Optional doesn't check field.data about wtforms HOT 1 OPEN

WTForms assumes that passed in data is already valid. So you should only expect to be able to validate formdata. After all it's a forms library, not a general purpose validation library.

The Optional/InputRequired validators generally should also be first in the validation chain. I think the reason that Optional clears errors is to avoid processing errors from surfacing to the application if the field was left empty, leaving the form in a faulty state where it can't be submitted. It unfortunately as a side-effect also hides incorrect validation chains. Generally it's weird that Optional is a validator to begin with, it really should've been a field attribute, considering it doesn't even work correctly with all fields. The issue is even more apparent with DataRequired.

If you want general purpose data validation then use something like pydantic. It should also be fairly easy to auto-generate a Form based on a pydantic Model and vice versa.

Your change will break existing code. You can always create your own internal validator, if you want to use WTForms this way, although I'd highly recommend against it.

from wtforms.