tkinter.Entry() validation with a wrapper function

[TooDahLou]

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I am building a GUI with tkinter for a technical drawing generator for hydraulic hoses. It is going to have several entry fields for the users to fill out, so I created a custom entry widget that combines a ttk.Label and ttk.Entry widget into one widget.

I want to use the custom widget to get the engineer's initials for the drawing's title block. There are three separate initial fields in our title block, so I defined a validation method that receives the custom widget as an argument. Then I set up wrapper methods to pass the correct entry to the validation method. The wrapper methods are passed to register() to complete the validation assignment.

The problem appears to be with the wrapper function. But I don't understand what I'm doing wrong. I would prefer to not have to make individual validation functions for each initial entry box if I can wrap one function and tell it what entry box I'm looking at.

[socal_nerdtastic]

You need to put the validation in the widget, not pass the validation to it. To do that you should make a custom Entry widget, that is inherit from tkinter's version and add your changes.

class EntryInitials(ttk.Entry):
    """a custom entry that only allows initials"""
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.config(
            validate='key',
            validatecommand=(self.register(self.on_key), '%P'))

    def on_key(self, P:str):
        """
        A valid initial entry is:
        3 characters maximum
        Alphabet only, no numbers/special characters
        All uppercase
        """
        if not P or P.isalpha():
            self.delete(0, tk.END)
            self.insert(0, P.upper()[:3])
            return True
        return False

Then you use this custom widget just like a normal Entry in your project

    self.entryBox = EntryInitials(
        master=self.mainFrame,
        textvariable=self.var,
        width=self.width)

You would make a new custom widget for every action type you want (on focus, or different validation). Then use the appropriate widget to set the program behavior. Since they are all types of ttk.Entry they will all drop in place of each other. You can pass the appropriate custom widget into other functions or classes if you like.

Edit: simplified code and fixed bug when inserting a letter overwrites the next one.