[Tkinter-discuss] validation woes

Guilherme Polo ggpolo at gmail.com
Mon Oct 6 22:22:28 CEST 2008


On Wed, Oct 1, 2008 at 12:15 AM, Jeff Cagle <jrcagle at juno.com> wrote:
> I want an Entry widget to accept a single digit only.  Here's the attempt:
>
> ---
> import Tkinter as Tk
>
> class MyEntry(Tk.Entry):
>
>   def __init__(self, parent, **kwargs):
>       kwargs['validate']='key'     # could also be 'all' -- has the same
> result
>       kwargs['vcmd']=self.validate
>       Tk.Entry.__init__(self, parent, **kwargs)
>
>   def validate(self):
>       print self.get()  # diagnostic only
>       if self.get() in [''] + list("123456789"):
>           return True
>       else:
>           self.bell()
>           return False
>
> if __name__=="__main__":
>   main = Tk.Tk()
>   main.e = MyEntry(main)
>   main.e.grid()
>   main.mainloop()
>
> ---
>
> I'd like to think this is straightforward, but No.
>
> I type in MyEntry: 1
> printed value: (empty string)
> contents of MyEntry: 1
>
> I type in MyEntry: 11
> printed value: 1
> contents of MyEntry: 11
>
> I type in MyEntry: 111 (bell rings)
> printed value: 11
> contents of MyEntry: 11
>
> I type in MyEntry: a
> printed value: (empty string)
> contents of MyEntry: a
>
> I type in MyEntry: a1 (bell rings)
> printed value: a
> contents of MyEntry: a
>
> Near as I can tell, validate() is called after a key is pressed and
> displayed in the Entry widget, but *before* the pressed key has been added
> to MyEntry's contents.  Hence, validate() has no way of knowing what the new
> keypress is -- it can only validate the past entry.

validate does have a way to know that.

> Once it does so, the
> new key is added to MyEntry's contents.
>
> How can validate() discover the new keypress in order to validate it prior
> to addition to the Entry widget?  Yikes!
>

Tkinter is a bit unfortunate in this situation, but Tk does provide
what you are after so you can use it in Tkinter.

You will have to change your:

kwargs['vcmd']=self.validate

to (after you call the __init__ method from Tk.Entry):

kwargs['vcmd'] = (self.register(self.validate), '%P') # '%P' pass the
possible new value for the entry, if it is validated

or to:

kwargs['vcmd'] = (self.register(self.validate), '%S') # '%S' will pass
the text being inserted/deleted

probably, based on what you described. There are some other
substitutions that could be applied and combined if you are
interested, they are all described in the entry widget manual in the
tcl/tk site.

Also, I believe if you use a textvariable with this entry then it will
contain the new text when the validatecommand callback is invoked.

> Thanks in advance,
> Jeff Cagle
>
>
> _______________________________________________
> Tkinter-discuss mailing list
> Tkinter-discuss at python.org
> http://mail.python.org/mailman/listinfo/tkinter-discuss
>



-- 
-- Guilherme H. Polo Goncalves


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