Abstract syntax tree traversal

An abstract syntax tree traversal systematically visits its nodes. A depth-first traversal of a tree that processes each child before its parent is called postorder. For the program expression 2+3*4, the schematic postorder is 2, 3, 4, Mult, Add. This order lets an evaluator obtain each operand value before applying its operation. A traversal of the entire Python schema additionally visits marker nodes such as Add and Store; this shorter list concerns only the expression nodes drawn in the figures. Visiting an abstract syntax tree is a structural operation, and is not a general model of execution: a conditional may select only one branch, while a structural visitor examines both.