Additional features

Several mypy features are not currently covered by this tutorial, including the following:

  • inheritance between generic classes
  • compatibility and subtyping of generic types, including covariance of generic types
  • super()

The attrs package

attrs is a package that lets you define classes without writing boilerplate code. Mypy can detect uses of the package and will generate the necessary method definitions for decorated classes using the type annotations it finds. Type annotations can be added as follows:

import attr
@attr.s
class A:
    one: int = attr.ib()          # Variable annotation (Python 3.6+)
    two = attr.ib()  # type: int  # Type comment
    three = attr.ib(type=int)     # type= argument

If you’re using auto_attribs=True you must use variable annotations.

import attr
@attr.s(auto_attribs=True)
class A:
    one: int
    two: int = 7
    three: int = attr.ib(8)

Typeshed has a couple of “white lie” annotations to make type checking easier. attr.ib and attr.Factory actually return objects, but the annotation says these return the types that they expect to be assigned to. That enables this to work:

import attr
from typing import Dict
@attr.s(auto_attribs=True)
class A:
    one: int = attr.ib(8)
    two: Dict[str, str] = attr.Factory(dict)
    bad: str = attr.ib(16)   # Error: can't assign int to str

Caveats/Known Issues

  • The detection of attr classes and attributes works by function name only. This means that if you have your own helper functions that, for example, return attr.ib() mypy will not see them.

  • All boolean arguments that mypy cares about must be literal True or False. e.g the following will not work:

    import attr
YES = True
@attr.s(init=YES)
class A:
    ...

  • Currently, converter only supports named functions. If mypy finds something else it will complain about not understanding the argument and the type annotation in __init__ will be replaced by Any.

  • Validator decorators and default decorators are not type-checked against the attribute they are setting/validating.

  • Method definitions added by mypy currently overwrite any existing method definitions.