Comment by js2

The most important thing about writing code is that you write it idiomatically for the language it is in. With Python, idiomatic code is known as Pythonic.

  def rename_user(user_id: str, name: str) -> User | Exception:
    # Consume the function
    user = get_user(user_id)
    if isinstance(user, Exception):
        return user
    user.name = name
    return user

This is not Pythonic. Don't do it. Like it or not, Python uses exceptions. How do I know that some other code that get_user calls isn't going to raise an exception? What if the program receives an interrupt signal during the get_user call? When I'm writing Python code, I have to think about exception handling, and now when I use this library, I have to add a bunch of isinstance calls too?

Perspective: I've written primarily in Python for over 20 years, as well as coded extensively in many other languages (C, Objective-C, JavaScript, Java, Bash) and have familiarity with a bunch more (Go, Ruby, Gradle, TCL, Lua, Kotlin, C++).

In practice, exception handling, declared exceptions or not, just isn't that big of a problem. When I'm writing code, I have to reason about it, and exceptions and return/error values are just one aspect of that reasoning.