Mutable global state (ARCH2187)

Key python.quality.mutable-global-state · Severity warning · Category quality

Module-level mutable global state creates hidden dependencies, is unsafe under concurrent access, and makes tests fragile. Encapsulate the state inside a function or class.

Incorrect

cache = {}

def get(key):
    if key not in cache:
        cache[key] = load(key)
    return cache[key]

Correct

class Repository:
    def __init__(self):
        self._cache = {}

    def get(self, key):
        if key not in self._cache:
            self._cache[key] = load(key)
        return self._cache[key]

Fix

Encapsulate mutable state in a class instance and inject it explicitly.