Difference between json.dumps() and json.loads()

JSON is important for many backend devs. But sometimes playing with it can be confusing.

Being a backend dev or full stack dev you might have written APIs for your Web or Mobile application and I’m sure you might have come across json.dumps() and json.loads().

Let’s understand the difference between json.dumps() and json.loads() with an example.

json.dumps()

import json

user_details_dict = {
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}

user_details_string = """{
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}"""

user_details_dict_json_dumps = json.dumps(user_details_dict)
user_details_string_json_dumps = json.dumps(user_details_string)

# user_details_dict_json_loads = json.loads(user_details_dict)
# user_details_string_json_loads = json.loads(user_details_string)

print(user_details_dict)
print(type(user_details_dict))

print(user_details_string)
print(type(user_details_string))

print(user_details_dict_json_dumps)
print(type(user_details_dict_json_dumps))

print(user_details_string_json_dumps)
print(type(user_details_string_json_dumps))

# print(user_details_dict_json_loads)
# print(type(user_details_dict_json_loads))

# print(user_details_string_json_loads)
# print(type(user_details_string_json_loads))

In the above code example, I have 2 objects one of a type dictionary and the other of a type string.

json.dumps() returns a string for both dict and string objects, as you can see in the below response.

{'first_name': 'Chandan', 'last_name': 'Rajpurohit'}
<class 'dict'>
{
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}
<class 'str'>
{"first_name": "Chandan", "last_name": "Rajpurohit"}
<class 'str'>
"{\n    \"first_name\": \"Chandan\",\n    \"last_name\": \"Rajpurohit\"\n}"
<class 'str'>
>

So basically, json.dumps() serialize(or encode) and returns a Python object into a string formatted JSON object. In simple terms, it converts a Python object into a string-formatted JSON object.

json.loads()

import json

user_details_dict = {
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}

user_details_string = """{
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}"""

# user_details_dict_json_dumps = json.dumps(user_details_dict)
# user_details_string_json_dumps = json.dumps(user_details_string)

user_details_dict_json_loads = json.loads(user_details_dict)
user_details_string_json_loads = json.loads(user_details_string)

print(user_details_dict)
print(type(user_details_dict))

print(user_details_string)
print(type(user_details_string))

# print(user_details_dict_json_dumps)
# print(type(user_details_dict_json_dumps))

# print(user_details_string_json_dumps)
# print(type(user_details_string_json_dumps))

print(user_details_dict_json_loads)
print(type(user_details_dict_json_loads))

print(user_details_string_json_loads)
print(type(user_details_string_json_loads))

Now when I run the above code I get a TypeError: the JSON object must be str, bytes or bytearray, not dict.

Traceback (most recent call last):
  File "<string>", line 16, in <module>
  File "/usr/local/lib/python3.11/json/__init__.py", line 339, in loads
ERROR!
raise TypeError(f'the JSON object must be str, bytes or bytearray, '
TypeError: the JSON object must be str, bytes or bytearray, not dict
>

Why this happened?

Because json.loads() takes string-formatted JSON and as you can see in the above error it says that the JSON object must be str, bytes or bytearray, not a dict.

import json

user_details_dict = {
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}

user_details_string = """{
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}"""

# user_details_dict_json_dumps = json.dumps(user_details_dict)
# user_details_string_json_dumps = json.dumps(user_details_string)

# user_details_dict_json_loads = json.loads(user_details_dict)
user_details_string_json_loads = json.loads(user_details_string)

print(user_details_dict)
print(type(user_details_dict))

print(user_details_string)
print(type(user_details_string))

# print(user_details_dict_json_dumps)
# print(type(user_details_dict_json_dumps))

# print(user_details_string_json_dumps)
# print(type(user_details_string_json_dumps))

# print(user_details_dict_json_loads)
# print(type(user_details_dict_json_loads))

print(user_details_string_json_loads)
print(type(user_details_string_json_loads))

So when I pass string-formatted JSON to json.loads I get a response which is of type dict.

{'first_name': 'Chandan', 'last_name': 'Rajpurohit'}
<class 'dict'>
{
    "first_name": "Chandan",
    "last_name": "Rajpurohit"
}
<class 'str'>
{'first_name': 'Chandan', 'last_name': 'Rajpurohit'}
<class 'dict'>
>

json.loads() deserialize(or decode) and returns a string-formatted JSON to a Python object. In simple terms, it converts string-formatted JSON to a Python object.

In conclusion, when you want to convert string to dict use json.loads(), and when you want to convert dict to string use json.dumps().

Try the above code yourself and let me know your thoughts.