BUG: Fix broken multiple inheritance in ScriptedLoadableModule classes

[allemangD]

Missing calls to super().__init__() in the ScriptedLoadableModule classes break multiple inheritance. The init method is skipped in any derived classes that come after the ScriptedLoadableModule class in the MRO.

Adding super calls to each of the ScriptedLoadableModule classes fixes this issue and correctly supports multiple inheritance.

• ScriptedLoadableModule.__init__
• ScriptedLoadableModuleWidget.__init__
• ScriptedLoadableModuleLogic.__init__

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For example, this class fails to initialize VTKObservationMixin:

class MyModuleLogic(ScriptedLoadableModuleLogic, VTKObservationMixin):
  def __init__(self):
    super().__init__()
    
    self.addObserver(...)  # AttributeError on self.Observations

In this case, swapping the base class order avoids the error as VTKObservationMixin does include a super call, however that solution is not perfect. MyModuleLogic.__base__ is not ScriptedLoadableModuleLogic as it should be, and it is impossible to pass the optional parent parameter to ScriptedLoadableModuleLogic.__init__.

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This change should not have any backwards compatibility issues, as currently the only way to support mixins is to explicitly call the mixin initializer, in which case the behavior of super() is ignored.

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ScriptedLoadableModuleTest cannot be fixed because its base class, unittest.TestCase, does not have a super call. For such cases it would still be necessary to explicitly call the mixin initializer.

[jcfr]

Thanks for the detailed explanation 🙏

Following b2ba482 (ENH: Add support for building Python 3.6) removing support for python 2.7, only new style python classes are supported and using super() makes sense.

For references:

• https://docs.python.org/3/library/functions.html#super
• https://www.python.org/doc/newstyle/
• https://docs.python.org/3/tutorial/classes.html
• https://wiki.python.org/moin/NewClassVsClassicClass?action=recall&rev=20

[allemangD]

ScriptedLoadableModuleTest cannot be fixed because its base class, unittest.TestCase, does not have a super call. For such cases it would still be necessary to explicitly call the mixin initializer.

https://docs.python.org/3/library/functions.html#super

Reading through that documentation more carefully, it seems it is possible to fix the behavior for ScriptedLoadableModuleTest as well. @jcfr thanks for posting the link! It helps to read things even if you already know the bulk of it, I learned something new!

The object-or-type determines the method resolution order to be searched. The search starts from the class right after the type.

This indicates that super(UnitTest, self) would take the next base type of self after UnitTest. Below is a minimal example of the class hierarchy:

class Mixin:
    def __init__(self):
        super().__init__()
        
        print('Mixin')


class UnitTest:
    def __init__(self):
        # super call cannot be added.
        
        print('UnitTest')


class ModuleTest(UnitTest):
    def __init__(self):
        # next class in MRO after UnitTest (Mixin.__init__)
        super(UnitTest, self).__init__() 
        
        # immediate next class in MRO (UnitTest.__init__) 
        super(ModuleTest, self).__init__()

        print('ModuleTest')


class MyModuleTest(ModuleTest, Mixin):
    def __init__(self):
        super().__init__()

        print('MyModuleTest')

_ = MyModuleTest()  # Mixin / UnitTest / ModuleTest / MyModuleTest

There is a risk in this implementation: if a super call is ever added to the UnitTest class, then Mixin.__init__ (and object.__init__) would be invoked twice.

I will add commits to this branch introducing the above change, along with a test to check for regression that ensures Mixin.__init__ is only ever called once.