python: Calling derived __init__ from the base class, multiple constructors problem

I'm trying to do protocol decoding in Python.

Short version of my question:

• I have received an array of bytes that is a packet of data.

  • Using the first byte of the packet, I know what type of packet it is.
  • Using that - I want to construct a packet via a class.

• When i want to send a packet, really need to reuse about 90% of the processing done in the same packet class so that I can construct and send a packet.

I can do one of the above, I can't seem to do both

For example - something like this:

class PacketType123(BasePacketType):

    def __init__fromparams( self, field1, field2 ):
        # called when you want to send a packet
        # ... initialize like I'am sending the packet
        self.field1 = field1
        self.field2 = field2

    def encode_and_send(self,destination):
        # encode into raw byte
        databytes = self.encode()
        destination.send( databytes )

    # called when a packet is received on the wire
    def __init__frombytearray( self, bytearray ):
        self.decode_bytearray( bytearray )
        print("Hi Field2 = %d" % self.field2 )

# create packet decode lookup table.
lookuptable = dict()
# populate it with classes
lookuptable[123] = PacketType123
# other packets would added to the lookup table
lookuptable[44] = PacketType44
lookuptable[12] = PacketType12

To send

   p1 = PacketType1( 42, "dogs and cats" )
   p1.encode_and_send( server )

To process incomming data, do this:

   databytes = receive_message()

   packet = lookuptable[ datbytes[0] ].__init__frombytearray(databytes)

My problem is I can do either the ToSend or the ProcessIncomming, I don't know how to do both with the same class.

I've looked over:

• May __init__ be used as normal method for initialization, not as constructor?
• Multiple constructors in python?

• What is a clean, pythonic way to have multiple constructors in Python?

• I've seen this but I'm confused by it: https://hg.python.org/cpython/file/3.5/Lib/multiprocessing/heap.py#l223

It seems the only way to make this work is with keyword args.. Yuck that complicates other things for me... Looking for other way of doing this.

Suggestions please?