the this argument to watch
callback to invoke when the expression changes
The newly installed watch. Note that the only reference held to the watch will be released when the watch is unwatched which can happen automatically, and not just via gtk.expression_watch.ExpressionWatch.unwatch. You should call gtk.expression_watch.ExpressionWatch.ref_ if you want to keep the watch around.
Watch the given expression for changes.
The notify function will be called whenever the evaluation of self may have changed.
GTK cannot guarantee that the evaluation did indeed change when the notify gets invoked, but it guarantees the opposite: When it did in fact change, the notify will be invoked.