Unhashable Type 'dict'

In Python, a dictionary refers to an unordered collection of key-value pairs. Each key is mapped to a specific value within the dictionary. A dictionary's key is unique and is used to access the corresponding value.

However, a key of a Python dictionary must be hashable. A hashable object means that the object is immutable and can produce a unique hash value.

Hence, using an unhashable object as a dictionary's key will result in an error.

Let us discuss how we can resolve this error.

Example Error

Consider the code below that illustrates how this type of error occurs.

>>> developers = {"games": 200, "database": 100, "full_stack": 500}
>>> employees = {developers: 800}

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'dict'

In the example above, we start by defining a dictionary that holds the type of developer and the total number.

We create an employee dictionary that takes the developer's dictionary as the key in the second part.

Since a dictionary is unhashable, Python cannot use it as a dictionary's key.

Python Check If Object is Hashable

The question comes, how do I know if an object is hashable or not? In Python, you can use the hash function to determine if an object is hashable.

An example is as shown:

>>> hash("key")
-4481039435095198476

If an object is hashable, it should return a unique hash as shown in the example above. Since a string is hashable, it returns a unique hash.

If you attempt the same operation on a dictionary or a list, you will get an error as shown:

>>> hash([1,2,3,4,5])

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

It is not hashable since a list is mutable, as shown in error above.

Fix: TypeError: unhashable type: 'dict'

If you want to use a dictionary as a key for another dictionary, you can do this by first converting it into a tuple.

A tuple is hashable as shown in the example below:

>>> hash((1,2,3,4,5))
-5659871693760987716

Let us solve our first error by converting the developer's dictionary into a tuple:

>>> developers = tuple(developers)
>>> print(developers)
('games', 'database', 'full_stack')

Once we convert it into a tuple, we can use it as key in the dictionary as shown:

>>> employees = {developers: 800}
>>> print(employees)
{('games', 'database', 'full_stack'): 800}

Fix 2

Another solution is using the dictionary as a value. For example, we can use the developer's dictionary as a value for the employees' dictionary.

Take the code illustration below:

>>> developers = {"games": 200, "database": 100, "full_stack": 500}
>>> employees = {"employees": developers}

>>> print(employees)
{'employees': {'games': 200, 'database': 100, 'full_stack': 500}}

In this case, the code does not return an error as we are using the dictionary as value for a new dictionary.

Using a For Loop

Another unconventional method is using a for loop. You can iterate over each key and value pairs of one dictionary and add them to a new dictionary.

Consider the example below:

>>> developers = {"games": 200, "database": 100, "full_stack": 500}
>>> for k, v in developers.items():
... employees[k] = v
...
>>> employees
{'employees': {'games': 200, 'database': 100, 'full_stack': 500}, 'games': 200, 'database': 100, 'full_stack': 500}

In this case, we use a for loop to iterate over the developers' dictionary items and append them to the employees' dict.

Conclusion

This article explored three methods of fixing the TypeError: Unhashable type: 'dict' error in Python.