Mypy has supported all language features new in Python 3.6 starting with mypy 0.510. This section introduces Python 3.6 features that interact with type checking.

Python 3.6 introduced a new syntax for variable annotations (in global, class and local scopes). There are two variants of the syntax, with or without an initializer expression:

from typing import Optional
foo: Optional[int]  # No initializer
bar: List[str] = []  # Initializer

You can also mark names intended to be used as class variables with :py:data:`~typing.ClassVar`. In a pinch you can also use :py:data:`~typing.ClassVar` in # type comments. Example:

from typing import ClassVar

class C:
    x: int  # Instance variable
    y: ClassVar[int]  # Class variable
    z = None  # type: ClassVar[int]

    def foo(self) -> None:
        self.x = 0  # OK
        self.y = 0  # Error: Cannot assign to class variable "y" via instance

C.y = 0  # This is OK

Python 3.6 allows coroutines defined with async def (PEP 492) to be generators, i.e. contain yield expressions. It also introduced a syntax for asynchronous comprehensions. This example uses the :py:class:`~typing.AsyncIterator` type to define an async generator:

from typing import AsyncIterator

async def gen() -> AsyncIterator[bytes]:
    lst = [b async for b in gen()]  # Inferred type is "List[bytes]"
    yield 'no way'  # Error: Incompatible types (got "str", expected "bytes")

Python 3.6 supports an alternative, class-based syntax for named tuples. See :ref:`named-tuples` for the details.