Handling Flask Forms: WTForms Fields and Form Submission

This article introduces the core knowledge points of handling forms in Flask using `Flask-WTF` (based on WTForms). First, forms rely on the `Flask-WTF` extension, which needs to be installed via `pip install flask-wtf`. WTForms provides various field types (such as `StringField`, `PasswordField`, `SubmitField`, etc.) corresponding to HTML input controls. To define a form class, you need to inherit from `FlaskForm`, declare fields in the class, and set validators (e.g., `DataRequired`, `Length`, `Email`). In the example, the `LoginForm` includes username, email, password, and a submit button, with clear validation rules for each field. In view functions, you need to create a form instance, use `form.validate_on_submit()` to check POST requests and data validation. If validation passes, process the data (e.g., login verification); otherwise, render the template. The template should use `form.hidden_tag()` to generate CSRF tokens for security and display error messages using `form.errors`. The core principles include form definition, request handling, data validation, template rendering, and CSRF protection. Common issues such as missing CSRF tokens require checking `form.hidden_tag()` and `SECRET_KEY`. Validation failures can be troubleshooted via `form.errors`, and password inputs should use [note: the original text ends abruptly here, so the translation follows the provided content].