Python batched strict: reject an incomplete import group

Operation contract

A settlement import expects groups of three IDs. The first input has six IDs and produces two complete groups. A second input has seven; two groups can be yielded before iteration raises ValueError on the final lone ID. Validate the entire stream before committing irreversible group effects.

Failure boundary

Strict batching is not an atomic transaction. Previously yielded groups may already have been written when the final error appears. If all-or-nothing import is required, stage data in a transaction or temporary store first. The iterator also has no built-in total input limit.

Working program

from itertools import batched

accepted_ids = (f"receipt-{number}" for number in range(47, 53))
groups = list(batched(accepted_ids, 3, strict=True))
print("groups", len(groups))
print("first", groups[0])

incomplete_ids = (f"receipt-{number}" for number in range(47, 54))
complete_before_error = 0
try:
    for settlement_group in batched(incomplete_ids, 3, strict=True):
        complete_before_error += 1
except ValueError:
    print("complete_before_error", complete_before_error)

Output

groups 2
first ('receipt-47', 'receipt-48', 'receipt-49')
complete_before_error 2

Costs and limits

Iteration takes O(n) time and retains up to three items per current batch. Materializing all groups with list, as in the first check, takes O(n) additional space.

Common Mistakes

• A late strict error does not undo earlier side effects.
• Strict mode checks final tuple length, not business validity of each item.
• An infinite input never reaches a final-batch check.

Connected lessons

• Python iterators: exhaustion and repeatable collection ownership
• Python generators: lazy iteration does not make retained output free
• Python CSV imports: parse quoted fields before validating rows

Test this contract.