Select Page

How to Solve Python TypeError: Object of type Decimal is not JSON serializable

by | Programming, Python, Tips

This error occurs when you try to serialize a Decimal object to a JSON string using the json.dumps() method. You can solve this error by setting the default keyword argument to str when calling the json.dumps() method. For example,

json_str = json.dumps(decimal_value, default=str)

You can also define a function that converts the Decimal object to a string and set the default keyword argument to the name of that function.

This tutorial will go through how to solve the error with code examples.

Table of contents

TypeError: Object of type Decimal 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 Decimal” tells us the error is due to an illegal operation with a Decimal object.

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 Decimal type.

Example

Let’s look at an example of JSON serializing a Decimal object. First, we will import the classes from the decimal module and then create a Decimal object containing Euler’s number.

import json
from decimal import *

euler_num = Decimal('2.71828')

Next, we will attempt to serialize the Decimal object using json.dumps(). 

json_str = json.dumps(euler_num)

print(json_str)

Let’s run the code to see what happens:

TypeError: Object of type Decimal is not JSON serializable

The error occurs because euler_num is an object of type Decimal and cannot be serialized into a JSON string.

Solution #1: Set default to str

The simplest way to solve this error is to set the default keyword argument of the json.dumps() method to the str class. The default value for the keyword argument default is None. We can set default to a function that gets called for objects that are not serializable. When we set the default keyword argument to str , it converts the Decimal object to a string.

Let’s look at the updated code:

import json
from decimal import *

euler_num = Decimal('2.71828')

json_str = json.dumps(euler_num, default=str)

print(json_str)

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

"2.71828"

Solution #2: Define a custom function for default kwarg

We can also solve this error by defining a custom function that converts a Decimal object to a string. Let’s look at the custom function:

from datetime import datetime, date
import json

def serialize_decimal(obj):

    if isinstance(obj, Decimal):

        return str(obj)

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

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

euler_num = Decimal('2.71828')

json_str = json.dumps(euler_num, default=serialize_decimal)

print(json_str)
"2.71828"

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. 

from decimal import *
import json

class decimal_encoder(json.JSONEncoder):

    def default(self, obj):

        if isinstance(obj, Decimal):

            return str(obj)

        return json.JSONEncoder.default(self, obj)

Similar to the custom function, the default method of the decimal_encoder class checks if the object is of type Decimal, converts it to a string 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:

euler_num = Decimal('2.71828')

json_str = json.dumps(euler_num, cls=decimal_encoder)

print(json_str)

Let’s run the code to see the result:

"2.71828"

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

PythonJSON
dictobject
list, tuplearray
strstring
int, float, int- & float- derived Enumsnumber
Truetrue
Falsefalse
Nonenull
JSONEncoder Supported objects and types by default

What is Python Decimal?

The Python decimal model provides support for fast correctly-rounded decimal floating-point arithmetic. Decimal objects have higher precision than floating-point numbers, making them particularly useful for financial calculations.

Summary

Congratulations on reading to the end of this tutorial.

For further reading on errors involving JSON serialization, go to the articles:

Go to the online courses page on Python to learn more about Python for data science and machine learning.

Have fun and happy researching!