gistfile1.txt

class _HashSumProtocol(Protocol):
    def __init__(self, kind='sha256'):
        self._hash = hashlib.new(kind)

    def dataReceived(self, data):
        self.bytes_received += len(data)
        self._hash.update(data)

then you'd do something like this with a twisted.web.client Agent:

        resp = yield agent.request('GET', uri)
        hasher = _HashSumProtocol()
        resp.deliverBody(hasher)