Overhaul the python typing system to make typing a 'first class citizen' with a dedicated typing syntax, like match patterns

Pythons implementation of typing at the language level has been very limited, with only the absolute minimal changes to the grammar being implemented:

• type annotations: a: int
• return type annotations: def () -> int:

Both of these syntaxes are just normal expressions that are evaluated (or are just strings if __future__.annotations is on).

That has meant that the entirety of the typing landscape has been implemented 'in the language' using existing syntax and functionality to try and emulate typing as a first class language feature.

This has lead to modifications to the standard library to attempt to better support these usages (eg: special casing type[...], defining __or__ on type and the introduction of __class_getitem__ among others). So all these objects now have a dual nature, they have functions to support their 'normal' usages, and also a bunch of functions to support building type annotations.

The authors of PEP-586 have commented on this problem(thanks @sobolevn):

we feel overhauling the syntax of types in Python is not within the scope of this PEP: it would be best to have that discussion in a separate PEP

Due to the nature of typing being 'implemented in the language' instead of in the grammar/interpreter has lead to a myriad of complexities, edge cases, limitations and confusion. I will list a few of these side effect here. also showing a more optimal syntax that would be possible if typing were to be overhauled:

Fake types and type alias

Fake types such as None and ForwardRefs lead to runtime errors(either in annotations or upon calling get_type_hints):

from typing import TypeAlias

Str: TypeAlias = "str"
Int: TypeAlias = "int"

a: Str | Int = 0  # SUS ALERT

There could be a dedicated typealias keyword like other languages

typealias foo = int | str

(https://github.com/python/mypy/issues/11582)

Shout out to NewType

MyStr = NewType("MyStr", str)

Could be

newtype MyStr = str

Typing imports

A large amount of types need to be imported from typing, leading to a boilerplate of imports in every module:

from typing import NoReturn, TypedDict, TypeAlias, TypeVar, Generic, Protocol, etc, etc, etc, etc

There could be a 'builtins' for type annotations that are usable by default.

Also there is no way to import a type statically, see #928

Type parameters

Are one of the worst incantations imaginable:

from typing import TypeVar, Generic

T_cont = TypeVar("T_cont", contravariant=True, bound=str)

class A(Generic[T_cont]):
    ...

Could be

class A[in T_cont: str]:
    ...

Also you can't specify type parameters explicitly on function call sites:

foo_result: int | str = foo(1, "a")
bar(foo_result)

Could be:

bar(foo[int | str](1, "a"))

Literal syntax

a: Literal[1, 2] = 1

Could be

a: 1 | 2 = 1

cast

Yet another import, and very clunky to boot.
And why is the type first? everywhere else(isinstance, issubclass, type annotations) types go second.

cast(int, a)

Could be as or similar like other languages:

a as int

Callable

In error messages mypy shows this as (int, str) -> bool

a: Callable[[int, str], bool]

Could be

a: (int, str) -> bool

Confusion

Because all these type machineries exist at runtime, it creates very confusing situations where an annotation is used in a value position:

A = int | str
A()  # is this valid?
foo(A)  # is this valid?
B: type[int | str] = int | str  # is this valid?

https://github.com/microsoft/pyright/issues/2522 shows that it's very easy to overlook this situation.

Match Patterns

Enter match/case statements, which have their own unique language level syntax for dealing with all the concepts needed in a comprehensive way. For example matching a union of strings: "a" | "b" which in a typing annotation would just be evaluated as a normal expression and raise a TypeError.

match "a":
    case "a" | "b":  # 😳😳😳😳😳😳😳😳😳😳😳😳😳😳😳😳
          print("AMONGUS")

Instead of endlessly implementing special casing and workarounds, please overhaul the entire implementation of typing in the language from the ground up. Currently typing in python is very rough around the edges and could be vastly improved based upon the experience and usages gathered over the years since it's inception.

Proposal

Add the concept of a 'type context' that has it's own unique semantic meaning, separating the type realm functionality from the value(value context) realm functionality. This would make things such as the type union | much simpler and consistent (see issue where some types are not actually types at all, and don't have __or__). These operations would still produce type machinery representations:

class A:
    a: int | str
A.__annotations__  # {'a': UnionType[int, str]}

And these machinereies could be constructed manually in a 'value context' when needed:

a = UnionType(int, str)

Example

Bar = 1 | 2
    # ^ value context
typealias Foo = 1 | 2
              # ^ type context

Evaluating type syntax

from __future__ import annotations
from typing import get_type_hints

class A:
    x: "a" | "b"

get_type_hints(A)  # Currently will raise a TypeError

Under the hood, get_type_hints simply evals the annotation as a module level expression, I suggest adding a eval_type function that will evaluate it as a type context expression, not a value context

No se nombran archivos de implementación ni pruebas. Empieza leyendo el modelo de contexto de tipos propuesto junto con PEP-586, PEP-622, issue #928 y las discusiones enlazadas de mypy y pyright; un siguiente paso viable requeriría acordar una sintaxis y un diseño antes de implementar en la gramática y las herramientas de tipado.