Practical Programming in Tcl & Tk, Third Edition

Button Commands and Scope Issues

Perhaps the trickiest issue with button commands has to do with variable scoping. A button command is executed at the global scope, which is outside of any procedure. If you create a button while inside a procedure, then the button command executes in a different scope later. The commands used in event bindings also execute later at the global scope.

I think of this as the “now” (i.e., button definition) and “later” (i.e., button use) scope problem. For example, you may want to use the values of some variables when you define a button command but use the value of other variables when the button command is used. When these two contexts are mixed, it can be confusing. The next example illustrates the problem. The ...