How to Check if String is Valid JSON in Python

To check if a string is valid JSON in Python, use the json.loads() function inside a try-except block. If json.loads() parses the string without errors, it is valid JSON; otherwise, it is not.

Syntax

Use the json.loads() function to parse a JSON string. Wrap it in a try-except block to catch json.JSONDecodeError if the string is not valid JSON.

• json.loads(json_string): Parses the JSON string.
• try-except: Handles parsing errors gracefully.

python

import json

try:
    json.loads(json_string)
    # If no error, json_string is valid JSON
except json.JSONDecodeError:
    # json_string is not valid JSON

Example

This example shows how to check if a string is valid JSON. It prints Valid JSON if parsing succeeds, otherwise Invalid JSON.

python

import json

def is_valid_json(json_string):
    try:
        json.loads(json_string)
        return True
    except json.JSONDecodeError:
        return False

# Test with valid JSON string
valid_str = '{"name": "Alice", "age": 30}'
print("Valid JSON" if is_valid_json(valid_str) else "Invalid JSON")

# Test with invalid JSON string
invalid_str = '{name: Alice, age: 30}'
print("Valid JSON" if is_valid_json(invalid_str) else "Invalid JSON")

Output

Valid JSON
Invalid JSON

Common Pitfalls

Common mistakes when checking JSON validity include:

• Not catching json.JSONDecodeError, which causes the program to crash.
• Assuming a string is JSON just because it looks like one (e.g., missing quotes around keys).
• Confusing JSON format with Python dictionaries (JSON keys must be in double quotes).

python

import json

# Wrong way: no error handling
json_string = '{name: Alice}'
# This will raise an exception and stop the program
# json.loads(json_string)

# Right way: use try-except
try:
    json.loads(json_string)
    print("Valid JSON")
except json.JSONDecodeError:
    print("Invalid JSON")

Output

Invalid JSON

Quick Reference

Summary tips for checking JSON validity in Python:

• Use json.loads() to parse the string.
• Wrap parsing in try-except json.JSONDecodeError to catch invalid JSON.
• Remember JSON keys must be double-quoted strings.
• Test with both valid and invalid JSON strings to confirm your check works.

Key Takeaways

Use json.loads() inside try-except to check JSON validity safely.

Catch json.JSONDecodeError to handle invalid JSON strings without crashing.

JSON keys must be double-quoted strings, unlike Python dict keys.

Testing with both valid and invalid JSON strings helps verify your check.

Avoid assuming a string is JSON without parsing it first.