Which of the following built-in types are mutable, and which of them are immutable? And why?

• function (in particular, method)
• user-defined class
• class instance
• module
• file type (for example, _io.TextIOWrapper)

The official reference says the following here:

Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.

I think this implies that functions are mutable because function objects can contain references (directly or indirectly) to mutable objects. Also it is always possible to change the code of a function object without affecting its id (see example here).

Next, one of the most upvoted answers on the topic of mutability claims that user-defined class and class instance are mutable types. Unfortunately, it doesn’t explain why.