Tkinter.Button(... command) lambda and argument problem

Dustan DustanGroups at gmail.com
Sat Sep 16 05:02:29 CEST 2006


Jay wrote:
> Thanks for the tip, but that breaks things later for what I'm doing.
>
> bearophileHUGS at lycos.com wrote:
> > In that case you don't need a lambda:
> >
> > import Tkinter as tk
> >
> > class Test:
> >     def __init__(self, parent):
> >         buttons = [tk.Button(parent, text=str(x+1),
> > command=self.highlight(x)) for x in range(5)]
> >         for button in buttons:
> >             button.pack(side=tk.LEFT)

Well, actually, that's wrong. You obviously don't understand why lambda
is necessary for event binding; in this case (and many others, for that
matter), the button gets bound to the event *returned* by
self.highlight(x), which, since nothing gets returned, would be None.
Then when you click the button, Tkinter calls None(), and out of the
blue, an error is raised.

Lambda is the safe way around that error.

> >
> >     def highlight(self, x):
> >         print "highlight", x
> >
> > root = tk.Tk()
> > d = Test(root)
> > root.mainloop()
> > 
> > Bye,
> > bearophile




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