[Tkinter-discuss] validating an entry

Michael Lange klappnase at web.de
Tue Aug 22 11:49:28 CEST 2006

On Mon, 21 Aug 2006 23:03:57 -0400
Jeff Cagle <jrcagle at juno.com> wrote:

> So it turned out that validating was not too hard, but you had to Google 
> for the documentation.  Your name, Cameron, showed up a couple of times...
> Here's the code:
> # Testing an edit box with validation
> from Tkinter import *
> class MyEntry(Entry):
>     def __init__(self, master, maxchars):
>         Entry.__init__(self, master, validate = "key", 
> validatecommand=self.validatecommand)
>         self.MAX = maxchars
>     def validatecommand(self, *args):
>         return len(self.get()) < self.MAX 
> if __name__ == '__main__':
>     tkmain = Tk()
>     e = MyEntry(tkmain, 5)
>     e.grid()
>     tkmain.mainloop()
> The validatecommand has to return a Boolean.  I have no idea of the 
> following, and haven't been able to find answers yet online:
> 1) what args are passed to validatecommand?  Obviously *args scoops them 
> up, but would one ever want to use them?
> 2) what values other than "key" are acceptable to validate?

Hi Jeff,

unfortunately these things do not seem to be documented for Tkinter, the Tk manpages
are your friend. From the entry man page:

       Command-Line Name:-validate
       Database Name:  validate
       Database Class: Validate

              Specifies  the  mode  in  which validation should operate: none,
              focus, focusin, focusout, key, or all.   It  defaults  to  none.
              When  you  want validation, you must explicitly state which mode
              you wish to use.  See Validation below for more.

       Validation  works  by  setting  the  validateCommand option to a script
       which will be evaluated according to the validate option as follows:

       none      Default.  This means no validation will occur.

       focus     validateCommand will be called when  the  entry  receives  or
                 loses focus.

       focusin   validateCommand will be called when the entry receives focus.

       focusout  validateCommand will be called when the entry loses focus.

       key       validateCommand will be called when the entry is edited.

       all       validateCommand will be called for all above conditions.

       It is possible to perform percent substitutions on the  validateCommand
       and  invalidCommand, just as you would in a bind script.  The following
       substitutions are recognized:

       %d   Type of action: 1 for insert, 0  for  delete,  or  -1  for  focus,
            forced or textvariable validation.

       %i   Index of char string to be inserted/deleted, if any, otherwise -1.

       %P   The value of the entry if the edit is allowed.  If you are config-
            uring  the  entry  widget to have a new textvariable, this will be
            the value of that textvariable.

       %s   The current value of entry prior to editing.

       %S   The text string being inserted/deleted, if any, {} otherwise.

       %v   The type of validation currently set.

       %V   The type of validation that triggered the callback (key,  focusin,
            focusout, forced).

       %W   The name of the entry widget.

Passing these percent substitutions in Tkinter is a little tricky. Here is a code
snippet I used to validate a Spinbox widget, it should work with an Entry just the
same I think:

        vcmd = (self.register(self._validate), '%s', '%P')

    def _validate(self, old, new):

Remember that Tk will pass any values to the validatecommand as strings, even if you
would expect an integer (as in '%i').

I hope this helps


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