How to make a field conditionally optional in WTForms?

Question

My form validation is working nearly complete, I just have 2 cases I don\'t know exactly how to solve: 1) The password field should be required of course but I also provide

Answer 1

The answer from @dcrosta is great, but I think some things have changed in wtforms since this answer. Inheriting from DataRequired adds a required attribute to the form field, so the conditional validator never gets called. I made a minor change to the class from @dcrosta that works with wtforms 2.1. This only over-rides field_flags so that browser validation is not done.

from wtforms.validators import DataRequired


class RequiredIf(DataRequired):
    """Validator which makes a field required if another field is set and has a truthy value.

    Sources:
        - http://wtforms.simplecodes.com/docs/1.0.1/validators.html
        - http://stackoverflow.com/questions/8463209/how-to-make-a-field-conditionally-optional-in-wtforms

    """
    field_flags = ('requiredif',)

    def __init__(self, other_field_name, message=None, *args, **kwargs):
        self.other_field_name = other_field_name
        self.message = message

    def __call__(self, form, field):
        other_field = form[self.other_field_name]
        if other_field is None:
            raise Exception('no field named "%s" in form' % self.other_field_name)
        if bool(other_field.data):
            super(RequiredIf, self).__call__(form, field)

A more ideal solution would manage to do the validation in the browser, like the current behavior of DataRequired.