python - Get and Set specific bits of a value

Question

For a given value, I need to be able to read and write to specific bits of said value.

Example :

n = 341
# bin(341) is '0b101010101'

get_bits(n, start = 2, end = 6)
# Returns 10 = '0b1010'

n = set_bits(n, start = 2, end = 6, newValue = 6)
# Replaces bits [2:6] of n with 0b0110
# Makes n = 309 = '0b100110101'

question from:https://stackoverflow.com/questions/66045290/get-and-set-specific-bits-of-a-value

Answer 1

Normally bits are numbered from right to left. If you used that standard convention and a start-stop value (like a range), your functions would be simpler to implement with bitwise operators:

def get_bits(n, start, end):
    return (n&((1<<end)-1))>>start

def set_bits(n, start, end, value):
    mask = (1<<end) - (1<<start) 
    return (n&~mask) | (value<<start)&mask

output:

n = 341  # 101010101
         # 876543210 bit numbers

                           # 101010101 = 341 
f"{get_bits(n,3,7):b}"     # ..1010... = 10
                           # 876543210

                           # 101010101 = 341 
f"{set_bits(n,3,7,12):b}"  # 101100101 = 357
                           # ..^^^^...
                           # ..1100... = 12
                           # 876543210