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PY-COLL0002 · literal_pair_sequence_mapping_candidate

Count fixed pair sequences whose complete use has unique-key mapping semantics.

This is a deterministic rule for python. Read its implementation.

Definition

Section titled “Definition”

Inspect names one callable body initializes with a list or tuple literal holding at least two two-tuples. Require every key to be a unique literal of one exact supported kind. Require the name to have one assignment and every read to be the iterable of a lookup loop that unpacks key, value, compares the key exactly once, and directly returns the matching value. The result is the number of sequences for which a dictionary states the observed contract more clearly.

Evidence

Section titled “Evidence”

Each finding identifies the fixed table, proven unique key count, and every lookup-loop line. No automatic fix is offered because callers, public annotations, and missing-key behavior may require a coordinated refactor even when the local data structure is unambiguous. The value is the number of pair sequences a dictionary would state more clearly.

Exceptions

Section titled “Exceptions”

Preserve sequences when keys repeat, key expressions are dynamic, key kinds mix, order is read, pairs escape, the sequence is returned, a loop consumes whole pairs, or any use has effects beyond exact lookup. A module constant is never a candidate, because its readers are every file that imports it and the file declaring it cannot see them. A sequence supplied by a caller is not assumed unique from its annotation. Ruff C406 continues to own literal sequences passed directly to dict, while Ruff SIM116 owns repeated if return tables. This rule covers neither form.

Examples

Section titled “Examples”

Bad

`rows = [("open", True), ("closed", False)]` followed only by loops that return the value for
a matching key behaves as a dictionary and is reported.


Good

Repeated keys, ordered priority rules, a returned pair list, and a loop that processes every pair remain sequences. A literal passed directly to dict remains Ruff’s diagnostic.

References

Section titled “References”
  • Cites “The Python Language Reference”, mapping types. Open reference
  • Cites “The Python Language Reference”, dictionary displays. Open reference
  • Cites Ruff C406 unnecessary-literal-dict. Open reference
  • Cites Ruff SIM116 if-else-block-instead-of-dict-lookup. Open reference