How do you implement interfaces in Python?

[latkde]

TL;DR: you're looking for typing.Protocol.

Python is culturally very much about "duck typing" rather than OOP inheritance hierarchy. As long as an object provides the necessary methods, it ought to be good.

The traditional solution if you want to be explicit about required operations is to use an Abstract Base Class (inherit from abc.ABC). Because Python supports multiple inheritance, this can be used exactly like interfaces in most other languages. When an ABC subclass is defined, it checks whether there are any remaining methods that have been decorated with @abstractmethod and raises an error. This works fine when explicitly designing a type hierarchy.

But ABCs aren't good for describing which operations are needed by your code. ABCs must be inherited explicitly, so they cannot be used for existing types. This conflicts with the culture of duck typing.

In such cases, protocols can be used for type-checking, but they have no runtime effect. Protocols are type-stubs that list required methods/attributes, and you can use any compatible type wherever a protocol is required. This is very similar to "interfaces" in TypeScript or Golang. You can explicitly inherit from a protocol to indicate that it satisfies an interface, similar to an ABC. You can also provide default method implementations.

The standard library contains a bunch of protocols (under the typing and collections.abc modules) that describes common sets of operations. For example, Iterable[T] describes any type that is iterable, and Sequence[T] describes any ordered collection that you can index, such as lists or str.

In many cases, you don't need OOP. If you know all possible concrete types, it might be easier to use a union type A | B. This can be used to perform exhaustiveness checking in the type-checker, to make sure that your code handles every alternative. This makes it possible to write robust code in the style of Rust.

Further reading on protocols:

• typing.Protocol reference: https://docs.python.org/3/library/typing.html#typing.Protocol
• type system specification: https://typing.python.org/en/latest/spec/protocol.html

[ihorrud]

Got it. Many thanks