Instance and class attributes¶
Mypy type checker detects if you are trying to access a missing attribute, which is a very common programming error. For this to work correctly, instance and class attributes must be defined or initialized within the class. Mypy infers the types of attributes:
class A: def __init__(self, x: int) -> None: self.x = x # Attribute x of type int a = A(1) a.x = 2 # OK a.y = 3 # Error: A has no attribute y
This is a bit like each class having an implicitly defined
__slots__ attribute. This is only enforced during type
checking and not when your program is running.
You can declare types of variables in the class body explicitly using a type comment:
class A: x = None # type: List[int] # Declare attribute x of type List[int] a = A() a.x =  # OK
As in Python, a variable defined in the class body can used as a class or an instance variable.
Similarly, you can give explicit types to instance variables defined in a method:
class A: def __init__(self) -> None: self.x =  # type: List[int] def f(self) -> None: self.y = 0 # type: Any
You can only define an instance variable within a method if you assign
to it explicitly using
class A: def __init__(self) -> None: self.y = 1 # Define y a = self a.x = 1 # Error: x not defined
Overriding statically typed methods¶
When overriding a statically typed method, mypy checks that the override has a compatible signature:
class A: def f(self, x: int) -> None: ... class B(A): def f(self, x: str) -> None: # Error: type of x incompatible ... class C(A): def f(self, x: int, y: int) -> None: # Error: too many arguments ... class D(A): def f(self, x: int) -> None: # OK ...
You can also vary return types covariantly in overriding. For
example, you could override the return type
object with a subtype
You can also override a statically typed method with a dynamically typed one. This allows dynamically typed code to override methods defined in library classes without worrying about their type signatures.
There is no runtime enforcement that the method override returns a value that is compatible with the original return type, since annotations have no effect at runtime:
class A: def inc(self, x: int) -> int: return x + 1 class B(A): def inc(self, x): # Override, dynamically typed return 'hello' b = B() print(b.inc(1)) # hello a = b # type: A print(a.inc(1)) # hello
Abstract base classes and multiple inheritance¶
Mypy uses Python abstract base classes for protocol types. There are
several built-in abstract base classes types (for example,
Iterator). You can define abstract
base classes using the
abc.ABCMeta metaclass and the
abc.abstractmethod function decorator.
from abc import ABCMeta, abstractmethod import typing class A(metaclass=ABCMeta): @abstractmethod def foo(self, x: int) -> None: pass @abstractmethod def bar(self) -> str: pass class B(A): def foo(self, x: int) -> None: ... def bar(self) -> str: return 'x' a = A() # Error: A is abstract b = B() # OK
Unlike most Python code, abstract base classes are likely to play a significant role in many complex mypy programs.
A class can inherit any number of classes, both abstract and concrete. As with normal overrides, a dynamically typed method can implement a statically typed abstract method defined in an abstract base class.
There are also plans to support more Python-style “duck typing” in the type system. The details are still open.