Flask Forms & Validation

User input is the most dangerous part of a web app. Flask-WTF provides a secure, Pythonic layer of protection.

Form Definitions

• WTForms: Define forms as Python classes (e.g., LoginForm(FlaskForm)).
• Validators: Use DataRequired(), Email(), Length(), and custom methods to ensure data integrity.

CSRF Protection

Flask-WTF automatically handles Cross-Site Request Forgery tokens. Ensure {{ form.hidden_tag() }} is present in your templates.

File Uploads

Use FileField and FileRequired with secure_filename() to safely handle user-uploaded media.

Best Practices

• Feedback: Display clear error messages next to invalid fields using form.errors.
• POST/Redirect/GET Pattern: Always redirect after a successful form submission to prevent duplicate data.