Runtime error: super-class __init__() never called

[Ed Leafe]

As I mentioned in a previous message, I'm trying to get Dabo to work with Python3 and wxPython 4.x. Dabo works mainly by the use of mixin classes that add a consistent interface to all objects, but these mixins can make debugging difficult sometimes.

As I've progressed, I'm now seeing the error message in the subject appearing consistently. So I've created a small sample program that illustrates the problem using a very simple mixin class:

# simple.py

import wx

class SimpleMixin():

    def __init__(self, name=None):

        print("Mixin init")

        self._name = name or "Default"

        print("Mixin init DONE")

    def simple_method(self):

        print(id(self), self._name)

class SimpleFrame(SimpleMixin, wx.Frame):

    def __init__(self, parent, name=None):

        print("Subclass init")

        super(SimpleFrame, self).__init__(name)

        print("Subclass init DONE")

if __name__ == "__main__":

    app = wx.App()

    f = SimpleFrame(None, "Test")

    f.Show()

    app.MainLoop()

When I run this program, I get the following output:

(wxenv)ed@imac:~/projects/dabo(2phase)$ python simple.py

Subclass init

Mixin init

Mixin init DONE

Subclass init DONE

Traceback (most recent call last):

  File "simple.py", line 22, in <module>

    f.Show()

RuntimeError: super-class __init__() of type SimpleFrame was never called

(wxenv)ed@imac:~/projects/dabo(2phase)$

It shows that the __init__() methods of both the subclass and the mixin were called and completed in the expected order, so I don't understand why I'm seeing this error message.

-- Ed Leafe

[Robin Dunn]

Ed Leafe wrote on 1/26/19 1:18 PM:
[...]

When I run this program, I get the following output:

(wxenv)ed@imac:~/projects/dabo(2phase)$ python simple.py

Subclass init

Mixin init

Mixin init DONE

Subclass init DONE

Traceback (most recent call last):

  File "simple.py", line 22, in <module>

    f.Show()

RuntimeError: super-class __init__() of type SimpleFrame was never called

(wxenv)ed@imac:~/projects/dabo(2phase)$

It shows that the __init__() methods of both the subclass and the mixin were called and completed in the expected order, so I don't understand why I'm seeing this error message.

The mixin class also needs to call super(...).__init__ in order for the MRO chain to continue to be followed. See

https://stackoverflow.com/questions/3277367/how-does-pythons-super-work-with-multiple-inheritance

--
Robin

[Chris Barker]

yup -- there are three guidelines for using super -- and both seminal articles:

"Super Considered Harmful"

https://fuhm.net/super-harmful/

and 

"Super considered Super"

https://rhettinger.wordpress.com/2011/05/26/super-considered-super/

say similar things:

• the method being called by super() needs to exist
  
• the caller and callee need to have a matching argument signature
  
• and every occurrence of the method needs to use super()
  

That last one is killing you --  nothing is calling wx.Frame's __init__

SimpleFrame's __init__ is calling super()'s __init__, which calls SimpleMixin's __init__ -- but then it's done. SImpleMixin's __init__ isn't calling anything else, so wx.Frame's __init__ never gets called.

The way I think of it is this:

super() does not assure that all the superclasses' methods get called -- rather, what it does is assure that each one in the MRO gets called only once.

The basic process of only one method getting called still holds -- the whole stack only gets called if EVERY method in the stack calls its superclass method -- whether by super() or directly.

Maybe this will help:

https://uwpce-pythoncert.github.io/PythonCertDevel/modules/MultipleInheritance.html

If not -- let me know, I'd like to improve it :-) You can post issues or PRs on:

https://github.com/UWPCE-PythonCert/PythonCertDevel

-CHB

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Christopher Barker, Ph.D.
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