CellProlog

CellProlog
is an option to Cell which gives an expression to evaluate before each ordinary evaluation of the contents of the cell.

CellProlog:>expr evaluates expr for every ordinary kernel evaluation done in the cell, as initiated for example by Shift+Enter or by Evaluate in Place.

CellProlog is often set at a style or notebook level, rather than for individual cells.

Rules for temporarily setting $Post:

Rules for setting up local package contexts for a cell: