Can you do wrapping generic types properly without a mypy plugin? · python/typing · Discussion #1500

cjw296
Oct 30, 2023

I think it's a relatively common pattern to have some kind of object that represents a value at a later date, or some other kind of "wrapping" type, such as a Twisted Deferred, asyncio or concurrent Future, etc. But how do we get type checking of the results of getting attributes of these?

Concretely:

https://github.com/cjw296/generative_typing/blob/main/bag.py

...which can be used like:

@dataclass
class MyDataClass:
 foo: int
 bar: str
bag_dataclass: Bag[MyDataClass] = Bag[MyDataClass](MyDataClass(foo=1, bar='x'))
# Happy path examples
bag_dataclass_foo: Bag[int] = bag_dataclass.foo
foo: int = bag_dataclass.foo.get()
foo_: int = bag_dataclass_foo.get()
bag_dataclass_bar: Bag[str] = bag_dataclass.bar
bar: str = bag_dataclass.bar.get()
bar_: str = bag_dataclass_bar.get()
class MyTypedDict(TypedDict):
 foo: int
 bar: str
bag_typeddict: Bag[MyTypedDict] = Bag[MyTypedDict]({'foo': 1, 'bar': 'x'})
# Happy path examples
bag_typeddict_foo: Bag[int] = bag_typeddict.foo
dfoo: int = bag_typeddict.foo.get()
dfoo_: int = bag_typeddict_foo.get()
bag_typeddict_bar: Bag[str] = bag_typeddict.bar
dbar: str = bag_typeddict.bar.get()
dbar_: str = bag_typeddict_bar.get()

I've got this working with a mypy plugin, but how could I get this to work so that pylance, ruff, etc would also work?

Replies: 3 comments 4 replies

sobolevn
Oct 30, 2023

Nope, __getattr__ support without a plugin is not possible at the moment.
However, this pattern is quite common.

Maybe it is time to start a discussion about potential Proxy type? Like def __getattr__(self, val: str) -> Proxy[MyDataClass]: and then reveal_type(bag_typeddict.bar) would reveal "builtins.str".