How to Validate JSON in Python and Catch JSONDecodeError

A robust Python function that attempts to parse JSON strings and returns a boolean plus either the parsed data or a descriptive error message when decoding fails.

Easy Python 3.9+ Aug 9, 2026 Errors & debugging 12 views 0 copies

json validation jsondecodeerror error-handling try-except

Python code

23 lines

Python 3.9+

import json

def validate_json(json_string):
    """Try to parse JSON, return (is_valid, data_or_error)."""
    try:
        data = json.loads(json_string)
        return True, data
    except json.JSONDecodeError as e:
        return False, f"Invalid JSON: {e}"

if __name__ == "__main__":
    test_inputs = [
        '{"name": "Alice", "age": 30}',
        '{"name": "Bob", "age": }',  # Trailing comma inside
        '[1, 2, 3]',
        'not json at all'
    ]
    
    for json_string in test_inputs:
        is_valid, result = validate_json(json_string)
        print(f"Input: {json_string!r}")
        print(f"Valid: {is_valid}")
        print(f"Result: {result}\n")

Output

stdout

Input: '{"name": "Alice", "age": 30}'
Valid: True
Result: {'name': 'Alice', 'age': 30}

Input: '{"name": "Bob", "age": }'
Valid: False
Result: Invalid JSON: Expecting value: line 1 column 19 (char 18)

Input: '[1, 2, 3]'
Valid: True
Result: [1, 2, 3]

Input: 'not json at all'
Valid: False
Result: Invalid JSON: Expecting value: line 1 column 1 (char 0)

How it works

The json.loads() function parses a JSON string into Python's native data types, such as dictionaries and lists. When the input is not valid JSON, it raises a JSONDecodeError, a subclass of ValueError. By wrapping the call in a try/except block, the function returns a tuple: (True, data) on success or (False, error_message) on failure. This pattern lets you safely handle malformed JSON without crashing your application. The function also preserves the original error message, which helps with debugging by indicating the exact line and character position of the malformed content.

Common mistakes

Using `json.load()` on a string instead of `json.loads()` — the former expects a file object.
Catching the generic `Exception` instead of the specific `json.JSONDecodeError`, which can hide unrelated bugs.
Forgetting that `json.loads()` can return non-dictionary types like lists, numbers, or booleans for valid JSON.

Variations

Use `try/except` with `json.loads()` inline without a custom function for one-off checks.
For files on disk, use `json.load()` with a file handle managed by a `with` statement.

Real-world use cases

Validating API request bodies in a web framework before processing them to avoid data corruption.
Sanitizing user-submitted configuration files in a CLI tool to give clear error feedback.
Pre-checks in data pipelines to guard against malformed JSON records from external sources.

How to Validate JSON in Python and Catch JSONDecodeError

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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