I recommend taking this to typing-sig...
On Mon, Nov 23, 2020 at 19:18 David Foster <[email protected]> wrote:
> On 11/22/20 10:15 AM, Guido van Rossum wrote:
> > - We intentionally don't support things like `isinstance(x, List[str])`
> > because that would require checking all the items with `isinstance(item,
> > str)`, and that seems a speed trap. Reverting this decision would be
> > hard work.
>
> Aye. I imagine many folks would expect isinstance() to be "fast" and
> altering
> it to do recursive checks on its argument would lose its current O(1) time.
>
> I imagine I *could* implement my own kind of isinstance() that would
> work on
> TypedDict values that would still be recognized by typecheckers. Say I
> write
> an implementation for the following method:
>
> from typing import Optional, Type, TypeVar, TypedDict
>
> TD = TypeVar(bound=TypedDict)
>
> def try_cast(type: Type[TD], value: object) -> Optional[TD]:
> """Returns `value` if it can be parsed as a `type`, otherwise
> None."""
> raise NotImplementedError()
>
> Then I could use that method in a similar way as my earlier example to
> parse
> a value very concisely:
>
> if (shape := try_cast(Shape, request.json)) is not None:
> draw_shape(shape) # is narrowed to Shape
> else:
> return HTTPResponse(status=400) # Bad Request
>
> Going further, I could extend try_cast() to accept any (non-None) JSON-like
> value as the top-level object (not just TypedDicts):
>
> from typing import Dict, List, Optional, Type, TypeVar, TypedDict,
> Union
>
> TD = TypeVar('TD', bound=TypedDict)
> JsonValue = Union[
> TD,
> Dict[str, 'OptionalJV'],
> List['OptionalJV'],
> Dict, # heterogeneous Dict
> List, # heterogeneous List
> float,
> int, # because json.loads may return an int when parsing a number
> str,
> bool,
> ]
> JV = TypeVar('JV', bound=JsonValue)
> OptionalJV = TypeVar('OptionalJV', bound=Union[JsonValue, None])
>
> def try_cast(type: Type[JV], value: object) -> Optional[JV]:
> """Returns `value` if it can be parsed as a `type`, otherwise
> None."""
> raise NotImplementedError()
>
> Now, I'm not sure if mypy can handle that kind of recursive TypedDict
> definition :), but it *will* work at runtime.
>
> I'll see about implementing a function like try_cast() as a separate
> package.
> This should be fun. :)
>
> --
> David Foster | Seattle, WA, USA
> Contributor to TypedDict support for mypy
>
--
--Guido (mobile)
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