Protocols and structural subtyping¶

The Python type system supports two ways of deciding whether two objects are compatible as types: nominal subtyping and structural subtyping.

Nominal subtyping is strictly based on the class hierarchy. If class Dog inherits class Animal, it’s a subtype of Animal. Instances of Dog can be used when Animal instances are expected. This form of subtyping is what Python’s type system predominantly uses: it’s easy to understand and produces clear and concise error messages, and matches how the native isinstance check works – based on class hierarchy.

Structural subtyping is based on the operations that can be performed with an object. Class Dog is a structural subtype of class Animal if the former has all attributes and methods of the latter, and with compatible types.

Structural subtyping can be seen as a static equivalent of duck typing, which is well known to Python programmers. See PEP 544 for the detailed specification of protocols and structural subtyping in Python.

Predefined protocols¶

The collections.abc, typing and other stdlib modules define various protocol classes that correspond to common Python protocols, such as Iterable[T]. If a class defines a suitable __iter__ method, mypy understands that it implements the iterable protocol and is compatible with Iterable[T]. For example, IntList below is iterable, over int values:

from __future__ import annotations

from collections.abc import Iterator, Iterable

class IntList:
    def __init__(self, value: int, next: IntList | None) -> None:
        self.value = value
        self.next = next

    def __iter__(self) -> Iterator[int]:
        current = self
        while current:
            yield current.value
            current = current.next

def print_numbered(items: Iterable[int]) -> None:
    for n, x in enumerate(items):
        print(n + 1, x)

x = IntList(3, IntList(5, None))
print_numbered(x)  # OK
print_numbered([4, 5])  # Also OK

Predefined protocol reference lists various protocols defined in collections.abc and typing and the signatures of the corresponding methods you need to define to implement each protocol.

Note

typing also contains deprecated aliases to protocols and ABCs defined in collections.abc, such as Iterable[T]. These are only necessary in Python 3.8 and earlier, since the protocols in collections.abc didn’t yet support subscripting ([]) in Python 3.8, but the aliases in typing have always supported subscripting. In Python 3.9 and later, the aliases in typing don’t provide any extra functionality.

Simple user-defined protocols¶

You can define your own protocol class by inheriting the special Protocol class:

from collections.abc import Iterable
from typing import Protocol

class SupportsClose(Protocol):
    # Empty method body (explicit '...')
    def close(self) -> None: ...

class Resource:  # No SupportsClose base class!

    def close(self) -> None:
       self.resource.release()

    # ... other methods ...

def close_all(items: Iterable[SupportsClose]) -> None:
    for item in items:
        item.close()

close_all([Resource(), open('some/file')])  # OK

Resource is a subtype of the SupportsClose protocol since it defines a compatible close method. Regular file objects returned by open() are similarly compatible with the protocol, as they support close().

Defining subprotocols and subclassing protocols¶

You can also define subprotocols. Existing protocols can be extended and merged using multiple inheritance. Example:

# ... continuing from the previous example

class SupportsRead(Protocol):
    def read(self, amount: int) -> bytes: ...

class TaggedReadableResource(SupportsClose, SupportsRead, Protocol):
    label: str

class AdvancedResource(Resource):
    def __init__(self, label: str) -> None:
        self.label = label

    def read(self, amount: int) -> bytes:
        # some implementation
        ...

resource: TaggedReadableResource
resource = AdvancedResource('handle with care')  # OK

Note that inheriting from an existing protocol does not automatically turn the subclass into a protocol – it just creates a regular (non-protocol) class or ABC that implements the given protocol (or protocols). The Protocol base class must always be explicitly present if you are defining a protocol:

class NotAProtocol(SupportsClose):  # This is NOT a protocol
    new_attr: int

class Concrete:
   new_attr: int = 0

   def close(self) -> None:
       ...

# Error: nominal subtyping used by default
x: NotAProtocol = Concrete()  # Error!

You can also include default implementations of methods in protocols. If you explicitly subclass these protocols you can inherit these default implementations.

Explicitly including a protocol as a base class is also a way of documenting that your class implements a particular protocol, and it forces mypy to verify that your class implementation is actually compatible with the protocol. In particular, omitting a value for an attribute or a method body will make it implicitly abstract:

class SomeProto(Protocol):
    attr: int  # Note, no right hand side
    def method(self) -> str: ...  # Literally just ... here

class ExplicitSubclass(SomeProto):
    pass

ExplicitSubclass()  # error: Cannot instantiate abstract class 'ExplicitSubclass'
                    # with abstract attributes 'attr' and 'method'

Similarly, explicitly assigning to a protocol instance can be a way to ask the type checker to verify that your class implements a protocol:

_proto: SomeProto = cast(ExplicitSubclass, None)

Invariance of protocol attributes¶

A common issue with protocols is that protocol attributes are invariant. For example:

class Box(Protocol):
      content: object

class IntBox:
      content: int

def takes_box(box: Box) -> None: ...

takes_box(IntBox())  # error: Argument 1 to "takes_box" has incompatible type "IntBox"; expected "Box"
                     # note:  Following member(s) of "IntBox" have conflicts:
                     # note:      content: expected "object", got "int"

This is because Box defines content as a mutable attribute. Here’s why this is problematic:

def takes_box_evil(box: Box) -> None:
    box.content = "asdf"  # This is bad, since box.content is supposed to be an object

my_int_box = IntBox()
takes_box_evil(my_int_box)
my_int_box.content + 1  # Oops, TypeError!

This can be fixed by declaring content to be read-only in the Box protocol using @property:

class Box(Protocol):
    @property
    def content(self) -> object: ...

class IntBox:
    content: int

def takes_box(box: Box) -> None: ...

takes_box(IntBox(42))  # OK

Recursive protocols¶

Protocols can be recursive (self-referential) and mutually recursive. This is useful for declaring abstract recursive collections such as trees and linked lists:

from __future__ import annotations

from typing import Protocol

class TreeLike(Protocol):
    value: int

    @property
    def left(self) -> TreeLike | None: ...

