Python: How do I extract specific bits from a byte?

Question

I have a message which reads as 14 09 00 79 3d 00 23 27. I can extract each byte from this message by calling message[4], which will give me 3d for example. How do I extract the individual 8 bits from this byte? For example, how would I get bits 24-27 as as single message? How about just bit 28?

Asked By: Aditya Salapaka

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Answer 1

To answer the second part of your question, you can get specific bit values using bitwise operations

# getting your message as int
i = int("140900793d002327", 16)

# getting bit at position 28 (counting from 0 from right)
(i >> 28) & 1

# getting bits at position 24-27
bin((i >> 24) & 0b111)

Answered By: Anis

Answer 2

The easiest way to do this is to use the & operator. Convert your message to an int using int(str_msg, 16). convert int to binary string using bin(myint)

To get bits 4-6 (from left) in a byte:

>> msg = int("10110111", 2) # or 0b10110111
>> extractor = int("00011100", 2) # or 0b10110111
>> result = msg & extractor
>> print bin(result)
00010100

If you want, you can bit shift result using result >> 2. Obviously you will want to make this more dynamic but this is a dumbed down example.

Answered By: Ian Kirkpatrick

Python: How do I extract specific bits from a byte?

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