How does collections.deque work internally?

Question presented to candidate: "Everyone says use a deque instead of a list for a queue. Why? What does a deque look like in memory in CPython, and what does that layout make cheap and what does it make expensive?"

What a strong answer should cover:

• Layout: a doubly linked list of fixed-size blocks (64 item slots each in CPython), plus leftblock/leftindex and rightblock/rightindex pointing at the two ends; never one big contiguous array.
• Cost: append, appendleft, pop, popleft are O(1) and never move other items or call realloc; a new block is allocated only every 64 pushes on a side.
• Contrast with list: list.pop(0) and insert(0, x) shift every element (O(n)), but a list indexes any position in O(1) while deque[i] walks blocks, so the middle is O(n).
• Extras that fall out of the layout: maxlen (bounded, evicts from the opposite end), rotate(k), no slicing, and "deque mutated during iteration" detection.
• Why blocks instead of one node per item: far less pointer overhead and far fewer malloc calls; spare blocks are kept on a small free list.
• Thread safety: the docs describe appends and pops from either end as thread-safe.

Code / implementation expected: Optional — demonstrating O(1) ends vs list.pop(0), maxlen, or sketching the block structure.