# Flask Forms Validation

> Securely handling user input using Flask-WTF and robust validation patterns.

- Skill: `jcorpac/flask-forms-validation` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jcorpac/flask-forms-validation`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jcorpac/flask-forms-validation/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jcorpac (https://skillmd.com/u/jcorpac)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jcorpac/flask-forms-validation

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# 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.