IntVar - to detect a change in it

[Frank Niessink]

Ole-Kristian Villaboe <olevi at my-deja.com> wrote:
> Is there anyway an altering of a variable (Int-, String- or Double-)
> (by the user, via the GUI) could call a function in order to tell the
> environment that it has changed since the initial setting(or a static
> comparison value)

Assuming you're talking about Tkinter here: below's an example of using
trace() to trace a variable:

class VLabel(Label):
        """VLabel class - becomes red when textvariable.valid() returns
	false"""

	def __init__(self, master, textvariable, **cnf):
		apply(Label.__init__, (self, master), cnf)
		self.config(textvariable=textvariable)
		self.textvariable = textvariable
		textvariable.trace('w',self.notify)

	def notify(self, *args):
		if self.textvariable.valid():
			self.config(bg=self.config('bg')[3])
		else:
			self.config(bg='red')

Everytime the textvariable is changed notify() gets called, which checks
whether the textvariable is valid. If not, the background is colored red,
if it is valid, the background is set to its default color.

HTH, Frank
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