Object Oriented Tkinter Coding: Changing text and Updating - python-2.7

Question

So I have been teaching myself Object Oriented Programming for Tkinter projects as I clearly see that they are much more organized for large amounts of coding. However I must admit that I've been coasting by simply copying various bits of coding from online, not fully understanding what its purpose is.
This has lead me to the point that my code does not work at all and I have no idea why not. The first issue is an issue with simply changing an aspect of other widgets.
I have this sample code:
import Tkinter as tk
LARGE_FONT = ("Verdana", 12)
class SeaofBTCapp(tk.Tk):
def __init__(self, *args, **kwargs):
tk.Tk.__init__(self, *args, **kwargs)
container = tk.Frame(self)
container.pack(side="top", fill = "both", expand = True)
container.grid_rowconfigure(0, weight = 1)
container.grid_columnconfigure(0 , weight = 1)
self.frames = {}
frame = StartPage(container, self)
self.frames[StartPage] = frame
frame.grid(row = 0, column = 0, sticky = "nsew")
self.show_frame(StartPage)
def show_frame(self, cont):
frame = self.frames[cont]
frame.tkraise()
class StartPage(tk.Frame):
def __init__(self, parent, controller):
tk.Frame.__init__(self, parent)
label = tk.Label(self, text = "Start Page", font = LARGE_FONT)
label.pack(pady = 10, padx = 10)
button = tk.Button(self, text = "Change Label", command = self.change)
button.pack(pady = 10, padx = 10)
def change(self):
label["text"] = "It has changed"
app = SeaofBTCapp()
app.mainloop()
Which SHOULD be a simple enough code that, with a button press, change the label from "Start Page" to "It has changed". But whenever I run it, it says that the global variable "label" is not defined. Additionally, if I then change it to self.label, it states that StartPage instance has no attribute 'label'. I don't know what I'm doing wrong.
Additionally, in a similar vein, I'm working on a project that has a SideBar class and a Main class tied to one MainApplication class. The Main class takes a value and displays it on a Frame in the Main class. Following this, a button in the SideBar increases that value by 1. But the Main display doesn't update and I have no idea how to tie the Main updating with the button in the SideBar.
import Tkinter as tk
something = [0, 6]
class Main():
def __init__(self, root):
mainboard = tk.Frame(root, height = 100, width = 100)
self.maincanvas = tk.Canvas(mainboard, bd = 1, bg = "white")
mainboard.grid(row = 0, column = 0)
self.maincanvas.grid(row = 0, column = 0)
self.maincanvas.create_text(45, 50, anchor = "center", text = str(something[1]))
class SideBar():
def __init__(self, root):
sidebarframe = tk.Frame(root, height = 100, width = 100)
button = tk.Button(sidebarframe, width = 20, text = "Change Value", command = self.add)
sidebarframe.grid(row = 0, column = 1)
button.grid(row = 0, column = 0)
def add(self):
something[1] += 1
print something[1]
class MainApplication():
def __init__(self, parent):
self.parent = parent
self.sidebar = SideBar(self.parent)
self.main = Main(self.parent)
if __name__ == "__main__":
root = tk.Tk()
MainApplication(root)
root.mainloop()
All help would be appreciated, but please try and not use a lot of technical terms, as I am still learning.

Answer 1

In the first scenario replace:
label = tk.Label(self, text = "Start Page", font = LARGE_FONT)
label.pack(pady = 10, padx = 10)
With:
self.label = tk.Label(self, text = "Start Page", font = LARGE_FONT)
self.label.pack(pady = 10, padx = 10)
And also in the function change put it like this:
self.label["text"] = "It has changed"
And in your second problem i changed the code a little bit so it works:
import Tkinter as tk
something = [0, 6]
class Main():
def __init__(self, root):
mainboard = tk.Frame(root, height = 100, width = 100)
self.maincanvas = tk.Canvas(mainboard, bd = 1, bg = "white")
mainboard.grid(row = 0, column = 0)
self.maincanvas.grid(row = 0, column = 0)
self.maincanvas.create_text(45, 50, anchor = "center", text = str(something[1]))
class SideBar():
def __init__(self, root, main):
self.main = main # Putting the main object to self.main
sidebarframe = tk.Frame(root, height = 100, width = 100)
button = tk.Button(sidebarframe, width = 20, text = "Change Value", command = self.add)
sidebarframe.grid(row = 0, column = 1)
button.grid(row = 0, column = 0)
def add(self):
something[1] += 1
self.main.maincanvas.delete("all") # Removing everything from canvas
self.main.maincanvas.create_text(45, 50, anchor = "center", text = str(something[1])) # Inserting the new value
print something[1]
class MainApplication():
def __init__(self, parent):
self.parent = parent
self.main = Main(self.parent) # The main needs to run first
self.sidebar = SideBar(self.parent, self.main) # So that SideBar can use its canvas
if __name__ == "__main__":
root = tk.Tk()
MainApplication(root)
root.mainloop()