Working with JSON files in Python

Introduction

JSON files let us save and share data in a simple text format. Working with JSON files helps programs read and write data easily.

Saving user settings so they stay after closing the program

Sharing data between different programs or websites

Storing lists or details like contacts or tasks

Loading data from a file to use in your program

Backing up information in a readable format

Syntax

Python

import json

# To read JSON from a file:
with open('file.json', 'r') as file:
    data = json.load(file)

# To write JSON to a file:
with open('file.json', 'w') as file:
    json.dump(data, file, indent=4)

Use json.load() to read JSON data from a file and convert it into Python objects.

Use json.dump() to write Python objects as JSON into a file. The indent parameter makes the file easy to read.

Examples

This reads JSON from 'data.json' and prints the Python data.

Python

import json

# Reading JSON data from a file
with open('data.json', 'r') as f:
    info = json.load(f)
print(info)

This saves the person dictionary into 'person.json' with nice formatting.

Python

import json

# Writing Python data to a JSON file
person = {'name': 'Anna', 'age': 25}
with open('person.json', 'w') as f:
    json.dump(person, f, indent=2)

This reads JSON, updates it, then writes it back to a new file.

Python

import json

# Reading and writing JSON in one program
with open('input.json', 'r') as f:
    data = json.load(f)

# Change data
data['new_key'] = 'new value'

with open('output.json', 'w') as f:
    json.dump(data, f, indent=4)

Sample Program

This program saves a user dictionary to a JSON file, then reads it back and prints it.

Python

import json

# Sample data to save
user = {
    'username': 'johndoe',
    'active': True,
    'score': 42
}

# Write data to JSON file
with open('user.json', 'w') as file:
    json.dump(user, file, indent=2)

# Read data back from JSON file
with open('user.json', 'r') as file:
    loaded_user = json.load(file)

print('Loaded user data:')
print(loaded_user)

Important Notes

JSON files store data as text, so they are easy to share and read.

Python converts JSON objects to dictionaries and JSON arrays to lists.

Always close files or use with to handle files safely.

Summary

JSON files help save and load data in a simple text format.

Use json.load() to read JSON from files and json.dump() to write Python data to JSON files.

Working with JSON files makes programs share and keep data easily.

Practice

1.

What is the main purpose of using JSON files in Python programs?

easy

A. To create graphical user interfaces

B. To execute Python code faster

C. To save and load data in a simple text format

D. To compile Python programs into executables

Solution

1. Step 1: Understand JSON file usage

   JSON files store data in a text format that is easy to read and share.

2. Step 2: Identify the correct purpose

   Saving and loading data simply matches the main use of JSON files in Python.

3. Final Answer:

   To save and load data in a simple text format -> Option C

4. Quick Check:

   JSON files = data storage [OK]

Hint: JSON files store data simply as text [OK]

Common Mistakes:

• Thinking JSON speeds up code execution
• Confusing JSON with GUI creation
• Assuming JSON compiles code

2.

Which of the following is the correct way to open a JSON file named data.json for reading in Python?

easy

A. open('data.json', 'r')

B. open('data.json', 'w')

C. open('data.json', 'a')

D. open('data.json', 'x')

Solution

1. Step 1: Understand file modes

   'r' mode opens a file for reading, 'w' for writing, 'a' for appending, 'x' for creating new file.

2. Step 2: Choose mode for reading JSON

   To read JSON data, the file must be opened in 'r' mode.

3. Final Answer:

   open('data.json', 'r') -> Option A

4. Quick Check:

   Read mode = 'r' [OK]

Hint: Use 'r' mode to read files [OK]

Common Mistakes:

• Using 'w' which overwrites the file
• Using 'a' which appends data
• Using 'x' which fails if file exists

3.

What will be the output of the following Python code?

import json

with open('data.json', 'w') as f:
    json.dump({'name': 'Alice', 'age': 30}, f)

with open('data.json', 'r') as f:
    data = json.load(f)
print(data['age'])

medium

A. Alice

B. 30

C. {'name': 'Alice', 'age': 30}

D. Error: file not found

Solution

1. Step 1: Write dictionary to JSON file

   The code writes {'name': 'Alice', 'age': 30} to 'data.json' using json.dump.

2. Step 2: Read JSON file and access 'age'

   json.load reads the file back as a dictionary, then data['age'] accesses the value 30.

3. Final Answer:

   30 -> Option B

4. Quick Check:

   data['age'] = 30 [OK]

Hint: json.load returns Python dict from JSON file [OK]

Common Mistakes:

• Expecting string output instead of integer
• Confusing json.dump and json.load usage
• Assuming file read fails without writing first

4.

Identify the error in this code snippet that tries to read JSON data from a file:

import json

f = open('data.json', 'r')
data = json.load(f)
f.close()
print(data)

medium

A. Missing import statement for json

B. File not opened in write mode

C. File not closed properly

D. No error, code works correctly

Solution

1. Step 1: Check import and file open

   The code imports json and opens the file in 'r' mode correctly.

2. Step 2: Verify json.load and file close

   json.load reads JSON data properly, and file is closed with f.close().

3. Final Answer:

   No error, code works correctly -> Option D

4. Quick Check:

   Code reads JSON file correctly [OK]

Hint: json.load needs file opened in 'r' mode [OK]

Common Mistakes:

• Forgetting to import json
• Opening file in wrong mode
• Not closing the file after reading

5.

You want to save a Python dictionary {'a': 1, 'b': 2, 'c': 3} to a JSON file and then read it back. Which code snippet correctly does this and prints the value of key 'b'?

hard

A. import json
with open('file.json', 'w') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.load(f)
print(data['b'])

B. import json
with open('file.json', 'r') as f:
json.dump({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'w') as f:
data = json.load(f)
print(data['b'])

C. import json
f = open('file.json', 'w')
json.load({'a': 1, 'b': 2, 'c': 3}, f)
f.close()
f = open('file.json', 'r')
data = json.dump(f)
f.close()
print(data['b'])

D. import json
with open('file.json', 'w') as f:
json.load({'a': 1, 'b': 2, 'c': 3}, f)
with open('file.json', 'r') as f:
data = json.dump(f)
print(data['b'])

Solution

1. Step 1: Write dictionary to JSON file correctly

   json.dump writes Python dict to file opened in 'w' mode.

   import json
   with open('file.json', 'w') as f:
       json.dump({'a': 1, 'b': 2, 'c': 3}, f)
   with open('file.json', 'r') as f:
       data = json.load(f)
   print(data['b'])

   does this correctly.

2. Step 2: Read JSON file and access key 'b'

   json.load reads JSON back from file opened in 'r' mode, then data['b'] prints 2.

3. Final Answer:

   Code in Option A correctly saves and reads JSON, printing 2 -> Option A

4. Quick Check:

   json.dump to write, json.load to read [OK]

Hint: Use json.dump to write, json.load to read JSON files [OK]

Common Mistakes:

• Using json.load to write data
• Using json.dump to read data
• Opening files in wrong modes