Private attributes which are written but never read are a clear case of dead store. Changing their value is useless and most probably indicates a serious error in the code.

Python has no real private attribute. Every attribute is accessible. There are however two conventions indicating that an attribute is not meant to be "public":

This rule raises an issue when a class-private attribute (two leading underscores, max one underscore at the end) is never read inside the class. It optionally raises an issue on unread attributes prefixed with a single underscore. Both class attribute and instance attributes will raise an issue.

Noncompliant Code Example

class Noncompliant:
    _class_attr = 0  # Noncompliant if enable_single_underscore_issues is enabled
    __mangled_class_attr = 1  # Noncompliant

    def __init__(self, value):
        self._attr = 0  # Noncompliant if enable_single_underscore_issues is enabled
        self.__mangled_attr = 1  # Noncompliant

    def compute(self, x):
        return x * x

Compliant Solution

class Compliant:
    _class_attr = 0
    __mangled_class_attr = 1

    def __init__(self, value):
        self._attr = 0
        self.__mangled_attr = 1

    def compute(self, x):
        return x * Compliant._class_attr * Compliant.__mangled_class_attr * self._attr * self.__mangled_attr

See