Pyright: Support for typing_extension TypedDict

[nabilnaffar]

__I'm not sure if this is a new feature request or wrong usage/implementation__

3th party package returns object with __getItem__ implementation:

# object structure: {"ok": True, "error":"", result: "..."}
# usage:
if response['ok']:
    do_something()...

I'm writing the following type stub for the package and having difficulties to get the correct property type from __getItem__

from typing import Mapping

class ServerResponse(Mapping):
    ok: bool
    error: str
    result: str

Type of response['ok'] is __Any__ where it should be inferred from the class property type as __bool__

[erictraut]

If an object implements __getitem__ but returns different types based on the parameter, there's no correct way to annotate the type.

You might be able model it in the stub file as a data class or a NamedTuple. Pyright supports both of these constructs.

[nabilnaffar]

Seems like I can use NamedTuple to model the response type (since data class doesn't support __getitem__).

but the property type still inferred as Unknown:

Stub file:

from typing import NamedTuple
SlackClientActions = TypeVar('T', 'files.upload', 'chat.postMessage')

class ServerResponse(NamedTuple):
    ok: bool
    error: str
    result: str

class SlackClient(object):
    def api_call(self, action: SlackClientActions, channels: str, title: str, as_user: bool, content: str, initial_comment: str) -> ServerResponse: ...

Usage:

result = client.api_call('files.upload', ...)
result['ok'] # - type is Unknown, should be bool

[nabilnaffar]

see also typedDict from mypy_extensions

[erictraut]

That's because you're using a string as an index. Is there a reason you can't use result.ok to access the object member?

In the general case where a string expression is used as a index (e.g. result['val_{}'.format(index)'], there's no way for a static type checker to determine the type. I could add a special case for expressions that consist of a single static string. I already have similar logic in place for a single static number used to index into a NamedTuple (e.g. result[3]).

Thanks for pointing out typedDict. I wasn't aware of that.

[nabilnaffar]

The returned object is returned from json.loads, which can be accessed only as result['ok']

[erictraut]

I see. Yeah, then a TypedDict would be more appropriate in this case. I'll work on implementing support for TypedDict.

[erictraut]

I investigated this further. Turns out that there is no support for TypedDict in typing_extension. It's currently found only in mypy_extensions. I don't want to take a dependency on that.

There is an official spec for TypedDict as a proposed addition to Python 3.8.

[bfelbo]

TypedDict is now supported in typing_extension: https://github.com/python/typing/commit/6e6e4dd6c0b33087a39891ceb142c269f05e2be9#diff-a705b8de8437703698ee3f286d3005a4

Any plans to support it in pyright in the near future?

[erictraut]

Thanks for the heads up. I'll look into it.

[erictraut]

Support for TypedDict will be included in the next release of pyright.

[erictraut]

This is now implemented in 1.0.65, which I just published. I tried to implement all aspects of PEP 589, including the class-based representation of TypedDict as well as the alternate (backward-compatible) syntax that mirrors NamedTuple declarations. Let me know if you run into any problems. Thanks for your patience on this one. I was waiting for the support to make it into the official typeshed typing_extension.pyi file.

[bfelbo]

Amazing, thanks!