This rule raises an issue when the same class appears more than once in the inheritance list of a class definition.

Why is this an issue?

In Python, a class definition can inherit from one or more base classes. The syntax for this is straightforward:

class MyClass(BaseClass1, BaseClass2):
    pass

When the same base class appears multiple times in the inheritance list, it serves no purpose and is always a programming error. Python’s Method Resolution Order (MRO) ensures that each class appears only once in the inheritance chain, even if it is listed multiple times.

This typically happens due to:

• copy-paste errors when defining the class
• misunderstanding of how inheritance works
• refactoring mistakes where duplicate entries were not cleaned up

What is the potential impact?

This issue has minimal runtime impact since Python automatically removes duplicate base classes from the Method Resolution Order. However, it affects code quality:

• Maintainability: The duplicate entries make the code confusing and harder to understand
• Code clarity: It suggests the code has not been properly reviewed
• Future errors: It may indicate a misunderstanding of inheritance that could lead to more serious bugs

How to fix it

Remove the duplicate base class from the inheritance list, keeping only one occurrence.

Code examples

Noncompliant code example

class MyWidget(QWidget, Serializable, QWidget):  # Noncompliant
    pass

Compliant solution

class MyWidget(QWidget, Serializable):
    pass

Resources

Documentation

• Python Documentation - Multiple Inheritance
• Python Documentation - Method Resolution Order