Why must Python dictionary keys be immutable?

Why must Python dictionary keys be immutable?

Python Dictionary key must be immutable, why?

Gaurav Sharma's photo
Gaurav Sharma
·May 28, 2022·

2 min read

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Python dictionary keys must always be immutable, which means you can not update the dictionary key in runtime.

But the Questions is Why Python dictionary keys should always be immutable ?

Let’s see:

Let’s run the below code and understand its output:

mydict = { [1, 2]: '12' }
print(mydict[[1, 2]])
# Output: TypeError: unhashable type: 'list'

So the output is an Exception, What does it mean by unhashable ?

It simply means that the Python interpreter is not able to generate a hash for the key given in the dictionary, whose data type is a list.

Why Python is generating a Hash for the dictionary key?

As per the design implementation of Python dictionary, Python dictionaries are saved in the memory using hash tables.

And In most versions of hash tables, there is no way of changing the key of an entry. This is because the key's hash value is very important, changing the key almost always changes the hash value.

If the key were a mutable object, its value could change, and thus its hash could also change

Then, why python was not generating the hash for a list, used in a dictionary as a key?

Because Python list is a mutable data type, which means it can be modified during runtime, that's why it is not used as a key in a dictionary.

Imagine if we can change the list (dictionary key) in runtime, and if dictionary key changes then how can you retrieve its values because its hash value is not valid now.

So what can we use as a key in a Python dictionary?

We can use string, number, tuple as a key in Python dictionary because it can not be modified.

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