Tkinter: Entry box value doesn't store into StringVar() object

Question

I'm trying to see whether or not a value entered in the Entry widget is stored inside the StringVar() object but when I print the length of the string value object, it says 0.

class POS(tk.Tk):
def __init__(self,*args,**kwargs):
    tk.Tk.__init__(self, *args, **kwargs)
    """
    some code to build multiple frames here
    """

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

        frame5 = Frame(self, bg = "pink")
        frame5.pack(fill = BOTH)

        frame5Label1 = tk.Label(frame5, text = "Product Bar Code", font = NORMAL_FONT)
        frame5Label1.grid(row = 0, column = 0, padx=5, pady=5, sticky = W)

        barCode = StringVar()
        frame5EntryBox = ttk.Entry(frame5, textvariable = barCode, width = 40)
        frame5EntryBox.grid(row = 0, column = 1, padx = 5, pady = 5)

        frame5Button = ttk.Button(frame5, text = "Add Item", command = lambda: updateCustomerList(barCode))
        frame5Button.grid(row = 0, column = 2, padx = 130, pady = 10)


def updateCustomerList(barCode):
    print(len(barCode.get()))

app = POS()
app.geometry("700x700")
app.resizable(False,False)
app.mainloop()

Answer 1

Minimal working example which displays length of StringVar assigned to Entry after pressing Button

import tkinter as tk

# --- functions ---

def check():
    print('len:', len(var.get()))

# --- main ---

root = tk.Tk()

var = tk.StringVar()

ent = tk.Entry(root, textvariable=var)
ent.pack()

but = tk.Button(root, text="Check", command=check)
but.pack()

root.mainloop()

mainloop displays window with widgets so using len() before starting mainloop makes no sense because var is still empty.

---

EDIT: with your class and function outside class

import tkinter as tk
from tkinter import ttk

# --- classes ---

class MainPage(tk.Frame):

    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)

        frame5 = tk.Frame(self, bg="pink")
        frame5.pack(fill="both")

        frame5Label1 = tk.Label(frame5, text="Product Bar Code", font="NORMAL_FONT")
        frame5Label1.grid(row=0, column=0, padx=5, pady=5, sticky="w")

        barCode = tk.StringVar()
        frame5EntryBox = ttk.Entry(frame5, textvariable=barCode, width=40)
        frame5EntryBox.grid(row=0, column=1, padx=5, pady=5)

        frame5Button = ttk.Button(frame5, text="Add Item", command=lambda:updateCustomerList(barCode))
        frame5Button.grid(row=0, column=2, padx=130, pady=10)

# --- functions ---

def updateCustomerList(barCode):
    print(len(barCode.get()))

# --- main ---

root = tk.Tk()

main = MainPage(root, root)
main.pack()

root.mainloop()

Or using self. and putting function inside class as method

import tkinter as tk
from tkinter import ttk

# --- classes ---

class MainPage(tk.Frame):

    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)

        frame5 = tk.Frame(self, bg="pink")
        frame5.pack(fill="both")

        frame5Label1 = tk.Label(frame5, text="Product Bar Code", font="NORMAL_FONT")
        frame5Label1.grid(row=0, column=0, padx=5, pady=5, sticky="w")

        self.barCode = tk.StringVar()
        frame5EntryBox = ttk.Entry(frame5, textvariable=self.barCode, width=40)
        frame5EntryBox.grid(row=0, column=1, padx=5, pady=5)

        frame5Button = ttk.Button(frame5, text="Add Item", command=self.updateCustomerList)
        frame5Button.grid(row=0, column=2, padx=130, pady=10)

    def updateCustomerList(self):
        print(len(self.barCode.get()))

# --- functions ---


# --- main ---

root = tk.Tk()

main = MainPage(root, root)
main.pack()

root.mainloop()