Help me understand this... (Classes and self).

[Yoriz]

You need to make

tab_one = tk.Frame(nb) # <-- target actual # #

accessible by adding it as a variable of MainFrame

self.tab_one = tk.Frame(nb) # <-- target actual # #
nb.add(self.tab_one, text='Entries')

then you can use

tab1 = TabOne(main_nb.tab_one) #This doesn't work; "main_nb.tab_one" (or whatever) throws error.