Python JSON – Append JSON to a File

Learn to read a JSON file and append JSON data into a file in Python.

1. Steps for Appending to a JSON File

In Python, appending JSON to a file consists of the following steps:

  • Read the JSON in Python dict or list object.
  • Append the JSON to dict (or list) object by modifying it.
  • Write the updated dict (or list) object into the original file.

Refer to the following articles to learn how to read JSON from a file and write JSON to a file in Python.

2. Appending to a JSON Array using Python List

The users.json file has a list of 2 users. We will append a third user to it.

[
    {
        "Name": "Person_1",
        "Age": 11,
        "Email": "11@gmail.com"
    },
    {
        "Name": "Person_2",
        "Age": 22,
        "Email": "22@gmail.com"
    }
]
import json
from os import path
 
filename = 'c:/temp/users.json'
listObj = []
 
# Check if file exists
if path.isfile(filename) is False:
  raise Exception("File not found")
 
# Read JSON file
with open(filename) as fp:
  listObj = json.load(fp)
 
# Verify existing list
print(listObj)

print(type(listObj))
 
listObj.append({
  "Name": "Person_3",
  "Age": 33,
  "Email": "33@gmail.com"
})
 
# Verify updated list
print(listObj)
 
with open(filename, 'w') as json_file:
    json.dump(listObj, json_file, 
                        indent=4,  
                        separators=(',',': '))
 
print('Successfully appended to the JSON file')

The updated JSON file is:

[
    {
        "Name": "Person_1",
        "Age": 11,
        "Email": "11@gmail.com"
    },
    {
        "Name": "Person_2",
        "Age": 22,
        "Email": "22@gmail.com"
    },
    {
        "Name": "Person_3",
        "Age": 33,
        "Email": "33@gmail.com"
    }
]

3. Appending to a JSON Object using Python Dictionary

The users.json file has a list of 2 users. We will append a new attribute role and modify an existing attribute Age.

{
	"Name": "Person_1",
	"Age": 11,
	"Email": "11@gmail.com"
}
import json
from os import path
 
filename = 'c:/temp/users.json'
dictObj = []
 
# Check if file exists
if path.isfile(filename) is False:
  raise Exception("File not found")
 
# Read JSON file
with open(filename) as fp:
  dictObj = json.load(fp)
 
# Verify existing dict
print(dictObj)

print(type(dictObj))
 
dictObj.update({"Age": 12,"Role": "Developer"})
 
# Verify updated dict
print(dictObj)
 
with open(filename, 'w') as json_file:
    json.dump(dictObj, json_file, 
                        indent=4,  
                        separators=(',',': '))
 
print('Successfully written to the JSON file')

The updated JSON file is:

{
    "Name": "Person_1",
    "Age": 12,
    "Email": "11@gmail.com",
    "Role": "Developer"
}

4. AttributeError: ‘dict’ object has no attribute ‘append’

We may get this error if the JSON object read from the json.load() method is of type dict.

The first example reads a JSON list [...] so the loaded object is of type list. If we are reading a file that has JSON object {...} then the loaded object will be of type dictionary and the above code will give AttributeError while invoking the list operations.

{'Name': 'Person_1', 'Age': 11, 'Email': '11@gmail.com'}
<class 'dict'>
Traceback (most recent call last):
  File "C:\temp\temp.py", line 20, in <module>
    listObj.append({
AttributeError: 'dict' object has no attribute 'append'

To fix this error, be mindful of what JSON structure you are reading from the JSON file.

  • Read JSON object to Python dict object.
  • Read JSON array to Python list object.

Happy Learning !!

Sourcecode on Github

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