How to fix TypeError: unhashable type: 'dict'

While working with Python dictionaries, you might encounter the following error:

TypeError: unhashable type: 'dict'

This error usually occurs when you attempt to set a dictionary object as the key of a key-pair value.

This tutorial will show you an example that causes this error and how to fix it in practice

How to reproduce this error

Suppose you create a dictionary object as follows:

dict_a = {"last_name": "Doe"}

Next, you tried to set this dictionary as an item in another dictionary as follows:

dict_a = {"last_name": "Doe"}

dict_b = {
    "first_name": "Jane",
    dict_a: "Andy"
}

Output:

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    dict_b = {
TypeError: unhashable type: 'dict'

This error occurs because the dict_a object is being used as the key to the second item in the dict_b object.

In Python, dictionary keys must be a hashable type such as a string, a tuple, a boolean, or an integer. A dictionary is not hashable, so the error is raised.

How to fix this error

To resolve this error, you need to make sure that you’re using a hashable type as the key in a dictionary object.

If you need to add a value from another dictionary, you can assign the value you need to a string key as follows:

dict_a = {"last_name": "Doe"}

dict_b = {
    "first_name": "Jane",
    "last_name": dict_a["last_name"]
}

print(dict_b)

Output:

{'first_name': 'Jane', 'last_name': 'Doe'}

If you want to add dict_a into dict_b, then you need to assign the dictionary as a value to a key you specified in dict_b:

dict_a = {"first_name": "Jane", "last_name": "Doe"}

dict_b = {
    "last_login": "20th March 2023",
    "detail" : dict_a
}

print(dict_b)

Output:

{
    'last_login': '20th March 2023',
    'detail': {'first_name': 'Jane', 'last_name': 'Doe'}
}

In the example, the dict_a object is added under the detail key defined in dict_b.

You can’t add a dictionary as a key to another dictionary, but you can add one as a value.

If you want to add just the key-value pair without creating a nested dictionary, you can use a for loop and the dict.items() method.

The example below shows how to add all key-value pairs in dict_a to dict_b:

dict_a = {"first_name": "Jane", "last_name": "Doe"}

dict_b = {
    "last_login": "20th March 2023"
}

for key, value in dict_a.items():
    dict_b[key] = value

print(dict_b)

Output:

{
    'first_name': 'Jane',
    'last_name': 'Doe',
    'last_login': '20th March 2023'
}

By using the for loop, you don’t need to create a new key in dict_b as in the previous example.

If you absolutely need to use a dictionary as a key in your dictionary object, then you need to use the json.dumps() method to convert the dictionary into a string:

import json

dict_a = {"first_name": "Jane", "last_name": "Doe"}

dict_b = {
    json.dumps(dict_a): "user"
}

print(dict_b)

Output:

{
    '{"first_name": "Jane", "last_name": "Doe"}': 'user'
}

While it works, I think you have a mistake in your source code if you ever need to use a dictionary as a key in another dictionary.

Unless you have a really unique case, strings and integers should be used as keys in a dictionary object to avoid complexity.

I hope this tutorial is helpful. Until next time! 🙌