How to prevent a user from entering letters in a Python 3.8 tkinter Entry widget?-CodePudding

I want to restrict users to enter digits only in an Entry widget.

I have been able to capture the key after it has been entered and displayed as well as extract what is already in the Entry widget; but I have not found a way of preventing the key from being added to the entry box.

The manipulation I am able to do on self.entry.get(), appears to be restricted to being prior to event.char being added, rather than after.

For example, entering "12b" in the code below, I get "1b"; but what I want is "12".

I tried to change the value of event.char=""; but that did not work.

Any help will be appreciated.

# generic python libraries
import tkinter as tk
from tkinter import ttk, END


class MyWindow(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)

        self.entry = ttk.Entry(self)
        self.entry.grid(row=0, column=0, padx=5, pady=5)
        self.entry.bind('<Key>', self.handle_entry_key_press)

    def handle_entry_key_press(self, event) -> None:
        if not event.char.isdigit():
            print(self.entry.get(),  event.char)
            self.entry.delete(self.entry.index(END)-1)


if __name__ == "__main__":
    root = tk.Tk()
    MyWindow(root).grid()
    root.mainloop()

CodePudding user response：

You can use the following:

class MyWindow(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)
        sv = StringVar()
        sv.trace_add("write", lambda _a,_b,_c, sv = sv: self.delete_last_char(sv))
        self.entry = ttk.Entry(self, textvariable = sv)
        self.entry.grid(row=0, column=0, padx=5, pady=5)

    def delete_last_char(self, var):
        print(var.get())
        var.set("".join([c for c in self.entry.get() if c.isdigit()]))

Instead of catching key events, it checks variable each time it changes and removes any non-digit characters.