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Comment by thayne | Hacker News Reader

And you can use dict() for an empty dictionary, and list() for an empty list.

For reasons I don't think I understand, using the functions is "discouraged" because "someone might muck with how those functions work" and the python world, in it's perfect wisdom responded "Oh of course" instead of "That's so damn stupid, don't do that because it would be surprising to people who expect built in functions to do built in logic"

Yes but they are not equivalent. dict and list are factories; {} and [] are reified when the code is touched and then never reinitialised again. This catches out beginners and LLMs alike:

https://www.inspiredpython.com/article/watch-out-for-mutable...

That article is about how defaults for arguments are evaluated eagerly. It doesn't real have to do with dict vs {}.

However, using the literal syntax does seem to be more efficient. So that is an argument for having dedicated syntax for an empty set.

I am not replying to the article but to the poster.

I am talking about the article you linked to in your comment.

Your link doesn't support your argument.

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.

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.

They are equivalent. In function signatures (what your article is talking about), using dict() instead of {} will have the same effect. The only difference is that {} is a literal of an empty dict, and dict is a name bound to the builtin dict class. So you can reassign dict, but not {}, and if you use dict() instead of {}, then you have a name lookup before a call, so {} is a little more efficient.

Right, but it instantiates it _once_ on module load! That is the point I am making; nothing else.

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