Python - Elegant way to deal with binary data that is not a multiple of 2

Asked Oct 19 '19 at 12:55

Active Nov 10 '20 at 03:20

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I am working with binary data that is stored in a byte array and I need to access bits of arbitrary length inside this array. Performance is also a concern. Let me give a brief example:

# 11111111 | 11101111 | 01111100
byte_arr = bytearray([255, 239, 124])

# A corresponds to the first 6 bits, e.g. index 0-5
a = byte_arr[0]>>2 

# B corresponds to the next 2 bits, e.g. index 6-7
b = byte_arr[0] & 0x3

# C corresponds to the next 9 bits, e.g. index 8-16
c = int.from_bytes(byte_arr[1::], byteorder="big", signed=False)>>7

My above solution works, but a typical byte array has 20 bytes and therefore the code gets unreadable and is definitely not intuitive at a first glance.

Modules like struct won't help, because they ONLY can unpack bytes, e.g. 2^X

edited Nov 10 '20 at 03:20

sjakobi

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asked Oct 19 '19 at 12:55

Leonleon1

So, you'd like a function that returns an integer based on an arbitrary start index and length from your byte array? – Nick Martin Oct 19 '19 at 13:10
Why don't just use a bytearray of size 17? – Dani Mesejo Oct 19 '19 at 13:15
@NickMartin Yes, that's want I want – Leonleon1 Oct 19 '19 at 13:32
@DanielMesejo A byte array stores bytes but I need bits. – Leonleon1 Oct 19 '19 at 13:34
Can you use external libraries, if so check this answer: https://stackoverflow.com/questions/11669178/how-to-create-an-array-of-bits-in-python – Dani Mesejo Oct 19 '19 at 13:38
I will have a look at the bit array module and take some performance measures, thanks for now! – Leonleon1 Oct 19 '19 at 14:51

Python - Elegant way to deal with binary data that is not a multiple of 2

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