Tkinter: How to assign calculated value to a Label

[Yoriz]

Post the bare minimum runnable code showing the problem at hand.
Dont use * imports see Namespace flooding with * imports
Create the label once and then change it's text value in the buttons event handler.
Use a class to easily keep a reference to GUI widgets.

See below code

import tkinter as tk


class Main:
    def __init__(self, root):
        self.frame = tk.Frame(root)
        lblCo2 = tk.Label(self.frame, text="CO2 (ppm)")
        lblCo2.grid(row=4, sticky=tk.W)
        self.valCo2 = tk.Label(self.frame, bg="white", width=12)
        self.valCo2.grid(row=4, column=1, sticky=tk.W)

        btnStart = tk.Button(self.frame, text="Start",
                             command=self.on_btn_start)
        btnStart.grid(row=5, column=1, sticky=tk.W)
        self.frame.pack()

    def on_btn_start(self):
        Cmd1 = [0xFE, 0x44, 0x00, 0x08, 0x02, 0x9F, 0x25]
        high = Cmd1[3]
        low = Cmd1[4]
        co2 = (high*256) + low
        self.valCo2['text'] = co2


root = tk.Tk()
main = Main(root)
root.mainloop()