# Why must Python dictionary keys be immutable?

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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:

```python
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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