do so.

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Bare in mind this solution is only valid if the deque is being modified in the same thread (i.e. inside the loop). Else, be aware that list(my_deque) is not atomic, and is also iterating over the deque. This means that if another thread alters the deque while it runs, you end up with the same error. If you're in a multithreaded environment, use a lock.