How to add custom attributes to a Python list?

Question

I'm trying to associate additional data with a Python list. Ideally, I'd like to add custom attributes to a list object, similar to how you'd add attributes to a custom class. For example:

a = [1, 2, 4, 8]
a.x = "Hey!" # I'd like this to work without errors

However, when I attempt this, I get the following error:

AttributeError: 'list' object has no attribute 'x'

I understand that Python lists don't support adding arbitrary attributes out of the box. I also tried using setattr, but it gave me a similar error:

setattr(a, "k", 98) # Raises AttributeError

I cannot create a custom class DummyList(list) to work around this (for reasons). I also don't want to use some global variable that holds the attributes for the lists. Are there any workarounds to achieve this in Python?

• wjandrea: What's the end goal? Beware the XY problem
• deceze: Let's put it this way: in decades of programming, I have never needed to add attributes to a list instance, nor otherwise needed to bend over backwards to make a builtin do something it's clearly not designed to do. There are probably other, better, more out-of-the-box ways to do what you need to.
• wjandrea: What are the reasons for not creating a class? If it's because inheriting from the builtins is messy, you can use UserList instead.
• deceze: So you want to "smuggle" some metadata through a library together with your data, while the library doesn't explicitly support any such thing? And it's written in a hostile way that makes this smuggling of metadata difficult? Then yeah, I'd second the notion to subclass and override that library in a way that makes it possible to pass your own types with your own metadata through. If that proves too difficult because you'd need to override a ton of code, then I'd try to associate the data externally somehow, not passing it through at all. We'll need code samples for concrete advice.
• blhsing: Note that there's nothing wrong with your current solution of using a global dict with list identities as keys. It may not be as elegant as an idealistic yet unfortunately unattainable list with a custom attribute, but is simple and effective, and may be the best alternative if too much code needs to be overriden to make the aforementioned override approach work.