Question

Given a decimal integer (eg. 65), how does one reverse the underlying bits in Python? i.e.. the following operation:

65 → 01000001 → 10000010 → 130

It seems that this task can be broken down into three steps:

1. Convert the decimal integer to binary representation
2. Reverse the bits
3. Convert back to decimal

Steps #2 and 3 seem pretty straightforward (see this and this SO question related to step #2), but I’m stuck on step #1. The issue with step #1 is retrieving the full decimal representation with filling zeros (ie. 65 = 01000001, not 1000001).

I’ve searched around, but I can’t seem to find anything.

Answer 1

int('{:08b}'.format(n)[::-1], 2)

You can specify any filling length in place of the 8. If you want to get really fancy,

b = '{:0{width}b}'.format(n, width=width)
int(b[::-1], 2)

lets you specify the width programmatically.