typing_extensions.Self not working as expected

[adamjstewart]

I'm attempting to use typing_extensions.Self but finding that it isn't working as expected. I modified the example code here. First, create a test.py file containing:

from typing_extensions import Self

class Foo:
    a = 2
    def return_self(self) -> Self:
        return self

x = Foo()
reveal_type(x)
reveal_type(x.a)
y = x.return_self()
reveal_type(y)
reveal_type(y.a)

Then run:

$ mypy test.py
test.py:5: error: Variable "typing_extensions.Self" is not valid as a type  [valid-type]
test.py:5: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
test.py:9: note: Revealed type is "test.Foo"
test.py:10: note: Revealed type is "builtins.int"
test.py:12: note: Revealed type is "Self?"
test.py:13: error: Self? has no attribute "a"  [attr-defined]
test.py:13: note: Revealed type is "Any"
Found 2 errors in 1 file (checked 1 source file)

A few things seem wrong to me:

1. Why is typing_extensions.Self not valid as a type?
2. Why is the revealed type of y Self? and not test.Foo?
3. Why does mypy complain when I try to access y.a?

Some useful versions to reproduce this:

Python: 3.10.8
typing-extensions: 4.3.0

[hauntsaninja]

This technically isn't anything to do with the runtime implementation of typing_extensions (which is what this repo is).

mypy just doesn't yet know how to interpret the Self type hint. The next release of mypy should.

[hauntsaninja]

For now you can do:

Self = TypeVar("Self")
class Foo:
    def return_self(self: Self) -> Self: ...

(note that self needs an additional annotation here)

[adamjstewart]

Thanks for the clarification!

[liu123liu123liu]

Traceback (most recent call last):
  File "/tmp/pycharm_project_710/cnn/train.py", line 23, in <module>
    from model.crn import CRN
  File "/tmp/pycharm_project_710/cnn/model/crn.py", line 3, in <module>
    from torch_geometric.nn import GCNConv
  File "/root/miniconda3/lib/python3.8/site-packages/torch_geometric/__init__.py", line 9, in <module>
    import torch_geometric.data
  File "/root/miniconda3/lib/python3.8/site-packages/torch_geometric/data/__init__.py", line 5, in <module>
    from .data import Data
  File "/root/miniconda3/lib/python3.8/site-packages/torch_geometric/data/data.py", line 469, in <module>
    class Data(BaseData, FeatureStore, GraphStore):
  File "/root/miniconda3/lib/python3.8/site-packages/torch_geometric/data/data.py", line 641, in Data
    def update(self, data: Union[Self, Dict[str, Any]]) -> Self:
  File "/root/miniconda3/lib/python3.8/typing.py", line 261, in inner
    return func(*args, **kwds)
  File "/root/miniconda3/lib/python3.8/typing.py", line 358, in __getitem__
    parameters = tuple(_type_check(p, msg) for p in parameters)
  File "/root/miniconda3/lib/python3.8/typing.py", line 358, in <genexpr>
    parameters = tuple(_type_check(p, msg) for p in parameters)
  File "/root/miniconda3/lib/python3.8/typing.py", line 145, in _type_check
    raise TypeError(f"Plain {arg} is not valid as type argument")
TypeError: Plain typing_extensions.Self is not valid as type argument

Do you know how to slove the typeerror?

[AlexWaygood]

@liu123liu123liu, it's pretty impossible to tell you how you could "solve" this error without a minimal reproducible example. Even if we had one, it doesn't seem like this error has anything to do with the original issue reported here. If you are able to create a minimal, reproducible example and you're confident that it demonstrates a bug in typing_extensions rather than a bug in your code or another library, feel free to open a new issue :-)