How Python Prevents Us from Adding a List as a Dictionary's Key?

Exploring the technical reason.

We all know that Python raises an error when we add a list as a dictionary’s key:

Adding a list to a dictionary’s key raises an error

But have you ever wondered how exactly this is implemented in practice? How Python prevents us from adding a list as a dictionary’s key?

This is important to know because in Python, if we define a custom class, then by default, its objects can be added as a dictionary’s key, as shown below:

Adding a custom class object to a dictionary’s key works fine

This behavior may not be desired at times.

Let’s understand the internal mechanisms today!

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First, we must understand that EVERYTHING in Python is an object instantiated from some class:

• List, tuple, dictionary, etc. are class objects.

• Int, float, and string are class objects.

• Functions are class objects.

• In fact, even classes are also objects.

Adding an object as a dictionary’s key is like hashing the object.

Thus, whenever we add an object as a dictionary’s key, Python invokes the __hash__() magic method of that object’s class.

Now, the implementation of __hash__() magic method for classes of int, tuple, string, etc., allows its objects to be hashable.

The integer class returns a valid hash value for the object (Note: integer is just a dummy name here)

However, the List class raises an error:

The list class raises an error in the hash method (Note: PyList is just a dummy name here to explain the point)

That is why whenever we add a list as a dictionary’s key, Python raises an error:

Adding a list to a dictionary’s key raises an error

Thus, if needed, we can use the same idea on our custom classes and make its object unhashable. This is shown below:

Implement the hash magic method in a custom class

Now, if we add its objects to a dictionary, Python will raise an error:

Adding a custom class object to a dictionary’s key raises an error

Now you have the complete information (conceptual and practical) on why some objects can be added as a dictionary’s key while some cannot.

Here’s a full deep dive into Python OOP if you want to learn more about advanced OOP in Python: Object-Oriented Programming with Python for Data Scientists.

👉 Over to you: What are some other core yet overlooked concepts in Python?

Thanks for reading!

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