Python Forum

I'm writing a program in Tkinter.

One page is a form, where the user inputs various values.
How do I limit an Entry field to a specific size?

For example, I use the following code, so that the user can put in their zip code:

    # Zip code
    label_for_zipc = Label(main_window, text='Zip code: (First 5 digits)', fg="blue")
    label_for_zipc.config(font=('helvetica', 16))
    canvas.create_window(0.7*hshift*fnl_x, fnl_y+vertShift, window=label_for_zipc)
    canvas.create_window(0.7*hshift*fnl_x, fnl_y+vertShift+30, window=entry_for_zipc)

The function that includes the above lines includes many other entry fields.
There is a "Proceed" Button, which will write all the input to a file, as well as bringing up another
Tkinter window for more input.

The problem I have is that, even though I limited the field size/length of "entry_for_zipc" to 5,
the user can type more than 5 characters in the entry field.
How do I program it so that the user cannot type more than 5 characters in the entry field?

Thanks.

You could test the length of the input and throw an error and remind that only 5 characters are allowed.

---

Quick example

import tkinter as tk
from tkinter import messagebox

def callback(field, arg):
    arg = arg.get()
    if len(arg) != 5:
        messagebox.showerror('Error', 'field must contain 5 characters')
        field.delete(0, tk.END)
    else:
        messagebox.showinfo('Correct', 'This field has 5 characters')

root = tk.Tk()
var = tk.StringVar()
contain = tk.LabelFrame(root, text='Some label text')
contain.pack(fill='both', expand=True)

field = tk.Entry(contain, textvariable=var, bd=2)
field.pack(fill='x', expand=True)

btn = tk.Button(contain, text='Submit', command=lambda: callback(field, var))
btn.pack(side='left', fill='x', pady=5)

root.mainloop()

You can use a validation callback to limit what is typed in an entry.

import tkinter as tk
from tkinter import messagebox

class Window(tk.Tk):
    def __init__(self):
        super().__init__()

        label = tk.Label(self, text="Zip Code")
        reg = self.register(self.zip_validate)
        self.zip_code = tk.StringVar(self, '')
        entry = tk.Entry(
            self,
            textvariable=self.zip_code,
            width=7,
            justify=tk.RIGHT,
            validate='key',
            validatecommand=(reg, '%P'))
        button = tk.Button(self, text='Accept', command=self.submit)

        button.pack(side=tk.BOTTOM, padx=5, pady=5, expand=True, fill=tk.X)
        label.pack(side=tk.LEFT, padx=5, pady=5)
        entry.pack(side=tk.RIGHT, padx=5, pady=5)

    @staticmethod
    def zip_validate(zip_code):
        return zip_code.isdigit() and len(zip_code) <= 5

    def submit(self):
        if len(self.zip_code.get()) < 5:
            messagebox.showerror('Error', 'Zip code must be 5 digits', parent=self)


Window().mainloop()

This example registers a validation callback function named 'zip_validate'. The function is called whenever a key is pressed ('key'). The function is passed a preview of what the entry contents will be if the key press is accepted ('%P'). The validator returns True if the new content is acceptable, else False.

Note that the validate function does not force the zip code to be 5 digits long. '123' is valid. To ensure that the zip code is 5 digits long you would still need some type of form validation very much like in menator's post.

In this example the validate function is a static method, but it can be a standalone function.

Read about validators here:

https://anzeljg.github.io/rin2/book2/240...ation.html
https://www.geeksforgeeks.org/python-tki...ry-widget/

Thanks everyone. I'll probably try some version of menator's code.
It seems silly to me that Python/Tkinter doesn't have this built-in.