Python syntax reference: inspect shape, unpacking and mutation rules

Operation contract

The receipt batch uses enumerate for an index and value, zip with strict=True to reject mismatched sequence lengths, and a comprehension to build a new result list. The strict zip exception prevents a missing price from silently truncating a batch. The input lists are not modified by the comprehension.

Failure and ownership boundary

Concise syntax does not replace input validation. Unpacking a receipt tuple assumes its field count, and a comprehension can allocate all results even when written on one line. Python lists: slicing copies the outer sequence, not nested objects, Python functions: define units, validate input and return a result and Python generators: lazy iteration does not make retained output free determine the surrounding contract.

Working program

receipt_ids = [41, 42]
amounts = [125, 75]
print([f"{position}:{identifier}" for position, identifier in enumerate(receipt_ids, start=1)])
print(sum(amount for identifier, amount in zip(receipt_ids, amounts, strict=True)))
try:
    list(zip(receipt_ids, [125], strict=True))
except ValueError:
    print("length mismatch rejected")

Output

['1:41', '2:42']
200
length mismatch rejected

Costs and limits

Enumeration and pairing are linear in consumed values. Materializing n comprehension results uses O(n) result storage; strict zip does not bound input length.

Common Mistakes

• Use strict pairing when unequal lengths are invalid.
• A one-line comprehension can still retain a large result.

Connected lessons

Python functions: define units, validate input and return a result, Python lists: slicing copies the outer sequence, not nested objects, Python generators: lazy iteration does not make retained output free.