How Do I Properly Call Parent Class `__init__` Methods in Multiple Inheritance?

Calling Parent Class init with Multiple Inheritance

In scenarios where multiple inheritance is used, it's crucial to ensure that all parent class constructors are called. Two common approaches are:

• ParentClass.__init__(self) (old-style)
• super(DerivedClass, self).__init__() (newer-style)

However, if parent classes do not follow a consistent convention, these approaches may fail.

Determining the Correct Approach

The appropriate approach depends on whether the base classes are designed for multiple inheritance:

1. Standalone Base Classes

• Not designed for multiple inheritance.

• Manually call each parent constructor using either:

  • Without super: Foo.__init__(self), Bar.__init__(self)
  • With super: super().__init__() (for all constructors up to Foo), super(Foo, self).__init__(bar) (for all constructors after Foo)

2. Mixins

• Designed for multiple inheritance.
• Use super().__init__() in the mixin class, which automatically calls the next constructor.
• Inherit from the mixin first, e.g., class FooBar(FooMixin, Bar).

3. Classes Designed for Cooperative Inheritance

• Similar to mixins, but all arguments are passed as keyword arguments.
• Call super().__init__() in all classes.
• Order of base classes does not matter.

Additional Considerations

• For object subclasses, avoid calling super().__init__().
• For standalone classes, always provide an empty constructor if directly inheriting from object (e.g., class Base(object): def __init__(self): pass).

Ultimately, the correct implementation depends on the classes involved. If a class is designed for multiple inheritance, it should be documented accordingly. Otherwise, assume it's not designed for such scenarios.

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