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Flask error handler that returns a JSON response with a 404 status code when a requested resource is not found.

Purpose

This function serves as a custom error handler for Flask applications to provide consistent JSON-formatted error responses when users attempt to access non-existent routes or resources. It's registered as an error handler for HTTP 404 errors using the @app.errorhandler(404) decorator, ensuring all 404 errors return a standardized JSON response instead of Flask's default HTML error page.

Source Code

def not_found(error):
    return jsonify({'error': 'Not found'}), 404

Parameters

Parameter Details

error: The error object automatically passed by Flask when a 404 error occurs. This parameter contains information about the error but is not used in the function body. It's required by Flask's error handler signature.

Return Value

Returns a tuple containing: (1) a JSON object with structure {'error': 'Not found'} created by Flask's jsonify() function, and (2) the HTTP status code 404. Flask automatically converts this tuple into a proper HTTP response with JSON content-type headers.

Dependencies

• flask

Required Imports

from flask import Flask
from flask import jsonify

Usage Example

from flask import Flask, jsonify

app = Flask(__name__)

@app.errorhandler(404)
def not_found(error):
    return jsonify({'error': 'Not found'}), 404

@app.route('/')
def home():
    return 'Home Page'

if __name__ == '__main__':
    app.run()

# When accessing a non-existent route like '/nonexistent':
# Response: {'error': 'Not found'} with status code 404

Best Practices

• This error handler should be registered after all route definitions to ensure it catches all 404 errors
• Consider adding more detailed error information in production environments, such as timestamp or request path
• For APIs, ensure all error responses follow a consistent JSON structure across different error types
• The error parameter should be kept in the signature even if unused, as Flask requires it for error handlers
• Consider logging the error for debugging purposes before returning the response
• This pattern works well for RESTful APIs where JSON responses are expected for all endpoints

Tags

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