shoyer shoyer - 1 month ago 18
Python Question

Python type annotation for custom duck type

Python's

typing
module defines a number of duck types, e.g.,
typing.SupportsAbs
to represent any type that implements the
__abs__
special method.

Is it possible to define custom duck types in a way such that I can use them as valid type annotations?

For example, I would like to be able to annotate that an argument should be a duck-type equivalent of a
threading.Lock
, i.e., any object that implements
acquire
and
release
methods. Ideally, I could annotate such an argument as
SupportsAcquireAndRequire
or
DuckLock
, rather than
object
.

Answer

You can define an abstract base class (ABC) to specify the interface:

from abc import ABCMeta, abstractmethod

class SupportsAcquireAndRequire(metaclass=ABCMeta):
    @abstractmethod
    def acquire(self):
        pass

    @abstractmethod
    def release(self):
        pass

    @classmethod
    def __subclasshook__(cls, C):
        for method in ('release', 'acquire'):
            for B in C.__mro__:
                if method in B.__dict__:
                    if B.__dict__[method] is None:
                        return NotImplemented
                    break
            else:
                return NotImplemented
        return True

This is basically how the protocols (like typing.SupportsAbs) are implemented, albeit without directly using ABCMeta.

By giving the ABC a __subclasshook__ method, you can use it in isinstance() and issubclass() tests, which is more than good enough for tools like mypy:

>>> from threading import Lock
>>> isinstance(Lock(), SupportsAcquireAndRequire)
True