Python, unlike statically typed languages such as Java, allows complete freedom when calling methods during object destruction. However, standard object-oriented principles apply to Python classes using deep inheritance hierarchies. Therefore the developer has responsibility for ensuring that objects are properly cleaned up when there are multiple __del__ methods that need to be called.

If the __del__ method of a superclass is not called during object destruction it is likely that that resources may be leaked.

A call to the __del__ method of a superclass during object destruction may be omitted:

When a subclass calls the __del__ method of the wrong class.
When a call to the __del__ method of one its base classes is omitted.

Either be careful to explicitly call the __del__ of the correct base class, or use super() throughout the inheritance hierarchy.

Alternatively refactor one or more of the classes to use composition rather than inheritance.

In this example, explicit calls to __del__ are used, but SportsCar erroneously calls Vehicle.__del__. This is fixed in FixedSportsCar by calling Car.__del__.

Python Tutorial: Classes.

Python Standard Library: super.

Artima Developer: Things to Know About Python Super.

Wikipedia: Composition over inheritance.