Is using a TypeAliasType to simply forward a TypeVar supposed to be approved?

[cainmagi]

Hello! I am trying to define an annotated proxy class. Previously, the type checker did not approve all my run-time compatible tries. But recently, I found the following usage will satisfy my requirement without raising a type checker error:

from typing import TypeVar, Sequence
from typing_extensions import TypeAliasType, reveal_type

T = TypeVar("T")

proxy = TypeAliasType("proxy", T, type_params=(T,))


# No errors are raised, and I am happy to see that!
def test_func(val1: Sequence[proxy[int]], val2: Sequence[proxy[str]]) -> Sequence[str]:
    return val2


reveal_type(test_func)  # (val1: Sequence[int], val2: Sequence[str]) -> Sequence[str]

How test_func can accept arguments like val1=(1, 2,) without raising any errors. This is what I expect to get.

However, the usage of TypeAliasType here may be problematic. In most cases, TypeAliasType is not used like this but is used for providing abbr. of complicated compositional types. I am not sure whether defining an alias with a single TypeVar should be approved. If it can be approved, this will be my final solution for a "type proxy".

VS Code extension or command-line

• Pylance version: 2023.7.20
• Pyright version: 74d8f3c

[erictraut]

Yes, this should work fine. In fact, you shouldn't need to use typing.TypeAliasType. The older-style typing.TypeAlias should work fine here. There's currently a bug in pyright where proxy: TypeAlias = T does not work correctly. I'll fix this for the next release.

proxy: TypeAlias = T

[cainmagi]

Thank you! If so, I think I can use this usage to provide a solution to
python/mypy#15107

(Sorry, I misreferred another issue, I have changed it just now)

[cainmagi]

I just checked another thing:

from typing import TypeVar
from typing_extensions import TypeAlias


T = TypeVar("T")
proxy: TypeAlias = T
proxy[int]

The above codes raise an error in run time:

python3.8

Traceback (most recent call last):
  File "alpha-proxy.py", line 7, in <module>
    proxy[int]
TypeError: 'TypeVar' object is not subscriptable

python3.11

Traceback (most recent call last):
  File "C:\Users\Yuchen Jin\Documents\GitHRC\dash-ait-pvt-analysis\alpha-proxy.py", line 7, in <module>
    proxy[int]
    ~~~~~^^^^^
TypeError: 'TypeVar' object is not subscriptable

It seems that TypeAlias annotation behaves differently from defining TypeAliasType when it is used to annotate T. Maybe it should not be treated in the same way of TypeAliasType?

[erictraut]

Hmm, I can't explain why the runtime would reject that. Perhaps @JelleZijlstra could shed some light on it. I'm not aware of any documentation that indicates this should not be permitted.

One potential workaround is to define the proxy type alias as a union between T and typing.Never.

proxy: TypeAlias = T | Never

It's a bit of a hack, but this appears to work fine at runtime and type checks with both mypy and pyright.

[cainmagi]

Thank you! That's magic!

[hmc-cs-mdrissi]

There’s special name for generic type where type argument is unused, https://wiki.haskell.org/Phantom_type. I haven’t seen them before in python but some languages do interesting type level stuff with them.