How to Use json Module in Python: Syntax and Examples

Use the json module in Python to convert Python objects to JSON strings with json.dumps() and parse JSON strings back to Python objects with json.loads(). You can also read from and write to files using json.dump() and json.load().

Syntax

The json module provides functions to convert between Python objects and JSON format.

• json.dumps(obj): Converts a Python object obj to a JSON string.
• json.loads(json_string): Parses a JSON string back into a Python object.
• json.dump(obj, file): Writes a Python object as JSON to a file.
• json.load(file): Reads JSON from a file and converts it to a Python object.

python

import json

# Convert Python object to JSON string
json_string = json.dumps({'name': 'Alice', 'age': 30})

# Convert JSON string back to Python object
python_obj = json.loads(json_string)

# Write Python object to a file as JSON
with open('data.json', 'w') as f:
    json.dump({'name': 'Alice', 'age': 30}, f)

# Read JSON from a file and convert to Python object
with open('data.json', 'r') as f:
    data = json.load(f)

Example

This example shows how to convert a Python dictionary to a JSON string and back, then save it to a file and read it again.

python

import json

person = {'name': 'Bob', 'age': 25, 'city': 'New York'}

# Convert to JSON string
json_str = json.dumps(person)
print('JSON string:', json_str)

# Convert JSON string back to Python dict
person_dict = json.loads(json_str)
print('Python dict:', person_dict)

# Save JSON to file
with open('person.json', 'w') as file:
    json.dump(person, file)

# Load JSON from file
with open('person.json', 'r') as file:
    loaded_person = json.load(file)
print('Loaded from file:', loaded_person)

Output

JSON string: {"name": "Bob", "age": 25, "city": "New York"}
Python dict: {'name': 'Bob', 'age': 25, 'city': 'New York'}
Loaded from file: {'name': 'Bob', 'age': 25, 'city': 'New York'}

Common Pitfalls

Common mistakes when using the json module include:

• Trying to serialize objects that are not supported by JSON, like sets or custom classes, without converting them first.
• Confusing json.dumps() (returns a string) with json.dump() (writes to a file).
• Forgetting to open files in the correct mode ('w' for writing, 'r' for reading).
• Not handling exceptions when parsing invalid JSON strings.

python

import json

# Wrong: Trying to dump a set directly
try:
    json.dumps({1, 2, 3})
except TypeError as e:
    print('Error:', e)

# Right: Convert set to list first
json_str = json.dumps(list({1, 2, 3}))
print('JSON string:', json_str)

Output

Error: Object of type set is not JSON serializable
JSON string: [1, 2, 3]

Quick Reference

Here is a quick summary of the main json module functions:

Function	Description
json.dumps(obj)	Convert Python object to JSON string
json.loads(json_string)	Parse JSON string to Python object
json.dump(obj, file)	Write Python object as JSON to a file
json.load(file)	Read JSON from a file and convert to Python object

Key Takeaways

Use json.dumps() to convert Python objects to JSON strings.

Use json.loads() to parse JSON strings back to Python objects.

Use json.dump() and json.load() to work with JSON files.

Only JSON-serializable Python objects can be converted directly.

Always open files in the correct mode when reading or writing JSON.