    @property
    def right(self) -> TreeLike | None: ...

class SimpleTree:
    def __init__(self, value: int) -> None:
        self.value = value
        self.left: SimpleTree | None = None
        self.right: SimpleTree | None = None

root: TreeLike = SimpleTree(0)  # OK

Using isinstance() with protocols¶

You can use a protocol class with isinstance() if you decorate it with the @runtime_checkable class decorator. The decorator adds rudimentary support for runtime structural checks:

from typing import Protocol, runtime_checkable

@runtime_checkable
class Portable(Protocol):
    handles: int

class Mug:
    def __init__(self) -> None:
        self.handles = 1

def use(handles: int) -> None: ...

mug = Mug()
if isinstance(mug, Portable):  # Works at runtime!
   use(mug.handles)

isinstance() also works with the predefined protocols in typing such as Iterable.

Warning

isinstance() with protocols is not completely safe at runtime. For example, signatures of methods are not checked. The runtime implementation only checks that all protocol members exist, not that they have the correct type. issubclass() with protocols will only check for the existence of methods.

Note

isinstance() with protocols can also be surprisingly slow. In many cases, you’re better served by using hasattr() to check for the presence of attributes.

Callback protocols¶

Protocols can be used to define flexible callback types that are hard (or even impossible) to express using the Callable[...] syntax, such as variadic, overloaded, and complex generic callbacks. They are defined with a special __call__ member:

from collections.abc import Iterable
from typing import Optional, Protocol

class Combiner(Protocol):
    def __call__(self, *vals: bytes, maxlen: int | None = None) -> list[bytes]: ...

def batch_proc(data: Iterable[bytes], cb_results: Combiner) -> bytes:
    for item in data:
        ...

def good_cb(*vals: bytes, maxlen: int | None = None) -> list[bytes]:
    ...
def bad_cb(*vals: bytes, maxitems: int | None) -> list[bytes]:
    ...

batch_proc([], good_cb)  # OK
batch_proc([], bad_cb)   # Error! Argument 2 has incompatible type because of
                         # different name and kind in the callback

Callback protocols and Callable types can be used mostly interchangeably. Parameter names in __call__ methods must be identical, unless the parameters are positional-only. Example (using the legacy syntax for generic functions):

from collections.abc import Callable
from typing import Protocol, TypeVar

T = TypeVar('T')

class Copy(Protocol):
    # '/' marks the end of positional-only parameters
    def __call__(self, origin: T, /) -> T: ...

copy_a: Callable[[T], T]
copy_b: Copy

copy_a = copy_b  # OK
copy_b = copy_a  # Also OK

Predefined protocol reference¶

Iteration protocols¶

The iteration protocols are useful in many contexts. For example, they allow iteration of objects in for loops.

collections.abc.Iterable[T]¶

The example above has a simple implementation of an __iter__ method.

def __iter__(self) -> Iterator[T]

collections.abc.Iterator[T]¶

def __next__(self) -> T
def __iter__(self) -> Iterator[T]

Collection protocols¶

Many of these are implemented by built-in container types such as list and dict, and these are also useful for user-defined collection objects.

collections.abc.Sized¶

This is a type for objects that support len(x).

def __len__(self) -> int

collections.abc.Container[T]¶

This is a type for objects that support the in operator.

def __contains__(self, x: object) -> bool

collections.abc.Collection[T]¶

def __len__(self) -> int
def __iter__(self) -> Iterator[T]
def __contains__(self, x: object) -> bool

One-off protocols¶

These protocols are typically only useful with a single standard library function or class.

collections.abc.Reversible[T]¶

This is a type for objects that support reversed(x).

def __reversed__(self) -> Iterator[T]

typing.SupportsAbs[T]¶

This is a type for objects that support abs(x). T is the type of value returned by abs(x).

def __abs__(self) -> T

typing.SupportsBytes¶

This is a type for objects that support bytes(x).

def __bytes__(self) -> bytes

typing.SupportsComplex¶

This is a type for objects that support complex(x). Note that no arithmetic operations are supported.

def __complex__(self) -> complex

typing.SupportsFloat¶

This is a type for objects that support float(x). Note that no arithmetic operations are supported.

def __float__(self) -> float

typing.SupportsInt¶

This is a type for objects that support int(x). Note that no arithmetic operations are supported.

def __int__(self) -> int

typing.SupportsRound[T]¶

This is a type for objects that support round(x).

def __round__(self) -> T

Async protocols¶

These protocols can be useful in async code. See Typing async/await for more information.

collections.abc.Awaitable[T]¶

def __await__(self) -> Generator[Any, None, T]

collections.abc.AsyncIterable[T]¶

def __aiter__(self) -> AsyncIterator[T]

collections.abc.AsyncIterator[T]¶

def __anext__(self) -> Awaitable[T]
def __aiter__(self) -> AsyncIterator[T]

Context manager protocols¶

There are two protocols for context managers – one for regular context managers and one for async ones. These allow defining objects that can be used in with and async with statements.

contextlib.AbstractContextManager[T]¶

def __enter__(self) -> T
def __exit__(self,
             exc_type: type[BaseException] | None,
             exc_value: BaseException | None,
             traceback: TracebackType | None) -> bool | None

contextlib.AbstractAsyncContextManager[T]¶

def __aenter__(self) -> Awaitable[T]
def __aexit__(self,
              exc_type: type[BaseException] | None,
              exc_value: BaseException | None,
              traceback: TracebackType | None) -> Awaitable[bool | None]