Distinguishing Between Base Class and Derived Class in Python Generics Using Typing and Mypy

Learn how to enforce strict type checks in Python generics with Typing and Mypy, differentiating between base and derived classes for better type safety.
---

Visit these links for original content and any more details, such as alternate solutions, comments, revision history etc. For example, the original title of the Question was: How to distinguish between Base Class and Derived Class in generics using Typing and Mypy

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Base and Derived Classes in Python Generics

When working with Python's type system, especially in relation to generics, distinguishing between base classes and derived classes can sometimes be tricky. In this post, we will explore a common problem encountered in Python code: ensuring that two instances passed to a function, which could be of different classes within the same inheritance hierarchy, are of the same type. We will demonstrate how to implement a solution using the Typing library and Mypy for type checking.

The Problem

Let's consider a basic example with two classes, A (the base class) and B (the derived class). We want to enforce that any two instances of these classes passed to our function must be of the same type. The naive implementation might fail to catch errors when different subtypes are provided, leading to runtime errors that could have been avoided. Here’s the foundational code:

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

In the example above, Mypy does not catch errors on the last two calls because it checks for the type of the base class A without regard for the actual subclass instance, allowing for potential type errors at runtime.

The Solution

To ensure type safety in our generic function, we can use overloading in Python. The overload decorator allows us to define multiple signatures for the same function. This way, Mypy can distinguish between different calls based on argument types.

Step-by-step Implementation

Defining the Classes

We start by defining the classes A and B as before:

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

Using Overloads

We can now set up overloaded function signatures to handle this type checking:

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

Defining the Function

Next, we define the actual function logic, ensuring to implement the type checking inside it:

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

Example Function Calls

Finally, let's look at function calls:

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

Any attempt to call the function with a mix of types will now raise a TypeError at runtime and be flagged by Mypy during static type checking.

Conclusion

With the mentioned techniques, we can create a generic function in Python that enforces strict type conformity between base and derived classes. By leveraging overloading and the TypeVar from the Typing module, we enhance the reliability and clarity of our type checks, resulting in code that's both safer and easier to maintain.

We hope you found this breakdown helpful for working with generics in Python! Happy coding!