How do I type hint a method with the type of the enclosing class?

python

type-hinting

annotations

generics

byAlex Kataev·Oct 13, 2024

To hint a method's return type as the enclosing class, use the string literal of the class name ("MyClass") or in Python 3.7 and later, __future__.annotations, to avoid quotes. In Python 3.10 and later, use the typing.Self shortcut:

# Python 3.7+ with future annotations
from __future__ import annotations

class MyClass:
    def my_method(self) -> MyClass:
        # "Who am I? Oh, right - I'm MyClass!"
        return self 

# Python 3.10+ with Self
from typing import Self

class MyClass:
    def my_method(self) -> Self:
        # "Self-awareness level: Pythonic!"
        return self

Choose the one in-sync with your Python version for a more streamlined syntax.

If times they are a-changin'

Handling Divergence: Backwards Compatibility

Python's progression has provided us with many ways to hint at the containing class within annotations. Regardless, not every developer has the luxury to immediately hop onto latest updates. It's essential to cherish older versions by using typing_extensions.Self where typing.Self hasn't entered the scene or simply stick to string literals.

Tapping into the Power of TypeVar

There is also the method of defining a TypeVar that is bound to the containing class. This is smoke and mirrors for generics.

from typing import TypeVar

T = TypeVar('T', bound='MyClass')

class MyClass:
    def my_method(self: T) -> T:
        # "I may not know T, but I know it's all about ME!"
        return self

This maneuver is like a Swiss Army knife in generic programming, allowing your class to be extensible while making sure the annotations remain shipshape.

When `type(self)` is the MVP

Storming the ramparts is the use of type(self) in the return type annotation. This dynamically indicates the actual runtime type and is particularly handy in managing inheritance hierarchies.

class MyBaseClass:
    def clone(self) -> 'MyBaseClass':
        # "I may be base class, but watch me multiply!"
        return type(self)()

The polymorphic essence of object-oriented programming is thus reinforced as derived classes return their own type when cloned - not the mundane base class.

Casting a wider net

"The future is here, old man!" - Postponed Evaluation

With the advent of Python 3.7 and thanks to PEP 563's postponed evaluation of annotations, you don't need to squint and see forward references using strings. Import from __future__ import annotations, and now all your annotations behave like good kids who don't throw tantrums (aka they are stored as strings until called upon by type checkers and fancy IDEs).

PEP 673: All About Self Importance

PEP 673 is the new cool kid on the block in Python 3.11, introducing the Self type. It not only gives methods that return instances of their class, it also gives projections a clear picture, especially when dealing with inheritance and factory methods.

PEP 649: Decorators Reloaded

PEP 649 comes with a Matrix-like twist and provides another solution where annotations, like Neo, dodge bullets and are wrapped in descriptors that can wade through even when decorators pose challenges.

Watch Your Step! Pitfalls Ahead

Depending too much on string annotations without robust linting is like walking a tightrope without a safety net. It could lead to documentation that misguides. To harness the full potential of type hinting in reinforcing readability and waving the flag of documentation, developers need to marry consistency with accuracy.

explain-codes / Python / How do I type hint a method with the type of the enclosing class?

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