How to Validate JSON in Python

The Basic Approach: json.loads + try/except

json.loads() raises json.JSONDecodeError if the input string is not valid JSON. Catch it to validate:

import json

def is_valid_json(text: str) -> bool:
    try:
        json.loads(text)
        return True
    except json.JSONDecodeError:
        return False

# Usage
is_valid_json('{"name": "Alice"}')   # True
is_valid_json("{'name': 'Alice'}")   # False — single quotes
is_valid_json('{"a": 1,}')          # False — trailing comma
is_valid_json('null')               # True — null is valid JSON
is_valid_json('')                   # False — empty string is not valid JSON

Get the Error Message and Line Number

json.JSONDecodeError exposes the lineno, colno, and msg attributes for detailed debugging:

import json
from dataclasses import dataclass
from typing import Any

@dataclass
class JsonResult:
    valid: bool
    data: Any = None
    error: str = None
    line: int = None
    col: int = None

def validate_json(text: str) -> JsonResult:
    try:
        data = json.loads(text)
        return JsonResult(valid=True, data=data)
    except json.JSONDecodeError as e:
        return JsonResult(
            valid=False,
            error=e.msg,
            line=e.lineno,
            col=e.colno,
        )

# Usage
result = validate_json('{"name": "Alice",}')
if not result.valid:
    print(f"Invalid JSON at line {result.line}, col {result.col}: {result.error}")
    # Invalid JSON at line 1, col 18: Expecting property name enclosed in double quotes

Validate a JSON File

Use json.load()(without the 's') to validate directly from a file object. It also raises json.JSONDecodeError on invalid JSON:

import json

def validate_json_file(path: str) -> bool:
    try:
        with open(path, "r", encoding="utf-8") as f:
            json.load(f)
        print(f"✓ {path} is valid JSON")
        return True
    except json.JSONDecodeError as e:
        print(f"✗ {path}: line {e.lineno}, col {e.colno} — {e.msg}")
        return False
    except FileNotFoundError:
        print(f"✗ File not found: {path}")
        return False

# Usage
validate_json_file("config.json")
validate_json_file("data/payload.json")

json.loads vs json.load: Use json.loads(string) for strings and json.load(file) for file objects.

Validate an API Response (requests)

When using the requests library, response.json() handles parsing automatically. But for raw webhook payloads or message queue messages, validate first:

import json
import requests

# Using requests — .json() raises requests.exceptions.JSONDecodeError on failure
response = requests.get("https://api.example.com/data")
try:
    data = response.json()
except requests.exceptions.JSONDecodeError as e:
    print(f"API returned invalid JSON: {e}")

# Raw webhook payload (e.g. from Flask/FastAPI)
def handle_webhook(raw_body: str):
    try:
        payload = json.loads(raw_body)
    except json.JSONDecodeError as e:
        print(f"Invalid webhook JSON at line {e.lineno}: {e.msg}")
        return None
    return payload

Structural Validation with jsonschema

For validating structure — not just syntax — use the jsonschema library. Install it with pip install jsonschema:

import json
import jsonschema
from jsonschema import validate, ValidationError

# Define the expected structure
schema = {
    "type": "object",
    "required": ["name", "age", "email"],
    "properties": {
        "name":  {"type": "string"},
        "age":   {"type": "integer", "minimum": 0},
        "email": {"type": "string", "format": "email"},
    },
    "additionalProperties": False,
}

def validate_user(raw_json: str):
    # Step 1: Check syntax
    try:
        data = json.loads(raw_json)
    except json.JSONDecodeError as e:
        return f"Syntax error at line {e.lineno}: {e.msg}"

    # Step 2: Check structure
    try:
        validate(instance=data, schema=schema)
        return "Valid"
    except ValidationError as e:
        return f"Schema error: {e.message}"

print(validate_user('{"name": "Alice", "age": 30, "email": "[email protected]"}'))
# Valid

print(validate_user('{"name": "Bob", "age": -1, "email": "[email protected]"}'))
# Schema error: -1 is less than the minimum of 0

Python JSON Edge Cases

Frequently Asked Questions