How to Solve Python TypeError: Object of type dict_keys is not JSON Serializable

This error occurs when we try to serialize a dict_keys object to a JSON string using the json.dumps() method. You can solve this error by converting the dict_keys object to a list using the built-in list() method. For example,

import json 

my_dict = {'particle':'electron', 'mass':0.511, 'charge': -1}

keys = list(my_dict.keys())

json_str = json.dumps(keys)

This tutorial will go through the error in detail and how to solve it with code examples.

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TypeError: Object of type dict_keys is not JSON Serializable

Let’s break up the error message to understand what the error means. TypeError occurs whenever you attempt to use an illegal operation for a specific data type. The part “Object of type dict_values” tells us the error is due to an illegal operation with an object of the dict_keys class. dict_keys is a view object that displays all of the keys in the given dictionary. The view object changes as the dictionary changes.

Serialization in Python refers to converting a Python object into a transmittable format that we can recreate when needed using deserialization. JSON serialization returns a human-readable string form called a JSON string. The JSON encoder json.dump() and json.dumps() can only serialize certain object types like dictionaries, lists, or strings.

“is not JSON serializable” informs us that the JSON serialization is an illegal operation for the dict_keys type.

Example

Let’s look at an example of serializing a dict_keys view object. First, we will import the json module and then define a dictionary.

import json 

my_dict = {'pizza':'tartufo', 'is_vegetarian':False, 'price': 13.99}

The above dictionary contains information about a pizza. Next, we will call the keys method on the dictionary object to get the dict_keys view object.

keys = my_dict.keys()

Next, we will attempt to serialize the dict_keys object to a JSON string by passing it to the json.dumps() method.

json_str = json.dumps(keys)

print(json_str)

Let’s run the code to see what happens:

TypeError: Object of type dict_keys is not JSON serializable

The error occurs because keys is a dict_keys object, which is not JSON serializable. We can check the type of an object by using the built-in type() method as follows:

print(type(keys))

<class 'dict_keys'>

Solution #1: Convert dict_keys to list

The simplest way to solve this error is to convert the dict_keys object to a list using the built-in list() method. The list data type is JSON serializable.

Let’s look at the revised code:

import json 

my_dict = {'pizza':'tartufo', 'is_vegetarian':False, 'price': 13.99}

keys = list(my_dict.keys())

json_str = json.dumps(keys)

print(json_str)

Let’s run the code to get the JSON string containing the list of dictionary keys:

["pizza", "is_vegetarian", "price"]

Solution #2: Define a custom function for the default kwarg

We can also solve this error by defining a custom function that converts a dict_keys object to a list. We can then pass the function as the default argument for the json.dumps() method. The function we set as the default keyword argument gets called for objects that are not JSON serializable. Let’s look at the custom function:

import json

from collections import abc

def serialize_dict_keys(obj):

    if isinstance(obj, abc.KeysView):

        return list(obj)

    raise TypeError ("Type %s is not serializable" % type(obj))

To check if an object is an instance of dict_keys we can use the collections.abc module, which provides abstract base classes to test whether a class provides a particular interface.

In this case, the Abstract Base Class (ABC) we need is dictionary keys views.

Note that if the object obj is not an instance of KeysView the function will raise a TypeError. Let’s set the default keyword argument to our custom function and run the code:

my_dict = {'pizza':'tartufo', 'is_vegetarian':False, 'price': 13.99}

keys = my_dict.keys()

json_str = json.dumps(keys, default=serialize_dict_keys)

print(json_str)

Let’s run the code to see the JSON string:

["pizza", "is_vegetarian", "price"]

We can also set default to list. Let’s look at the revised code:

my_dict = {'pizza':'tartufo', 'is_vegetarian':False, 'price': 13.99}

keys = my_dict.keys()

json_str = json.dumps(keys, default=list)

print(json_str)

Let’s run the code to see the JSON string:

["pizza", "is_vegetarian", "price"]

Solution #3: Define a JSONEncoder subclass for the cls kwarg

We can also solve this error by building a custom JSONEncoder subclass. This subclass will override the default method to serialize additional types.

import json

from collections import abc

class dict_keys_encoder(json.JSONEncoder):

    def default(self, obj):

        if isinstance(obj, abc.KeysView):

            return list(obj)

        return json.JSONEncoder.default(self, obj)

Similar to the custom function, the default method of the dict_keys_encoder class checks if the object is an instance of dict_keys using the KeysView ABC and converts it to a list and returns it.

We have to specify the custom JSONEncoder subclass with the cls keyword argument. Otherwise, JSONEncoder is used. Let’s look at the updated code:

my_dict = {'pizza':'tartufo', 'is_vegetarian':False, 'price': 13.99}

keys = my_dict.keys()

json_str = json.dumps(keys, cls=dict_keys_encoder)

print(json_str)

Let’s run the code to see the result:

["pizza", "is_vegetarian", "price"]

Below is the collection of objects that the JSONEncoder class supports by default, and their JSON equivalent.

Python	JSON
dict	object
list, tuple	array
str	string
int, float, int- & float- derived Enums	number
True	true
False	false
None	null

Summary

Congratulations on reading to the end of this tutorial!

For further reading on “not JSON serializable” TypeErrors, go to the articles:

• How to Solve Python TypeError: Object of type dict_values is not JSON Serializable
• How to Solve Python TypeError: Object of type dict_items is not JSON serializable
• How to Solve Python TypeError: Object of type bool_ is not JSON serializable

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