You can use slicing on byte objects:
>>> value = b'\x00\x01\x00\x02\x00\x03'
>>> value[:2]
b'\x00\x01'
>>> value[2:4]
b'\x00\x02'
>>> value[-2:]
b'\x00\x03'
When handling these frames, however, you probably also want to know about memoryview() objects; these let you interpret the bytes as C datatypes without any extra work on your part, simply by casting a ‘view’ on the underlying bytes:
>>> mv = memoryview(value).cast('H')
>>> mv[0], mv[1], mv[2]
256, 512, 768
The mv object is now a memory view interpreting every 2 bytes as an unsigned short; so it now has length 3 and each index is an integer value, based on the underlying bytes.