How Should Parent Class Constructors Be Called in Multiple Inheritance?

Calling Parent Class Constructors in Multiple Inheritance: The Right Approach

Understanding the Issue

In multiple inheritance, multiple parent classes contribute their attributes and methods to a child class. To initialize the child class properly, the constructors of all parent classes need to be invoked. However, choosing the right way to do this can be challenging, especially when the parent classes have different conventions.

Approaches and Considerations

There are two typical approaches to invoking parent class constructors:

• Old-Style: Call ParentClass.__init__(self) explicitly.
• Newer-Style: Use super(DerivedClass, self).__init__().

The choice depends on whether the parent classes are designed for multiple inheritance.

Scenarios and Procedures

1. Unrelated Base Classes: If the parent classes are standalone and not designed for multiple inheritance, you must call each constructor manually using either of the above approaches.
2. One Parent Class as a Mixin: If one parent class is designed as a mixin (intended for multiple inheritance), it will call the next constructor automatically. In this case, use only super().__init__() in the child class, which will invoke all constructors (starting from the mixin).
3. Cooperative Base Classes: If all parent classes are designed for cooperative inheritance, they pass through unused arguments to subsequent constructors. Here again, use super().__init__() in the child class to invoke all constructors.

Inferred Recommendations

• If possible, ensure that the parent classes follow the same convention.
• If using separate parent classes, employ explicit constructor calls without super.
• If using a mixin or cooperative classes, use super with proper inheritance order (mixin first or all arguments as keyword arguments).

Conclusion

Choosing the right approach for calling parent class constructors in multiple inheritance depends on the design of the parent classes. By understanding these scenarios and procedures, you can ensure proper initialization and avoid common pitfalls.

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