Question

I guess this is best explained by code and comments:

import struct

class binary_buffer(str):
    def __init__(self, msg=""):
        self = msg
    def write_ubyte(self, ubyte):
        self += struct.pack("=B", ubyte)
        return len(self)

Output
>> bb = binary_buffer()
>> bb # Buffer starts out empty, as it should
''
>> bb.write_ubyte(200)
1   # We can see that we've successfully written one byte to the buffer
>> bb
''  # Huh? We just wrote something, but where has it gone?

Answer 1

strs are immutable. Therefore,

self += struct.pack("=B", ubyte)

is evaluated as

self = self + struct.pack("=B", ubyte)

This assigns a new value to the name self, but self is just a name like any other. As soon as the method exits, the name (and the associated object) are forgotten.

You want a bytearray:

>>> bb = bytearray()
>>> bb.append(200)
>>> bb
bytearray(b'\xc8')