Re-implementing NewType in python

[lovasoa]

Python provides a handy NewType function that allows the creation of new types. If I'm not mistaken, this function's behavior is hardcoded in mypy and it's not possible to reproduce it in python.

There are several cases where it would be useful to be able to extend this function, and still have the code typecheck.
One example use case is the NewType function in marshmallow-dataclass (defined here)

• Please insert below the code you are checking with mypy,

from typing import Type, TypeVar

T = TypeVar("T")
def MyNewType(name: str, tp: Type[T]) -> Type[T]:
    def new_type(x):
        return x

    new_type.__name__ = name
    new_type.__supertype__ = tp
    return new_type

Email = MyNewType("Email", str)

class MySchema:
    email: Email

• What is the actual behavior/output?

/tmp/test.py:9: error: "Callable[[Any], Any]" has no attribute "__supertype__"
/tmp/test.py:10: error: Incompatible return value type (got "Callable[[Any], Any]", expected "Type[T]")
/tmp/test.py:15: error: Variable "test.Email" is not valid as a type

• What is the behavior/output you expect?
  The code would typecheck
• What are the versions of mypy and Python you are using?
  mypy 0.720
  Python 3.7.5
• What are the mypy flags you are using? (For example --strict-optional)
  none

[ilevkivskyi]

Yes, typing.NewType is a special syntactic form mypy's point of view (it never parses its source code). I don't think we will ever support type system feature that would be able to express this kind of dynamic type creation. Your best bet is to write a small mypy plugin using the get_dynamic_class_hook() (you can use e.g. plugin that ships with sqlalchemy-stubs as an example).