• Inspired by ASP.NET validation.
• Works primarily with Textbox control.
• Can recurively call validate() on any control.
• .is_valid property stores the last validation result.
• Textbox implements .validate() and .is_valid defined on Control level.
• ...or user provides on_validate handler which returns a message if the value is invalid, for example:

first_name = Textbox(label="First name", on_validate=lambda v: "Please enter your first name" if v == "" else None)

or providing a Validator class:

first_name = Textbox(label="First name", validator=RequiredFieldValidator("Please enter your first name", focus_on_error=True))