Completion does not work for generic type aliases

[pappasam]

Generic type aliases are not currently supported for autocompletion. For context, for a codebase I'm developing, I'm working with a generic type alias that helps me handle TypeScript interface undefined values (which differ from null, a value that corresponds to Python's None).

See below for a simple example of the problem.

from dataclasses import dataclass
from typing import TypeVar, Union

T = TypeVar("T")

MyType = Union[None, T]

d = {"t": "hello"}

@dataclass
class MyDataclass:

    t: MyType[str]

result = MyDataclass(**d)
result.t.

To remain consistent with jedi's behavior concerning Union types, this should provide completions for both string and None, but it completes nothing with version 0.17.2. For comparison, things work fine for Optional, which is basically a special case of Union[None, T]:

from dataclasses import dataclass
from typing import Optional

d = {"t": "hello"}

@dataclass
class MyDataclass:

    t: Optional[str]

result = MyDataclass(**d)
result.t.

[PeterJCLaw]

Thanks for the report, however unfortunately I think you've hit one of the limitations of Python type annotations themselves rather than Jedi specifically.

The limitation is that user defined generic aliases (such as your MyType) do not preserve their generic nature, so in fact your MyType alias is in fact equivalent to Optional[Any].

You can see this in action if you add in a reveal_type(MyType) or reveal_type(MyType[str]) and then type-check using either mypy or pyre-check.

I'm pretty sure this is an known issue with the type system, though I can't find an exact reference for it.
There are a couple of mypy issues which are similar though: python/mypy#7092 and python/mypy#7084.

[pappasam]

Hmm, I'm not completely understanding your point. User defined generic aliases appear to be supported by mypy and type checking the above example works as expected with mypy (version 0.790). Try changing example 1 to be:

from dataclasses import dataclass
from typing import TypeVar, Union

T = TypeVar("T")

MyType = Union[None, T]


@dataclass
class MyDataclass:

    t: MyType[str]


a = MyDataclass(t=None)
b = MyDataclass(t="hello")
c = MyDataclass(t=12)

and observe that mypy now returns:

test.py:17: error: Argument "t" to "MyDataclass" has incompatible type "int"; expected "Optional[str]"
Found 1 error in 1 file (checked 1 source file)

This indicates that mypy is understanding the generic type alias as-intended.

I do see that reveal_type returns builtins.object for MyType, but it seems like it does that for any Union type.

[PeterJCLaw]

Huh, looks like you're right. I was fairly sure that this didn't work in the past, though I agree it definitely does now.

[davidhalter]

@PeterJCLaw Interested in solving this? Seems like something tailored for you :)

[PeterJCLaw]

Looks like this is possible with Callable too, which Jedi also doesn't support:

from typing import Callable, TypeVar
T = TypeVar('T')
MyCallable = Callable[..., T]
x: MyCallable[str]
x().<nothing>

I suspect the fix for Callable is slightly different, though potentially also worth fixing.