I'm starting to work with classes with a Tkinter app, but I don't seem to understand how classes work, especially the relationship parent-controller. As you can see in the code down below, I was planning to have an outer class for a whole section, then 4 inner classes for every section within that frame. However, I cannot call those classes from the initial frame. Is there any better way to do this? What is it that I'm doing wrong?

```
class MainScreenFrameCenter(tk.Frame):
    def __init__(self, parent, controller,*args,**kwargs):
        tk.Frame.__init__(self,parent, bg="white",height=680, width=640,highlightbackground="black", highlightthickness=1)
        self.controller = controller
        self.pack(side="top",  fill="both", expand=True)

        self.widgets_nw = MainScreenFrameCenterNW(parent=self,controller=self)
        self.widgets_sw = MainScreenFrameCenterSW(parent=self,controller=self)
        self.widgets_ne = MainScreenFrameCenterNE(parent=self,controller=self)
        self.widgets_se = MainScreenFrameCenterSE(parent=self,controller=self)

    class MainScreenFrameCenterNW(tk.Frame):
        def __init__(self, parent, controller,*args,**kwargs):
            tk.Frame.__init__(self,parent,height=350,width=640,bg="white",highlightbackground="black",highlightthickness=1)
            self.controller = controller
            self.grid(row=0,column=0,sticky="nsew")
    class MainScreenFrameCenterSW(tk.Frame):
        def __init__(self, parent, controller,*args,**kwargs):
            tk.Frame.__init__(self,parent,height=350,width=640,bg="white",highlightbackground="black",highlightthickness=1)
            self.controller = controller
            self.grid(row=1,column=0,sticky="nsew")
    class MainScreenFrameCenterNE(tk.Frame):
        def __init__(self, parent, controller,*args,**kwargs):
            tk.Frame.__init__(self,parent,height=350,width=640,bg="white",highlightbackground="black",highlightthickness=1)
            self.controller = controller
            self.grid(row=0,column=1,sticky="nsew")
    class MainScreenFrameCenterSE(tk.Frame):
        def __init__(self, parent, controller,*args,**kwargs):
            tk.Frame.__init__(self,parent,height=350,width=640,bg="white",highlightbackground="black",highlightthickness=1)
            self.controller = controller
            self.grid(row=1,column=1,sticky="nsew") 

```