Python defaultdict: missing-key reads can create state

Operation contract

The batch groups accepted receipt IDs by region using a list factory. Calling get for an unknown region does not invoke the factory, while indexing that region creates an empty group. The fixture makes both behaviors visible because a supposedly read-only inspection can otherwise change exported group keys.

Failure and ownership boundary

The factory must produce independent mutable containers. Reusing one shared list would couple unrelated groups. This helper assumes records have already been validated; grouping is not a schema or authorization check. Python dictionaries: insertion order and duplicate-key replacement and Python aggregation exercise: validate records before returning group totals cover those boundaries.

Working program

from collections import defaultdict

groups = defaultdict(list)
for region, receipt_id in [("DEL", "R-0041"), ("BOM", "R-0042"), ("DEL", "R-0043")]:
    groups[region].append(receipt_id)
print(dict(groups))
print(groups.get("BLR"))
print("BLR" in groups)
print(groups["BLR"])
print("BLR" in groups)

Output

{'DEL': ['R-0041', 'R-0043'], 'BOM': ['R-0042']}
None
False
[]
True

Costs and limits

Grouping n accepted records takes expected O(n) mapping operations and O(n+g) references for g groups. A missing-key factory can perform arbitrary work; the list factory here creates one empty container.

Common Mistakes

• Get and missing subscript lookup have different mutation behavior.
• Return a fresh mutable value from each factory call.

Connected lessons

Python dictionaries: insertion order and duplicate-key replacement, Python shallow and deep copies: preserve aliases deliberately, Python aggregation exercise: validate records before returning group totals.

Trace the next boundary

Python dict.setdefault: the returned collection remains mutable shared state.