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What if my function can raise an exception?

Question

def fetch_user(user_id: str) -> User:
    user = database.fetch_user(id=user_id)
    if user is None:
        raise LookupError(user_id)
    return user

This function either returns a user or raises an exception. How to annotate it correctly?

Exception means NoReturn, right?

from typing import Union, NoReturn


def fetch_user(user_id: str) -> Union[User, NoReturn]:
    user = database.fetch_user(id=user_id)
    if user is None:
        raise LookupError(user_id)
    return user
This doesn't break any rules per se, but this means exactly the same as just returning a User: 
def fetch_user(user_id: str) -> User:

What about Union with the exception type?

from typing import Union


def fetch_user(user_id: str) -> Union[User, LookupError]:
    user = database.users.where(id=user_id, deleted=False)
    if user is None:
        raise LookupError(user_id)
    return user
This annotation means that the function can return a LookupError object, not raise it. This is not right, and it doesn't force the caller to handle the exception with a try-except block. 
user = fetch_user("u:cba43b8f:42")
# user: User | LookupError

How to specify that I'm raising an exception, then?

There isn't a way to mark that a function raises something in Python's type system. In Python, pretty much anything can raise a variety of exceptions.

If raising an exception is part of the function's contract, note it in the docstring: 

from typing import Union


def fetch_user(user_id: str) -> User:
    """
    Raises:
        LookupError: If the user is not found
    """
    user = database.users.where(id=user_id, deleted=False)
    if user is None:
        raise LookupError(user_id)
    return user

If you want a more type-safe way to handle errors, consider returning an error value. The simplest example is None: if you return a value that can be None, the type checker will force the caller to handle the error case.

If you want something more sophisticated than None, perhaps take a look at the returns library.