[Typing-sig] Thoughts on a typing.Self?

2 Jun 2021

      I would like to draft a PEP for a typing.Self class which would perform similarly to rust's Self keyword in function annotations.

Examples
```python
from typing import TypeVar

F = TypeVar("F", bound="Foo")

class Foo:
    def parse(self: F, data: bytes) -> F:
        ...
        return self
```
vs
```python
from typing import Self

class Foo:
    def parse(self, data: bytes) -> Self:
        ...
        return self
```
Another example of where this could be useful is classmethods:
```python
class Foo:
    @classmethod
    def bar(cls: type[F], *args: Any, **kwargs: Any) -> F:
        return cls()
```
vs
```python
class Foo:
    @classmethod
    def bar(cls, *args: Any, **kwargs: Any) -> Self:
        return cls()
```

Implementation wise it would be a copy of NoReturn, i.e. not subscriptable, a _SpecialForm and should only be valid as a return type.

I haven't seen any discussion of the idea on the mailing list, so I wanted to run it by everyone.

Thanks,
James.

[Typing-sig] Thoughts on a typing.Self?

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