Formencode validator that will require a minimum of one field

require at least one.py

from formencode import Schema, validators, Invalid
from formencode.validators import FormValidator


class RequireAtLeastOne(FormValidator):
    choices = []
    __unpackargs__ = ('choices',)
    def _convert_to_python(self, value_dict, state):
        for each in self.choices:
            if value_dict.get(each) is not None:
                return value_dict
        raise Invalid('You must give a value for %s' % repr(self.choices), value_dict, state)
       
        
class ValidateThings(Schema):
    field1 = validators.String(if_missing=None)    
    field2 = validators.String(if_missing=None)
    field3 = validators.String(if_missing=None)
    chained_validators = [RequireAtLeastOne(['field1', 'field2', 'field3'])]
        

""" Success """        
params = ValidateThings().to_python({"field2": 12})
params = ValidateThings().to_python({"field2": 12, "field3": 126})
params = ValidateThings().to_python({"field1": "foo", "field2": 12, "field3": 126})

""" Failure """
params = ValidateThings().to_python({})