Cannot type Flask view functions return types in the natural way #7218

Flask handles a variety of return types from view functions -- for instance, you can return a Response object, a str, or a tuple of str and int, etc. These are documented at https://github.com/python/typeshed/blob/master/third_party/2and3/flask/app.pyi#L48-L58.

If you need to return more than one type from a given view function, mypy gives you an error (as of python/typeshed#3003). Here's a small reproducing case, which also shows why we might want to do this:

from typing import Union
from flask import Flask, redirect, render_template
from werkzeug import Response

app = Flask(__name__)

@app.route("/")
def index() -> Union[Response, str]:
    if True:
        return redirect("/foo")           # is a Response
    return render_template("index.html")  # is a str

When we run mypy, we see:

$ mypy flaskapp.py
flaskapp.py:9: error: Value of type variable "_VT" of function cannot be "Callable[[], Union[Response, str]]"

I've logged this here, as opposed to in typeshed, as I think that mypy ought to be able to figure out from the ViewFunc definition that my particular view function is one of two valid values there, but apparently it can't.