This section will help get you started annotating your
classes. Built-in classes such as
int also follow these same
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 # Aha, 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 annotation:
class A: x: List[int] # Declare attribute 'x' of type List[int] a = A() a.x =  # OK
As in Python generally, a variable defined in the class body can used as a class or an instance variable.
Type comments work as well, if you need to support Python versions earlier than 3.6:
class A: x = None # type: List[int] # Declare attribute 'x' of type List[int]
Note that attribute definitions in the class body that use a type comment
are special: a
None value is valid as the initializer, even though
the declared type is not optional. This should be used sparingly, as this can
None-related runtime errors that mypy can’t detect.
Similarly, you can give explicit types to instance variables defined in a method:
class A: def __init__(self) -> None: self.x: List[int] =  def f(self) -> None: self.y: Any = 0
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
int. Similarly, you can vary argument types
contravariantly – subclasses can have more general argument types.
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.
As always, relying on dynamically typed code can be unsafe. 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' # Incompatible with 'A', but no mypy error
Abstract base classes and multiple inheritance¶
Mypy supports Python abstract base classes (ABCs). Abstract classes
have at least one abstract method or property that must be implemented
by a subclass. You can define abstract base classes using the
abc.ABCMeta metaclass, and the
abc.abstractproperty function decorators. Example:
from abc import ABCMeta, abstractmethod 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
Note that mypy performs checking for unimplemented abstract methods
even if you omit the
ABCMeta metaclass. This can be useful if the
metaclass would cause runtime metaclass conflicts.
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 method defined in any base class, including an abstract method defined in an abstract base class.
You can implement an abstract property using either a normal property or an instance variable.