[radiowave]

I'm not disputing how or why it works, just saying that I think it is poor design, as it causes a statement that looks like variable assignment to actually produce overriding. I think this violates the principle of least astonishment, and I suspect there is no good reason (beyond implementation simplicity) why Python class variables do behave this way.

(edit: replaced "an expression" with "a statement")

[macbony]

No, the reason it exists is so you can override functionality of subclasses at run time, otherwise things like monkey patching would be impossible. I guess it COULD do a pass to see if the attr is an immutable or some form of primative type when __new__ is called and force those to be instantiated as instance attributes at the cost of internal consistency, but it seems pretty straight forward to me the way it works now. You don't need to be an expert, you just need a basic understand of how Python evaluates code and the difference between tags and variables.