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.

Answer 2

I'm not sure this quite fits your needs, but I've used a RequiredIf custom validator on fields before, which makes a field required if another field has a value in the form... for instance, in a datetime-and-timezone scenario, I can make the timezone field required to have a value if the user has entered a datetime.

class RequiredIf(Required):
    # a validator which makes a field required if
    # another field is set and has a truthy value

    def __init__(self, other_field_name, *args, **kwargs):
        self.other_field_name = other_field_name
        super(RequiredIf, self).__init__(*args, **kwargs)

    def __call__(self, form, field):
        other_field = form._fields.get(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)

The constructor takes the name of the other field that triggers making this field required, like:

class DateTimeForm(Form):
    datetime = TextField()
    timezone = SelectField(choices=..., validators=[RequiredIf('datetime')])

This could be a good starting point for implementing the sort of logic you need.

Answer 3

I found this question helpful and based on the answer of @dcrosta I created another validator which is optional. The benefit is that you can combine it with other wtforms validators. Here is my optional validator which checks another field. Because I needed to check the value of the other field against some certain value I added a custom check for value:

class OptionalIfFieldEqualTo(wtf.validators.Optional):
    # a validator which makes a field optional if
    # another field has a desired value

    def __init__(self, other_field_name, value, *args, **kwargs):
        self.other_field_name = other_field_name
        self.value = value
        super(OptionalIfFieldEqualTo, self).__init__(*args, **kwargs)

    def __call__(self, form, field):
        other_field = form._fields.get(self.other_field_name)
        if other_field is None:
            raise Exception('no field named "%s" in form' % self.other_field_name)
        if other_field.data == self.value:
            super(OptionalIfFieldEqualTo, self).__call__(form, field)