[Python-ideas] Add pop_callback to deque signature

[Michel Desmoulin]

My initial proposal would have been to add a data structure to allow
something like deque, but allowing cheaper random accesses. However I
realized it was a use case more suitable for an external lib.

Thinking about how I would implement such a new data structure, I
imagined several possibilities, one of them would be to couple a deque
with a dict or a list.

However, sharing data between a deque and another data structure is hard
because, while you can easily hook on the element going in (since you
put them in yourself), there is no efficient way to get back an element
on its way out.

On lists or dicts, if you remove an element, you can pop() it and you
get back the removed element.

On deque, if you set a maxlen of 5 and add a 6th element, if you want to
get the element that has been removed, you need to check if the maxlen
has been reached, and if yes, get a reference to the first element, then
add the new one. It's inconvenient and of course slower than it needs to
be given that deque are quite fast.

So my more modest and realistic proposal would be to add a callback on
deque signature:

collections.deque(iterable, maxlen, pop_callback)

pop_callback would accept any callable, and call it everytime an element
is removed from the deque, allowing third party libraries to then do
whatever they need with it.