I've never thought of this as a footgun. It's useful to accumulate state in a function, sort of like a static variable in c. E.g., def func(x=[]): x.a

I've never thought of this as a footgun. It's useful to accumulate state in a function, sort of like a static variable in c. E.g., def func(x=[]): x.append(5) return x I've always been annoyed that pylint yells at you for this.