-- Common handler --

class Server:
    ...
    self.file = None
class ServerConnectionHandler(...):
    def data_received(self, recv):
        if recv == b'OPEN' and not self.server.file:
            self.server.file = open('myfile.ext')
        else:
            self.server.file.seek(struct.unpack("!I", recv));
            data = self.server.file.read(512 * 1024)
            self.transport.write(data)

-- Own handler --
class ServerConnectionHandler(...):
    def data_received(self, recv):
        if recv == b'OPEN':
            self.file = open('myfile.ext')
        else:
            self.file.seek(struct.unpack("!I", recv));
            data = self.file.read(512 * 1024)
            self.transport.write(data)