namedtuple_builder.py

def builder(cls, allow_default=True):
    class Builder(object):
        def __init__(self):
            self._state = cls(**{name: None for name in cls._fields})
            self._fields_set = set()

        def set(self, **kwargs):
            self._fields_set.update(set(dict(**kwargs).keys()))
            self._state = self._state._replace(**kwargs)
            return self

        def build(self):
            if not allow_default and self._fields_set != set(cls._fields):
                raise ValueError("Not all fields were set: %s",
                                 set(cls._fields) - self._fields_set)
            return self._state

    return Builder()

if __name__ == '__main__':
    import collections
    Foo = collections.namedtuple('Foo', ['x', 'y', 'z'])

    # simple usage
    print builder(Foo).set(x=2).set(y=5).build()

    # As a classmethod
    Foo.builder = classmethod(lambda cls: builder(cls))
    print Foo.builder().set(x=2).set(y=5).set(z=1).build()
    print Foo.builder().set(x=2, y=5).build()