Could you give some examples? What encapsulation features does Java have that Python doesn't? You can get private methods with double underscores. It's true that attributes in Python objects are all public by default and in Java you have to be more explicit to get public attributes, so maybe that counts, but otherwise what do you mean by "objects can pick up state in unexpected ways"?

So let's so you have:

  class Foo():
      def __init__(self):
          self.bar = 1

Then let's say you have foo_instance imported in your module, and you see that foo_instance.bar has a value of 2.

You can see that foo_instance is imported from module A. So you go there, and see that the instance wasn't created there, it was imported from module B. But wait, it wasn't created there either, it was imported from module C, etc.

Somewhere along the way bar picked up the value of 2. But it's almost impossible to figure out where because there isn't any requirement use a uniquely-named accessor to modify the instance, so you can't just grep through the code to see where that method was called. (And maybe the name of the instance has changed a few times along the way, for whatever reason.)

And what's worse, the code that modified the instance might not even be visible in any of the modules, it's possible that someone took advantage of Python's ability to change the way the importing system works to mess with either the class or the instance. Or maybe there is some global middleware that's messing with objects or instances, or unrelated module Z is secretly monkey patching module B somewhere in the middle of this process. Who knows, and good luck figuring out.

Whereas if you just write the user-code logic in functions then it's not possible for developers to create situations like this. That obviously isn't possible for a library where the entire point is to make everything extensible and where you don't know how people will want to use something in advance, but hopefully the people writing libraries and frameworks have good enough judgment not to completely abuse the tools.

I'm not sure if I can really articulate how this is different than Java or Swift or whatever, but for whatever reason it just feels different.

I understand your point. I think this is partly related to the fact that instance attributes are public by default and do not require accessor or mutator methods (though you can easily create them with the @property decorator) and partly due to the fact that Python is a very dynamic language.

You can make instance variables "culturally private" by pretending the variable name with a single underscore, and you can name-mangle (to make it effectively private, unless someone really goes out of their way) with double underscores, but I know that's annoying to do for most/all variables and that making them private by default has advantages.

Python is just not as strict a language as Java, Swift, C#, etc. That has advantages and disadvantages.

Personally, this hasn't generally been an issue for me, because I've rarely directly modified instance variables in my own code unless I'm intentionally doing something really "meta", and it's not a pattern I've seen much of in other codebases.