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

[Dustan]

James Stroud wrote:
> Dustan wrote:
> > 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
> >
> >
>
> Actually, lambda is not necessary for event binding, but a closure (if I
> have the vocab correct), is:

Of course. What I should have said was "lambda is the quickest and
easiest way". However, if you want your intent to be clear, lambda
isn't always the best option, but it is quite often the quickest.

>
> import Tkinter as tk
>
> def make_it(x):
>    def highliter(x=x):
>      print "highlight", x
>    return highliter
>
> class Test:
>      def __init__(self, parent):
>          buttons = [tk.Button(parent, text=str(x+1),
>                     command=make_it(x)) for x in range(5)]
>          for button in buttons:
>              button.pack(side=tk.LEFT)
>
> root = tk.Tk()
> d = Test(root)
> root.mainloop()
>
>
> --
> James Stroud
> UCLA-DOE Institute for Genomics and Proteomics
> Box 951570
> Los Angeles, CA 90095
> 
> http://www.jamesstroud.com/