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How does collections.deque work internally?

Q: How does collections.deque work internally?

A `deque` ('deck', double-ended queue) is implemented as a **doubly-linked list of fixed-size blocks** (small arrays), not one big contiguous array. That layout gives **O(1)** appends and pops at **both ends** — adding/removing at the head never shifts the other elements: block [.,.,.] block [.,.,.] <line class='d-edge' x1='248' y1='80' x2='210' y2='80' marker-end='url(#py-d

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Doubly-linked blocks.

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PYO(1) both ends, plus a maxlen sliding window

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