python - How to properly function annotate / type hint a list of strings

Question

I am trying to figure out how to properly function annotate or type hint a list of strings. For example, if I had a function like this:

def send_email(self, from_address: str, to_addresses: list[str]):
    pass

to_addresses should be a list of strings. But when I try to use that annotation I get the following error in my Python 3.4.3 interpreter:

TypeError: 'type' object is not subscriptable

I am positive the list[str] is causing the issue, because if I change it to str the error goes away, but that doesn't properly reflect my intentions for the parameter.

Answer 1

Python 3.4 doesn't specify a format for its function annotations, it merely provides a mechanism that allows you to use any expression as the annotation. How the annotations are interpreted is up to you and the libraries you use.

Python 3.5 standardizes the way function annotations are used for type hinting, as documented in PEP 484. To annotate a list of strings, you use List[str], where List is imported from the typing module. You can also use Sequence[str] if your function accepts any list-like sequence, or Iterable[str] for any iterable.

Starting with Python 3.9, you can use list[str] as a type annotation, which doesn't require importing anything.