python dictionary with key requirements

fields_dict.py

#! /usr/bin/env python
from abc import ABCMeta


class FieldsDict(dict):
    __metaclass__ = ABCMeta
    FIELDS = None
    REQUIRED = None

    def __init__(self, seq=None, **kwargs):
        # create a single dict
        seq = seq or {}
        mapping = dict(seq, **kwargs)
        # make sure all fields are expected
        try:
            unexpected = next(key for key in mapping if key not in self.FIELDS)
        except StopIteration:
            pass
        else:
            raise ValueError("The field '%s' was not expected" % unexpected)
        # and check the required fields
        if self.REQUIRED:
            try:
                req = next(r for r in self.REQUIRED if r not in mapping)
            except StopIteration:
                pass
            else:
                raise ValueError("The field '%s' is required" % req)
        super(FieldsDict, self).__init__(mapping)

# use as:
class Person(FieldsDict):
    FIELDS = (
        'title',
        'first_name',
        'last_name',
        'email',
        'phone',
        'birthday',
    )
    REQUIRED = (
        'title',
        'first_name',
        'last_name',
    )


# should give ValueError("The field 'title' is required")
me = Person(first_name='Roy')

# should give ValueError("The field 'last_name' is required")
me = Person({'title': 'MR', 'first_name': 'Roy'})

# should give ValueError("The field 'level' was not expected")
me = Person({'level': 'PRO', 'first_name': 'Roy'})

# should be fine
me = Person({'title': 'MR', 'first_name': 'Roy', 'last_name': 'Enjoy', 'email': 'roy@example.com'})
# or that is fine too
me = Person(title='MR', first_name='Roy', last_name='Enjoy', phone=123456)