Question

I have been having some real trouble with this for a while. I am receiving a string of binary data in python and I am having trouble unpacking and interpreting only a 5bit subset (not an entire byte) of the data. It seems like whatever method comes to mind just simply fails miserably.

Let’s say I have two bytes packed binary data, and I would like to interpret the first 10bits within the 16. How could I convert this to an 2 integers representing 5bits each?

Answer 1

Use bitmasks and bitshifting:

>>> example = 0x1234   # Hexadecimal example; 2 bytes, 4660 decimal.
>>> bin(example)       # Show as binary digits
'0b1001000110100'
>>> example & 31       # Grab 5 most significant bits
20
>>> bin(example & 31)  # Same, now represented as binary digits
'0b10100'
>>> (example >> 5) & 31 # Grab the next 5 bits (shift right 5 times first)
17
>>> bin(example >> 5 & 31)
'0b10001'

The trick here is to know that 31 is a 5-bit bitmask:

>>> bin(31)
'0b11111'
>>> 0b11111
31
>>> example & 0b11111
20

As you can see you could also just use the 0b binary number literal notation if you find that easier to work with.

See the Python Wiki on bit manipulation for more background info.