Generic methods and generic self

You can also define generic methods — just use a type variable in themethod signature that is different from class type variables. In particular,self may also be generic, allowing a method to return the most precisetype known at the point of access.

Note

This feature is experimental. Checking code with type annotations for selfarguments is still not fully implemented. Mypy may disallow valid code orallow unsafe code.

In this way, for example, you can typecheck chaining of setter methods:

  1. from typing import TypeVar
  2.  
  3. T = TypeVar('T', bound='Shape')
  4.  
  5. class Shape:
  6.     def set_scale(self: T, scale: float) -> T:
  7.         self.scale = scale
  8.         return self
  9.  
  10. class Circle(Shape):
  11.     def set_radius(self, r: float) -> 'Circle':
  12.         self.radius = r
  13.         return self
  14.  
  15. class Square(Shape):
  16.     def set_width(self, w: float) -> 'Square':
  17.         self.width = w
  18.         return self
  19.  
  20. circle = Circle().set_scale(0.5).set_radius(2.7)  # type: Circle
  21. square = Square().set_scale(0.5).set_width(3.2)  # type: Square

Without using generic self, the last two lines could not be type-checked properly.

Other uses are factory methods, such as copy and deserialization.For class methods, you can also define generic cls, using Type[T]:

  1. from typing import TypeVar, Tuple, Type
  2.  
  3. T = TypeVar('T', bound='Friend')
  4.  
  5. class Friend:
  6.     other = None  # type: Friend
  7.  
  8.     @classmethod
  9.     def make_pair(cls: Type[T]) -> Tuple[T, T]:
  10.         a, b = cls(), cls()
  11.         a.other = b
  12.         b.other = a
  13.         return a, b
  14.  
  15. class SuperFriend(Friend):
  16.     pass
  17.  
  18. a, b = SuperFriend.make_pair()

Note that when overriding a method with generic self, you must eitherreturn a generic self too, or return an instance of the current class.In the latter case, you must implement this method in all future subclasses.

Note also that mypy cannot always verify that the implementation of a copyor a deserialization method returns the actual type of self. Thereforeyou may need to silence mypy inside these methods (but not at the call site),possibly by making use of the Any type.

For some advanced uses of self-types see additional examples.