validate

Check the expression against the request's substitutions, before any of it is evaluated.

DynamoDB rejects an expression naming an undefined or malformed expression attribute value as a request validation error, whatever the item holds - so the check cannot ride on eval, where AND/OR short-circuiting decides whether a branch is reached at all. The default is a no-op; the nodes which resolve a substitution, and the combinators which have to pass the walk on to their sub-expressions, override it.