How does a Python list grow in memory?

Question presented to candidate: "A Python list can grow forever with append(), and yet append() is called O(1). What is a list in memory, what happens when it runs out of room, and which list operations are actually slow?"

What a strong answer should cover:

• A list is a dynamic array: a small header (ob_size = length, allocated = capacity) plus one contiguous array of pointers to the item objects, not the objects themselves.
• Over-allocation: when append() runs out of slots, CPython reallocates with extra room (about 12.5% plus a small constant, rounded to a multiple of 4), so resizes are rare and append() is amortized O(1).
• The observed growth sequence on CPython 3.11: capacities 4, 8, 16, 24, 32, 40, 52, 64, 76, 92, ... and shrinking only when fewer than half the slots are used.
• The costs: index and append()/pop() at the end are O(1); insert(0, x), pop(0) and del a[0] shift every pointer, O(n); use collections.deque for a queue.
• sys.getsizeof() counts only the header and pointer array, never the items; how a list is built ([0] * n, list(range(n)), a comprehension) changes its spare capacity.

Clarifying questions expected:

• Do you mean CPython specifically? (The growth rule is an implementation detail, not part of the language.)

Code / implementation expected: Optional — a sys.getsizeof() experiment that reveals the capacity steps, or an amortized-cost argument.