How to Alias an Annotated Type in Python

Learn how to create an `Array` type alias in Python to associate types with sizes using `Annotated`. This blog provides a clear solution for your typing dilemmas.
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How to Alias an Annotated Type in Python

In Python development, especially when using type hints, you may encounter several scenarios where you want to create a type that not only denotes a data type but also contains additional metadata, such as its size or constraints. One common problem is defining an Array type that associates a base type with a specific positive number.

In this post, we will explore the solution to this problem by creating an alias to form a type that effectively utilizes the Annotated feature in Python's typing module.

Understanding the Problem

The initial challenge involves creating an Array type that can help users document the intended use of a tuple with a specified length. The following code snippet outlines a desire to create an array of four floating point numbers:

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

While the intention is clear, the attempt to use Annotated directly in the following manner does not succeed:

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

Attempting to subscript Array leads to an error stating:

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

This indicates that our approach must be rethought.

Solution: Custom Class with __class_getitem__

The effective solution to creating an aliased typed array relies on defining a custom class that implements the __class_getitem__ method. This method allows the class to be used with the subscript notation:

Step 1: Define the Array Class

Here’s how you can implement the Array class:

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

Step 2: Use the Array Class in Your Code

Now, you can easily define the MyIp class that uses the Array type:

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

Important Considerations

While this solution works well for documentation purposes, it's essential to be aware of certain limitations:

Immediate Evaluation: The definition Array[float, 4] is evaluated immediately, and thus when you check MyIp.__annotations__, it will refer directly to Annotated[float, 4] instead of Array[float, 4].

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

Conclusion

Creating aliased types in Python can be a powerful way to add clarity and robustness to your code. By leveraging the custom __class_getitem__ method, you can define more expressive types that meet your documentation needs.

Feel free to implement this technique in your projects and enhance the way you handle typed structures in Python development!