monkeypatch: add support for TypedDict

[adamjstewart]

Closes #10999

This feature was added in Python 3.8. It looks like pytest still supports Python 3.7 (I'm guessing that will change in about 6 weeks). For now, it isn't easy to test or annotate this properly.

[RonnyPfannschmidt]

thank you for this excellent addition

lets add type triggers in https://github.com/pytest-dev/pytest/blob/main/testing/typing_checks.py as well to make sure type checks work as expected

[adamjstewart]

pre-commit errors are because TypedDict doesn't exist in 3.7. Solutions include bumping language_version in .pre-commit-config.yaml from python3 to python3.8 or waiting 6 weeks until Python 3.7 support is presumably dropped.

[RonnyPfannschmidt]

We just merge a pr to remove the default version,, does that help?

[bluetech]

The current approach wouldn't work; TypedDict[K, V] is not something that makes sense, since TypedDict is not generic in key and value types, its keys and values are per-determined. And TypedDict on its own is not a type you can refer to, i.e. there is no common TypedDict super-type of all typed dicts in Python (I'm like 90% sure of that, so not 100%).

So some other approach is needed. Off hand I'm not sure how to refer to all typed dicts. Maybe the solution is to relax the MutableMapping to Mapping and accept that passing readonly dicts will fail at runtime.

[bluetech]

See python/mypy#4976. Particularly the current last comment python/mypy#4976 (comment) matches the case here.

[adamjstewart]

I'd be fine with the Mapping solution if you want me to implement that.

[adamjstewart]

Confirmed that this solves the problem for me.

[bluetech]

AFAIK there is no type-safe way to do this in current python, so Mapping sounds OK to me -- for the monkeypatch use case, some type unsafety seems better than forcing users to cast or type-ignore.

I left some comments.

[adamjstewart]

Seems like Mapping doesn't work either since it's read-only and doesn't define __setitem__ or __delitem__. Could we use plain dict instead?

[bluetech]

We will need to type: ignore the errors inside the functions, i.e. encapsulate the type unsafety for the user. It would also be nice to add a code comment explaining why we're doing it (=> typed dicts).

[bluetech]

Thanks!