mypy says request.json returns Optional[Any], how do I solve?

Question

I am trying to understand mypy a little better. For the following line of code:

request_body: dict = {}
request_body = request.get_json()

mypy returns an error:

error: Incompatible types in assignment (expression has type "Optional[Any]", variable has type "Dict[Any, Any]")

What is the correct fix for this?

Answer 1

As you can see in the following code, taken from /wekzeug/wrappers/request.py, the function get_json doesn't always return a dictionary. I would suggest removing the type hinting from the variable, as it can be None or a dictionary.

def get_json(
        self, force: bool = False, silent: bool = False, cache: bool = True
    ) -> t.Optional[t.Any]:
        """Parse :attr:`data` as JSON.

        If the mimetype does not indicate JSON
        (:mimetype:`application/json`, see :meth:`is_json`), this
        returns ``None``.

        If parsing fails, :meth:`on_json_loading_failed` is called and
        its return value is used as the return value.

        :param force: Ignore the mimetype and always try to parse JSON.
        :param silent: Silence parsing errors and return ``None``
            instead.
        :param cache: Store the parsed JSON to return for subsequent
            calls.
        """
        if cache and self._cached_json[silent] is not Ellipsis:
            return self._cached_json[silent]

        if not (force or self.is_json):
            return None

        data = self.get_data(cache=cache)

        try:
            rv = self.json_module.loads(data)
        except ValueError as e:
            if silent:
                rv = None

                if cache:
                    normal_rv, _ = self._cached_json
                    self._cached_json = (normal_rv, rv)
            else:
                rv = self.on_json_loading_failed(e)

                if cache:
                    _, silent_rv = self._cached_json
                    self._cached_json = (rv, silent_rv)
        else:
            if cache:
                self._cached_json = (rv, rv)

        return rv

This line specifically causes the method to return None:

except ValueError as e:
            if silent:
                rv = None```