How to Fix Custom Validators Not Working in Flask-WTF Forms

Learn how to resolve issues with custom validators in Flask-WTF forms by understanding the validation process and how to properly display error messages.
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This video is based on the question https://stackoverflow.com/q/68724586/ asked by the user 'Hamza Mir' ( https://stackoverflow.com/u/9885924/ ) and on the answer https://stackoverflow.com/a/68727037/ provided by the user 'Menno Hölscher' ( https://stackoverflow.com/u/5189255/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving Custom Validator Issues in Flask-WTF Forms

When working with Flask and the WTForms library, many developers face challenges involving form validation, especially with custom validators. A common issue arises when using both login and signup forms on a single HTML page, leading to unexpected behaviors where validations do not trigger correctly. In this post, we'll discuss how to resolve issues with custom validators not working properly in your Flask application forms.

The Problem

Imagine you have a login and signup form displayed on the same page. You’ve created custom validators to ensure that usernames and emails are unique. However, after modifying your routes to separate the handling of these forms, you notice that the custom validators no longer seem to work. Instead of showing validation errors when inputs are invalid, the forms display errors from the other form, like "input required". This can lead to confusion and unnecessary frustration.

The Root Cause

The primary reason for this issue is how Flask-WTF handles form validation and error handling. Let's break it down:

Different Form Instances: When you redirect to a different route after a form submission, Flask creates a new instance of that form. For example, in your register route, if the validation fails and you use redirect(url_for('home')), you're essentially throwing away the original form's state, including its errors.

Error Information Lost: Since the form validation failed, the error messages are associated with the initial form instance, which gets lost when you create a new instance after the redirect. This is why the custom error messages for the signup form are not displayed.

The Solution

To properly manage this, you should render the template directly, passing in your form instances instead of using a redirect. Here's how to adjust the routes in your routes.py:

Modified Routes Example

[[See Video to Reveal this Text or Code Snippet]]

Key Changes Made:

Rendering the Template: Instead of redirecting after a failed validation, we return render_template('home.html', login_form=login_form, regis_form=regis_form). This allows us to keep the form state and error messages intact, showing the user which fields need correction.

Form Instantiation: Ensure that you're instantiating both forms on every request so they contain the correct data or errors.

Conclusion

By directly rendering your template instead of redirecting after a failed form submission, you can maintain the form state and display error messages correctly. This approach ensures a smoother user experience and improves the clarity of feedback provided to users filling out forms on your application.

With these changes, your custom validators should work seamlessly in the Flask-WTF forms! Keep experimenting and refining your forms for better validation workflows in your apps.