how to implement multiple inheritance?

Question

please help to fix the script.

import tkinter
import sys


class mainMenu(tkinter.Frame):

    def __init__(self, parent):
        tkinter.Frame.__init__(self, parent)
        self.pack(side = 'top', fill = 'x')
        self.parent = parent  
        self.make_menu_bar()

    def make_menu_bar(self):
        menubar = tkinter.Menu(self.parent) 
        self.parent.config(menu = menubar)                           

        file = tkinter.Menu(menubar, tearoff = False)
        file.add_command(label = 'Quit', command = sys.exit())
        menubar.add_cascade(label = 'File', menu = file)  


class MainFrame(tkinter.Frame, mainMenu):

    def __init__(self, parent):
        tkinter.Frame.__init__(self, parent)
        self.pack(side = 'top', fill = 'x', expand = 'yes')
        self.parent = parent  
        self.make_elements()

    def make_elements():
        self.menu = TextPadMenu.__init__(self, parent)


root = MainFrame(tkinter.Tk())
root.mainloop()

the problem is that the class MainFrame can not inherit from: tkinter.Frame, mainMenu. error message is:

Traceback (most recent call last): File "C:\Python33\projects\TEXTPADS\textPad_OOP\q.py", line 22, in class MainFrame(tkinter.Frame, mainMenu): TypeError: Cannot create a consistent method resolution order (MRO) for bases Frame, mainMenu

Why do you want MainFrame to inherit from Frame and mainMenu? mainMenu already inherits from Frame, so there's no need to inherit from Frame again. — BrenBarn
– BrenBarn, Commented Feb 7, 2014 at 9:02
this simple case study, it has no practical value. important for me to understand the principle — Sergey
– Sergey, Commented Feb 7, 2014 at 9:37
The principle is, don't make a class that inherits from the same class multiple times. If class B inherits from A, then don't make a class C that explicitly inherits from A and B. Just make it inherit from B, and that will automatically also make it inherit from A. — BrenBarn
– BrenBarn, Commented Feb 7, 2014 at 9:43

Bryan Oakley · Accepted Answer · 2014-02-07 15:39:38Z

This is not a tkinter problem, this is simply how classes work in python. The short answer to your question is, you can't do what you want. It's possible to hack a workaround, but why? You shouldn't ever inherit both from a class, and from the base class of that class.

To illustrate the fact it's not a Tkinter program, here's a minimal solution that gives the same error:

class Base(object): pass
class Example1(Base): pass
class Example2(Base, Example1): pass

When run, this yields the following output:

bash-3.2$ python example.py
Traceback (most recent call last):
  File "example.py", line 3, in <module>
    class Example2(Base, Example1): pass
TypeError: Error when calling the metaclass bases
    Cannot create a consistent method resolution
order (MRO) for bases Example1, Base

Understanding the method resolution order can be difficult when you step out of the bounds of normal use. If you want a deeper answer to the question "why can I not do this?", start here:

http://www.python.org/download/releases/2.3/mro/

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