Wired that ttk Label textvariable can't be displayed in class properly

Question

I am totally newbie of Python. However i have to use python 3 to do some projects. I am really confused by ttk.Label with textvariable. Below is my codes.

from tkinter import *
from tkinter import ttk

class new_label:

    def __init__(self, master):
        self.master = master
        self.label_var = StringVar()

        ttk.Label(self.master, text="iii").grid(row=0, sticky = "w")
        self.create_label()

    def create_label(self):
        self.l1 = ttk.Label(f,
                           textvariable = self.label_var,
                           foreground = "red",)
        self.l1.grid(row=1)
        self.label_var.set("First Label")
        print(self.l1.cget("text"))

r=Tk()
r.title("My Label Update")

f=ttk.Frame(r)
f.grid(row=0)

new_label(f)
r.mainloop()

In my codes I add a print and it can print the text well. However the text can't display out and there is no any message of any errors. I do need someone to help for this issue. Thank you very much in advance.

Answer 1

You aren't keeping a reference so the instance of new_label, so the python garbage collector is collecting it. The ttk widgets are particularly sensitive to the use of garbage-collected StringVar instances.

The simple solution is to keep a reference to your instance of new_label:

x = new_label(f)