在python中使用解包

Question

I have a list of data where in each member in the list contains a byte value.

list data = [0x00, 0x42, 0x00, 0xAD, 0x42, 010, 0xE0, 0xA2 ....]

This is the data that i receive from some external API. I need to map this data to fixed format.

I am writing a class where in each field value can be retrieved .

class FmtData
    def __init__(self, data):
        self.data = data

    def getParam1(self):
       //This a to return a value of 8 bytes starting from say index 1 in data

    def getParam2(self):
       // return 12 bits [20-31] corresponding bits from {byte 3 and 4 - 0x00 and 0xAD}

What is the best way to do it in python.

i am trying to use unpack from struct, but not sure how to extract specific bits ?

Answer 1

First, you probably want to convert the list of bytes to bytearray(list_data) .

However, the struct module doesn't handle sub-byte packing. You could do it manually, but at that point, why even use struct ?

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The ctypes module lets you define C-style structures, including bit fields. See Structures and unions in the docs. It's not as convenient for some use cases, but I think it will work for you here. For example:

from ctypes import *

class FmtStruct(Structure):
    _fields_ = [('padding', c_byte),
                #...
               ]

Then you construct a bytearray from the data, cast it to a FmtStruct , and just access the fields.

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You can also look at third-party modules which let you turn a string of bytes into a string of bits (without wasting 8x the storage), which you can then slice like any other string, and also let you turn any slice into an integer, which sounds like exactly what you want. I don't have a specific module to recommend, but from a quick search, bitstring and bitarray look promising.

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Or, if speed and size aren't that important, just manually explode the list of bytes into a list of bits, slice it, and manually convert the slices to integers bit by bit (eg, via << and | ).