WTForms `RequiredIf` validator.

wtf_required_if.py

class RequiredIf(object):
    """Validates field conditionally.

    Usage::

        login_method = StringField('', [AnyOf(['email', 'facebook'])])
        email = StringField('', [RequiredIf(login_method='email')])
        password = StringField('', [RequiredIf(login_method='email')])
        facebook_token = StringField('', [RequiredIf(login_method='facebook')])
    """
    def __init__(self, *args, **kwargs):
        self.conditions = kwargs

    def __call__(self, form, field):
        for name, data in self.conditions.iteritems():
            if name not in form._fields:
                Optional(form, field)
            else:
                condition_field = form._fields.get(name)
                if condition_field.data == data and not field.data:
                    Required()(form, field)
        Optional()(form, field)

@mcelhenny, how to support FieldList(FormField() fields? Basically importing a different form from another class. Die form[name] part breaks as the name field is unknown.