Class basics¶
This section will help get you started annotating your
classes. Built-in classes such as int
also follow these same
rules.
Instance and class attributes¶
The 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 = [1] # OK
As in Python generally, a variable defined in the class body can be used
as a class or an instance variable. (As discussed in the next section, you
can override this with a ClassVar
annotation.)
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 self
:
class A:
def __init__(self) -> None:
self.y = 1 # Define 'y'
a = self
a.x = 1 # Error: 'x' not defined
Annotating __init__ methods¶
The __init__
method is somewhat special – it doesn’t return a
value. This is best expressed as -> None
. However, since many feel
this is redundant, it is allowed to omit the return type declaration
on __init__
methods if at least one argument is annotated. For
example, in the following classes __init__
is considered fully
annotated:
class C1:
def __init__(self) -> None:
self.var = 42
class C2:
def __init__(self, arg: int):
self.var = arg
However, if __init__
has no annotated arguments and no return type
annotation, it is considered an untyped method:
class C3:
def __init__(self):
# This body is not type checked
self.var = 42 + 'abc'
Class attribute annotations¶
You can use a ClassVar[t]
annotation to explicitly declare that a
particular attribute should not be set on instances:
from typing import ClassVar
class A:
x: ClassVar[int] = 0 # Class variable only
A.x += 1 # OK
a = A()
a.x = 1 # Error: Cannot assign to class variable "x" via instance
print(a.x) # OK -- can be read through an instance
It’s not necessary to annotate all class variables using
ClassVar
. An attribute without the ClassVar
annotation can
still be used as a class variable. However, mypy won’t prevent it from
being used as an instance variable, as discussed previously:
class A:
x = 0 # Can be used as a class or instance variable
A.x += 1 # OK
a = A()
a.x = 1 # Also OK
Note that ClassVar
is not a class, and you can’t use it with
isinstance()
or issubclass()
. It does not change Python
runtime behavior – it’s only for type checkers such as mypy (and
also helpful for human readers).
You can also omit the square brackets and the variable type in
a ClassVar
annotation, but this might not do what you’d expect:
class A:
y: ClassVar = 0 # Type implicitly Any!
In this case the type of the attribute will be implicitly Any
.
This behavior will change in the future, since it’s surprising.
An explicit ClassVar
may be particularly handy to distinguish
between class and instance variables with callable types. For example:
from typing import Callable, ClassVar
class A:
foo: Callable[[int], None]
bar: ClassVar[Callable[[A, int], None]]
bad: Callable[[A], None]
A().foo(42) # OK
A().bar(42) # OK
A().bad() # Error: Too few arguments
Note
A ClassVar
type parameter cannot include type variables:
ClassVar[T]
and ClassVar[list[T]]
are both invalid if T
is a type variable (see Defining generic classes
for more about type variables).
Overriding statically typed methods¶
When overriding a statically typed method, mypy checks that the override has a compatible signature:
class Base:
def f(self, x: int) -> None:
...
class Derived1(Base):
def f(self, x: str) -> None: # Error: type of 'x' incompatible
...
class Derived2(Base):
def f(self, x: int, y: int) -> None: # Error: too many arguments
...
class Derived3(Base):
def f(self, x: int) -> None: # OK
...
class Derived4(Base):
def f(self, x: float) -> None: # OK: mypy treats int as a subtype of float
...
class Derived5(Base):
def f(self, x: int, y: int = 0) -> None: # OK: accepts more than the base
... # class method
Note
You can also vary return types covariantly in overriding. For
example, you could override the return type Iterable[int]
with a
subtype such as list[int]
. Similarly, you can vary argument types
contravariantly – subclasses can have more general argument types.
In order to ensure that your code remains correct when renaming methods,
it can be helpful to explicitly mark a method as overriding a base
method. This can be done with the @override
decorator. @override
can be imported from typing
starting with Python 3.12 or from
typing_extensions
for use with older Python versions. If the base
method is then renamed while the overriding method is not, mypy will
show an error:
from typing import override
class Base:
def f(self, x: int) -> None:
...
def g_renamed(self, y: str) -> None:
...
class Derived1(Base):
@override
def f(self, x: int) -> None: # OK
...
@override
def g(self, y: str) -> None: # Error: no corresponding base method found
...
Note
Use –enable-error-code explicit-override to require
that method overrides use the @override
decorator. Emit an error if it is missing.
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 Base:
def inc(self, x: int) -> int:
return x + 1
class Derived(Base):
def inc(self, x): # Override, dynamically typed
return 'hello' # Incompatible with 'Base', 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 any concrete (non-abstract) subclass. You can define abstract base
classes using the abc.ABCMeta
metaclass and the @abc.abstractmethod
function decorator. Example:
from abc import ABCMeta, abstractmethod
class Animal(metaclass=ABCMeta):
@abstractmethod
def eat(self, food: str) -> None: pass
@property
@abstractmethod
def can_walk(self) -> bool: pass
class Cat(Animal):
def eat(self, food: str) -> None:
... # Body omitted
@property
def can_walk(self) -> bool:
return True
x = Animal() # Error: 'Animal' is abstract due to 'eat' and 'can_walk'
y = Cat() # 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.
Since you can’t create instances of ABCs, they are most commonly used in
type annotations. For example, this method accepts arbitrary iterables
containing arbitrary animals (instances of concrete Animal
subclasses):
def feed_all(animals: Iterable[Animal], food: str) -> None:
for animal in animals:
animal.eat(food)
There is one important peculiarity about how ABCs work in Python –
whether a particular class is abstract or not is somewhat implicit.
In the example below, Derived
is treated as an abstract base class
since Derived
inherits an abstract f
method from Base
and
doesn’t explicitly implement it. The definition of Derived
generates no errors from mypy, since it’s a valid ABC:
from abc import ABCMeta, abstractmethod
class Base(metaclass=ABCMeta):
@abstractmethod
def f(self, x: int) -> None: pass
class Derived(Base): # No error -- Derived is implicitly abstract
def g(self) -> None:
...
Attempting to create an instance of Derived
will be rejected,
however:
d = Derived() # Error: 'Derived' is abstract
Note
It’s a common error to forget to implement an abstract method. As shown above, the class definition will not generate an error in this case, but any attempt to construct an instance will be flagged as an error.
Mypy allows you to omit the body for an abstract method, but if you do so,
it is unsafe to call such method via super()
. For example:
from abc import abstractmethod
class Base:
@abstractmethod
def foo(self) -> int: pass
@abstractmethod
def bar(self) -> int:
return 0
class Sub(Base):
def foo(self) -> int:
return super().foo() + 1 # error: Call to abstract method "foo" of "Base"
# with trivial body via super() is unsafe
@abstractmethod
def bar(self) -> int:
return super().bar() + 1 # This is OK however.
A class can inherit any number of classes, both abstract and concrete. As with normal overrides, a dynamically typed method can override or 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.
Slots¶
When a class has explicitly defined __slots__,
mypy will check that all attributes assigned to are members of __slots__
:
class Album:
__slots__ = ('name', 'year')
def __init__(self, name: str, year: int) -> None:
self.name = name
self.year = year
# Error: Trying to assign name "released" that is not in "__slots__" of type "Album"
self.released = True
my_album = Album('Songs about Python', 2021)
Mypy will only check attribute assignments against __slots__
when
the following conditions hold:
All base classes (except builtin ones) must have explicit
__slots__
defined (this mirrors Python semantics).__slots__
does not include__dict__
. If__slots__
includes__dict__
, arbitrary attributes can be set, similar to when__slots__
is not defined (this mirrors Python semantics).All values in
__slots__
must be string literals.