Python collections reference: selection costs and retained ownership

Operation contract

The reference contrasts a list snapshot, set membership, dictionary defaults and a bounded deque. List copying separates the outer container, set membership answers whether an ID is present, and dictionary get reads a missing entry without creating it. Deque retains only the selected history length and intentionally discards older entries.

Failure and ownership boundary

This is a compact operation reference, not a universal complexity guarantee. Custom hashes, equality methods and nested aliases can change cost or behavior. A bounded history must not stand in for a ledger that must preserve every event. Python hash and equality: immutable dictionary keys, Python shallow and deep copies: preserve aliases deliberately and Python Counter and deque: counts, queues and bounded history provide the detailed contracts.

Working program

from collections import deque

receipt_ids = ["R-0041", "R-0042"]
outer_snapshot = receipt_ids.copy()
known = set(receipt_ids)
amounts = {"R-0041": 125}
history = deque(receipt_ids, maxlen=2)
history.append("R-0043")
print("member:", "R-0041" in known)
print("missing amount:", amounts.get("R-0099"))
print("key created:", "R-0099" in amounts)
print("retained history:", list(history))
receipt_ids.append("R-0044")
print("outer snapshot:", outer_snapshot)

Output

member: True
missing amount: None
key created: False
retained history: ['R-0042', 'R-0043']
outer snapshot: ['R-0041', 'R-0042']

Costs and limits

List snapshot construction costs O(n) references. Set/dictionary membership has expected constant lookup work under suitable hashes. Deque endpoint operations avoid list-front shifting, while middle indexing/traversal has different costs.

Common Mistakes

• An outer snapshot still shares mutable nested objects.
• A bounded deque discards old history by design.

Connected lessons

Python hash and equality: immutable dictionary keys, Python shallow and deep copies: preserve aliases deliberately, Python Counter and deque: counts, queues and bounded history.