How to define a property as part of a Protocol

Question

Could somebody please clarify the generally accepted way of defining a property as part of a Protocol?

I've been doing it like so:

from typing import Protocol

class MyProtocol(Protocol):
    @property
    def my_property(self):
        """ Classes should have a my_property to conform to MyProtocol """

But Pycharm's built-in linter keeps complaining that a "Getter should return or yield something", hence my confusion. Raising a NotImplementedError makes the linter message go away, but doing that seems to fit more with an abstract class than with a Protocol.

Monroe

why would you want to use @property in a protocol? I mean, it's a protocol, not a base class – Sydelle

@Marat, to me it seems useful to be specific about the intent of my protocol, i.e. I would want to signal to any interface that depends on this protocol that they should access a property instead of a method. – Monroe

From the outsider's perspective, it is just a property, and from the standpoint of protocol definition it should remain as such. Implementation details should be left to implementation or inherited classes. – Sydelle

If you could post the information above as an answer I will make sure to accept and upvote :) – Monroe

@Sydelle its not just a property, its a read only property. It makes a difference if the implementation allows a public facing property to be set. – Bluegreen

@Bluegreen it is a Protocol, a thing used for structural subtyping. Introducing implementation details into this abstraction kind of ruins its purpose. UPD: we had this exact discussion in the comments above - please read – Sydelle

In the abstract, properties can absolutely make sense in a protocol, without being some leak of implementation detail. For example, it’d be perfectly valid for a “WheeledVehicle” protocol to require conforming types to have a getter for a numberOfWheels. If that weren’t allowed, you’d have to use a function like getNumberOfWheels(), which you wouldn’t otherwise do. Though if the language’s definition of properties is only stored properties (and doesn’t support computed properties), then the function is a more flexible bet. – Paleface

Even the protocols PEP itself speaks of properties. On the other hand, this is at the time of writing still an open issue in PyCharm. – Nichol

Answer 1

Defining a property in a protocol is supported in the latest mypy: https://mypy.readthedocs.io/en/stable/common_issues.html?highlight=%40property#covariant-subtyping-of-mutable-protocol-members-is-rejected

In this case, you can simply define the property in the protocol and in the classes that conform to that protocol:

from typing import Protocol

class MyProtocol(Protocol):
    @property
    def my_property(self) -> str:
        ...

class MyClass(MyProtocol):
    @property
    def my_property(self) -> str:
        # the actual implementation is here

Cosmology

pycharm says "Getter should return or yield something" – Thistledown

@Thistledown I just tested this snippet in the latest pyright and the latest mypy, and I didn't get any errors or warnings. It looks like PyCharms implementation of the spec raises this false positive: youtrack.jetbrains.com/issue/PY-40180/… – Cosmology

yeah copy that. finding a lot of these inspection bugs recently – Thistledown

Answer 2

They are separate. Property is to allow a function to be called like an attribute. for example

class MyClassProtocol(Protocol):
    def example() -> int: ...

class MyClass(MyClassProtocol):
    @property
    def example() -> int:
        return 5

tmp = MyClass()
tmp.example

5

That is why your IDE is picking up the must return or yield something because a property is expecting a return value.

Hanaper

I downvote, because your sample won't pass type checking with any conformant checker. – Codfish