I'll use your PhoneInput example (from the RequiredIfMissing docstring) to explain:

I have a Schema with a phone number. I want to have an international phone number field that adds an international calling code to the existing phone number. You specify the field name and it adds the country code + phone number to itself.

The international phone number should be accessible though the schema's to_python() method

import formencode
import formencode.validators as validators

class InternationalPhoneNumber(validators.FancyValidator):
    phone = None
    def __init__(self, fieldname, **kwargs):
        super(InternationalPhoneNumber, self).__init__()
        self.phone = fieldname

    def _to_python(self, value, instance):
        # somehow find the schema's ``phone`` field and return it here:
        # phonenumber = get_field_value_from_schema(self.phone)
        return "1-" + value

class PhoneInput(formencode.Schema):
    phone = validators.PhoneNumber()
    phone_type = validators.String(if_missing=None)
    int_phone = InternationalPhoneNumber("phone", if_missing=None)

PhoneInput().to_python({"phone": "111-111-1111", "int_phone": "phone"})

This code will output: {'phone_type': None, 'phone': '111-111-1111', 'int_phone': '1-phone'} but I would want it to output: {'phone_type': None, 'phone': '111-111-1111', 'int_phone': '1-111-111-1111'}

How would I go about making that happen?

Using chained_validators is sort of what I'm looking for, but all it does is validate, not add the resulting validator value back into the schema.

Any help is appreciated