[mickeyp]

I wrote the link and yes it does. Module evaluations reify {}, [], etc. once. That is why people keep making subtle bugs when they do `def foo(a=[]):` unaware that this will in fact not give you a brand new list on every function call.

Factory functions like list/tuple/set are function calls and are executed and avoid this problem. Hence why professional python devs default to `None` and check for that and _then_ initialise the list internally in the function body.

Adding {/} as empty set is great, sure; but that again is just another reified instance and the opposite of set() the function.

[rand_r]

There is no difference between “def f(x={})” and “def f(x=dict())”, unless you have shadowed the dict builtin. They both have exactly the same subtle bug if you are mutating or return x later.