[tkinter] question about correct use of validation

[Peter Otten]

steve wrote:

> for determine the maximum number of characters in an entry
> 
> I have read several interpretations for the solution of the problem, but
> I wanted to find an alternative way (for convenience of the code)
> 
> I kindly ask for an opinion on the use of validation in this way.
> 
> -----
> 
> problem: limit number of characters in different entries of a form.
> 
> code:
> 
> #!/usr/bin/python
> # -*- coding: utf-8 -*
> 
> from Tkinter import *
> 
> 
> class View():
> 
>      def __init__(self, parent):
>          self.parent = parent
>          self.make_ui()
> 
>      def make_ui(self):
>          ''' create user interface '''
>          self.id_entry = Entry(self.parent, width=6)
>          # take out the properties for understanding
> 
>          vcmd = (self.parent.register(self.maxlength_validate), '%P', 4)
> # 4 is my question
> 
>          self.id_entry.configure(validate="key", validatecommand=vcmd)
>          self.id_entry.pack()
> 
>          self.name_entry = Entry(self.parent, width=30)
>          # take out the properties for understanding
> 
>          vcmd = (self.parent.register(self.maxlength_validate), '%P', 20)
> # 20 is my question
> 
>          self.name_entry.configure(validate="key", validatecommand=vcmd)
>          self.name_entry.pack()
> 
>      def maxlength_validate(self, value, maxlength):
>          ''' function validated for maximum number of characters '''
>          maxlength = int(maxlength)
>          if len(value) > maxlength:
>              value = value[:maxlength]
>              return (value == ' ')
>          return True
> 
> 
> def run():
>      root = Tk()
>      root.title('test')
>      View(root)
>      root.mainloop()
> 
> if __name__ == "__main__":
>      run()
> 
> The code works well :-) but...
> 
> in vcmd i use this:
> 
> vcmd = (self.parent.register(self.maxlength_validate), '%P', 20)
> # '20' argument is my question, is not default value (is max length of
> char, different for each entry... very comfortable for me)
> 
> is it all right, according to you, to pass a non-default argument? (no
> error from the interpreter)
> 
> Without this way I would not know how to pass the maximum number of
> characters to the validation function, I can not use one variable
> self.--- for each entry ... it would become aesthetically unattractive.
> 
> I would not even like to add a textvariable variable because anyway then
> 
> I should always pass the comparison value.
> 
> thank you in advance

A viable alternative may be to use functools.partial() to pass the 
maxlength:

from Tkinter import *

from functools import partial


class View():

    def __init__(self, parent):
        self.parent = parent
        self.make_ui()

    def make_ui(self):
        ''' create user interface '''

        def vcmd(maxlength):
            return self.parent.register(
                partial(self.maxlength_validate, maxlength=maxlength)
            ), "%P"

        self.id_entry = Entry(self.parent, width=6)
        # take out the properties for understanding

        self.id_entry.configure(validate="key", validatecommand=vcmd(4))
        self.id_entry.pack()

        self.name_entry = Entry(self.parent, width=30)
        # take out the properties for understanding

        self.name_entry.configure(validate="key", validatecommand=vcmd(20))
        self.name_entry.pack()

    def maxlength_validate(self, value, maxlength):
        return len(value) <= maxlength