StreamSource

def evolve(self: _B, **params) -> _B:
    """Return a copy of the instance with the passed values replaced.
    """
    return replace(self, **params)

@classmethod
def from_response(cls, resp: Dict[str, Any]):
    cls._convert(resp, 'external', ExternalStream)
    cls._convert(resp, 'subject_transforms', SubjectTransform)
    return super().from_response(resp)

def as_dict(self) -> Dict[str, object]:
    result = super().as_dict()
    if self.subject_transforms:
        result['subject_transform'] = [
            tr.as_dict() for tr in self.subject_transforms
        ]
    return result