List-Wrapping in sequence() and fallback()

Decision

sequence() returns list[results] on success — all child results collected up to the threshold. fallback() and selector() return [result] — a single-element list wrapping the first truthy result. On failure, both return the last falsy scalar result directly.

Context

A behavior tree node must return a truthy value on success and a falsy value on failure. For composite nodes that aggregate multiple children, returning a bare scalar on success loses information about which children ran and what they returned.

Why List-Wrapping

The return value is always truthy on success. An empty list [] is falsy in Python — returning a list guarantees that a non-empty result list is truthy without any special casing. sequence with one successful child returns [result]; with three, [r1, r2, r3].

All child results are preserved. Callers that need to inspect what happened (logging, debugging, conditional branching on results) have the full picture.

success_threshold generalizes sequence and fallback. With success_threshold=len(children), all children must succeed (classical sequence). With success_threshold=1, the first success wins (fallback). The list return covers both: it always contains exactly the results collected up to the threshold.

Trade-offs

Callers that only need a boolean must test bool(result) or if result: — the list is truthy, not True
Callers that expect a single value must unwrap: result[0] for fallback, result[-1] for the last sequence element
Failure returns a bare scalar (the last falsy result), so success and failure have different types — callers must check truthiness before indexing