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Learn how to create a custom class that inherits from `functools.cached_property` with proper type hinting in Python. Discover the steps to ensure your properties return the correct type.
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How to Properly Type Hint a Custom Class Inheriting from cached_property in Python

When working with Python, particularly with type hinting in modern frameworks, you may run into situations where you want to customize existing behaviors. One common scenario is wanting to create a custom property decorator that inherits from functools.cached_property. This is useful for encapsulating some logic under a different name while still benefiting from the caching behavior provided by the original implementation. However, type hinting can become a challenge, leading to confusion regarding the expected return types. In this guide, we'll address how to create your own my_property class, derived from cached_property, and ensure that it is correctly type-hinted for better code clarity and functionality.

The Problem with Type Hinting

In your custom property, you may find that the type hint for the attribute is inferred as Any, rather than the specific type you expect (e.g., str). This is a common issue when extending classes and might lead to misleading type guidance in your IDE, making it hard to maintain or use effectively. Below is the example issue that illustrates this problem:

[[See Video to Reveal this Text or Code Snippet]]

Observations

The foo attribute is recognized as a property but is incorrectly classified with an Any type.

The oof attribute, still utilizing cached_property, is correctly recognized as returning a str type.

The Solution

To overcome the type hinting problem, we need to leverage the capabilities of the typing module in Python. Particularly, the Generic type can be integrated to allow more specific type annotations. Let’s break down the adjusted version of your my_property class.

Step-by-Step Implementation

Use Generic: Import Generic from the typing module.

Define the Generic Type Variable: Use <T> to denote the type your property is expected to return.

Override the _init_ method: Ensure it takes a callable that resolves to the expected type.

Override the _get_ method: This should also return the correct type.

Here’s how the refactored version of your my_property would look:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of Key Changes

Generic[T]: By subclassing Generic[T], we signify that my_property can operate with any specified type T.

init method: Accepts a function that returns a value of type T. This establishes the expected output type at the moment of property creation.

get method: With a specified return type T, calling this method will now yield a result with correct typing, ensuring that IDEs like VS Code provide accurate information.

Advantages of This Approach

Better Type Inference: Your custom property now behaves like any standard property with clear typing; IDEs will provide accurate hints and validations.

Reusable Code: By using Generics, you can create properties with multiple types without duplicating code.

Improved Readability: Future developers (and your future self!) will find the code easier to understand and maintain.

Conclusion

Creating a custom property that inherits from cached_property in Python can improve the readability and organization of your code. By implementing type hinting with Generic, you ensure your properties are not only functional but also self-documenting, leading to better development experiences. With these adjustments, you're now equipped to create robust custom properties that are both efficient and easy to work with. Happy coding!