Shadowing a class variable with an instance variable

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Accepted Answer

Assigning to a.debug creates an instance variable on 'a' that shadows the class variable. Python's attribute lookup checks the instance first, then the class. So a.debug finds the instance variable and returns True. b.debug has no instance variable, so Python falls back to Config.debug which is still False. Config.debug itself is also still False - the class variable was never changed. This shadowing behavior trips up many developers who expect assignment to one instance to affect all instances. If you want to change the class variable, you must write Config.debug = True explicitly.