Understanding Type Representation in Python: Custom Classes vs. Built-ins

Explore how to create a type in Python that only accepts custom class instances and not built-in types like `int`, `str`, or `dict`. Learn about the limitations and distinctions in type representation.
---

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: What is the type that represents any object of any custom class?

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Type Representation in Python: Custom Classes vs. Built-ins

In the world of programming, particularly in Python, one persistent inquiry is the creation of type representations that can specify the kind of data a function is willing to accept. More specifically, developers often want to know how to define a type that allows instances of any custom class but categorically rejects built-in types like int, str, or list. This post will delve into this common question and clarify the solution to this type representation challenge.

The Core Question

To set the stage, let's break down the initial problem. Consider the following function definition:

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

The aim here is for MyType to act as a filter: it should produce a type error if an attempt is made to pass a built-in type:

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

The question that arises is: What would MyType be in this context?

The Solution: A Distinction that Doesn't Exist

Upon investigating this question, we arrive at an important realization: there is no such type in Python. Python does not make a distinction between built-in types and custom classes in the way that many developers might be seeking.

Here’s Why:

Python is Dynamically Typed: In Python, type checks are often performed at runtime. This means that all objects fall into the category of instances, whether they are built-in types or user-defined classes.

Lack of Type Discrimination: Because Python doesn't natively provide a way to declare a type that explicitly excludes built-in types, there’s no MyType that serves this purpose directly. The typing system allows for various customizable types, but it does not enable the exclusion of built-in types in the manner described.

Making the Best of Python's Type System

While we can’t create a type that exclusively accepts custom classes, there are some approaches you can take to achieve behavior that mimics this goal:

Custom Type Check in the Function: Use a runtime check within your function to ensure that the passed argument is an instance of a custom class. For example:

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

Utilizing Type Hints: While this doesn’t enforce restrictions, you can still use type hints to indicate that a function is designed to take a specific type, knowing that type safety checks won’t prevent built-in types from being passed:

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

Conclusion

In summary, while the desire to define a type that accepts only instances of custom classes and rejects built-in types is understandable, Python's type system does not support this differentiation directly. Understanding how Python handles types can help developers navigate limitations and adopt alternative strategies for type checking and validation.

By embracing the fluidity of Python’s type system, you can engage in effective programming practices while effectively managing types, ensuring that your functions work seamlessly with the intended data.

Now that we've tackled this intriguing question, feel free to explore the comments or ask about related topics! Let's continue the conversation about Python’s type system together!