CellProlog

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

Details

CellProlog:>expr evaluates expr for every ordinary kernel evaluation done in the cell, as initiated for example by or by Evaluate in Place.
CellProlog is not evaluated for evaluations associated with Dynamic objects.
The default setting is CellProlog->None.
CellProlog is often set at a style or notebook level, rather than for individual cells.

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Set the option CellProlog:>(x=42) on the second input cell in this example:

Though the variable x has not yet been set, it acquires the value 42 upon the evaluation of the input:

Set options on a cell to numericize output from a particular input cell using $Post:

This cell produces a numerical result:

Other cells are not affected:

Rules for setting up local package contexts for a cell:

Newly created symbols in the following cell will be in the context "c1`":

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