This rule raises an issue when code assigns a value to an attribute that is not listed in the class’s __slots__ declaration.

Why is this an issue?

In Python, the __slots__ class attribute is used to explicitly declare which instance attributes a class can have. When a class defines __slots__, Python restricts instances to only those attributes, preventing the creation of new attributes dynamically.

This restriction serves several purposes:

• memory efficiency: instances use less memory because Python does not create a __dict__ for each instance
• faster attribute access: attribute lookups are faster with __slots__
• explicit interface: the class clearly declares which attributes it supports

Assigning to an attribute that is not in __slots__ causes Python to raise an AttributeError at runtime:

class Point:
    __slots__ = ['x', 'y']

p = Point()
p.x = 10  # OK
p.z = 20  # Raises AttributeError: 'Point' object has no attribute 'z'

What is the potential impact?

When code attempts to assign to an attribute not in __slots__, the application will crash with an AttributeError at runtime. This can lead to:

• application crashes: the error will terminate the current operation unless properly handled
• data loss: if the assignment happens during a data processing operation, partial results may be lost
• poor user experience: users may encounter unexpected errors during normal operations
• difficult debugging: the error may only occur in specific scenarios, making it hard to reproduce and fix

How to fix it

If the attribute should be part of the class’s interface, add it to the __slots__ declaration.

Code examples

Noncompliant code example

class User:
    __slots__ = ['name', 'email']

    def __init__(self, name, email, age):
        self.name = name
        self.email = email
        self.age = age  # Noncompliant: "age" is not in __slots__

Compliant solution

class User:
    __slots__ = ['name', 'email', 'age']

    def __init__(self, name, email, age):
        self.name = name
        self.email = email
        self.age = age

Resources

Documentation

• Python Documentation - __slots__