Forms in Flask can be implemented by using an extension called Flask-WTF. Flask-WTF is created by integrating Flask with WTForms. WTForms is a python-based form rendering and validation library. It supports data validation, internationalization, and CSRF protection.
Flask-WTF also provides reCAPTCHA support along with file uploads when tied with Flask-Uploads. You also can handle JavaScript requests, and customize the error response.