Support for default types in TypedDict

This problem arose from trying to type the environ argument for a WSGI application. There are specific keys of the environ dict that have non-str types, and then all the other keys are HTTP_ variables (or server-defined variables, which can be ignored for this purpose) with type str (or Text, depending on the version of Python).

I would like to get stronger typing on environ than Dict[str, Any], but there isn't a way to do that with TypedDict currently because the set of possible keys is unbounded (the client can send any HTTP header).

Ideally, I'd like to be able to do something like:

EnvironDict = TypedDict("EnvironDict", {
    "wsgi.version": Tuple[int, int],
    "wsgi.input": IO[str],
# ...
}, default_type=str, total=False)

and have this mean that any key is allowed in the dictionary and all unknown keys get a value type of str.

この issue では、WSGI environ の例を動機として、それ以外は未知のキーに対するデフォルト値型を TypedDict に追加することを説明しています。まず mypy の既存の TypedDict サポートとそのテストを確認してください。提案する構文が定義され、任意のキーがデフォルト型を受け取り、明示的なキーがそれぞれの型を維持し、その動作がテストでカバーされれば完了です。