The cleanest way (I've ever seen) to implement state machines in Python

protocol_via_coroutines.py

#!/usr/bin/env python3

# Based on a great article from Eli Bendersky:
#
#     http://eli.thegreenplace.net/2009/08/29/co-routines-as-an-alternative-to-state-machines/
#
# I just "ported" to Python3 (still using plain old yield) and added the proper type annotations,
# since Python 3.5 can now support them (and mypy can check them):
#
#      $ mypy --disallow-untyped-defs protocol_via_coroutines.py 
#      $
#
from typing import Callable, Generator, List

# A protocol decoder:
#
# - yields Nothing
# - expects ints to be `send` in his yield waits
# - and doesn't return anything.
ProtocolDecodingCoroutine = Generator[None, int, None]

# A frame consumer (passed as an argument to a protocol decoder):
#
# - yields Nothing
# - expects List[int] to be `send` in his waiting yields
# - and doesn't return anything.
FrameConsumerCoroutine = Generator[None, List[int], None]


def unwrap_protocol(header: int=0x61,
                    footer: int=0x62,
                    dle: int=0xAB,
                    after_dle_func: Callable[[int], int]=lambda x: x,
                    target: FrameConsumerCoroutine=None) -> ProtocolDecodingCoroutine:
    """
    Simplified framing (protocol unwrapping) co-routine.
    """
    # Outer loop looking for a frame header
    #
    while True:
        byte = (yield)
        frame = []  # type: List[int]

        if byte == header:
            # Capture the full frame
            #
            while True:
                byte = (yield)
                if byte == footer:
                    target.send(frame)
                    break
                elif byte == dle:
                    byte = (yield)
                    frame.append(after_dle_func(byte))
                else:
                    frame.append(byte)


def frame_receiver() -> FrameConsumerCoroutine:
    """
    A simple co-routine "sink" for receiving full frames.
    """
    while True:
        frame = (yield)
        print('Got frame:', ''.join('%02x' % x for x in frame))

bytestream = bytes(
    bytearray((0x70, 0x24,
               0x61, 0x99, 0xAF, 0xD1, 0x62,
               0x56, 0x62,
               0x61, 0xAB, 0xAB, 0x14, 0x62,
               0x7)))

frame_consumer = frame_receiver()
next(frame_consumer)  # Get to the yield

unwrapper = unwrap_protocol(target=frame_consumer)
next(unwrapper)  # Get to the yield

for byte in bytestream:
    unwrapper.send(byte)