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

Exploring the technical reason.

What happens when we add a list as a dictionary’s key in Python?

We all know that Python raises an error.

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

Nothing new here. Everyone knows this.

But I think very few folks question the technical reason behind this.

More specifically:

• Why is a list unhashable?

• How is this implemented in practice?

• If needed, how can we make our custom class objects unhashable because, by default, we can add any custom class object as a dictionary’s key, as shown below:

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

Let’s understand this 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)

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.

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

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