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)

Miss import Optional and Required ?

from wtforms.validators import DataRequired, Optional


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
        - https://gist.github.com/devxoul/7638142#file-wtf_required_if-py
    """
    field_flags = ('requiredif',)

    def __init__(self, message=None, *args, **kwargs):
        super(RequiredIf).__init__()
        self.message = message
        self.conditions = kwargs

    # field is requiring that name field in the form is data value in the form
    def __call__(self, form, field):
        for name, data in self.conditions.items():
            other_field = form[name]
            if other_field is None:
                raise Exception('no field named "%s" in form' % name)
            if other_field.data == data and not field.data:
                DataRequired.__call__(self, form, field)
            Optional()(form, field)

My version, based on some examples...basically the same thing....also follows same 'usage' as this one

@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.