How do I do a bitwise Not operation in Python?

Question

In order to test building an Xor operation with more basic building blocks (using Nand, Or, and And in my case) I need to be able to do a Not operation. The built-in not only seems to do this with single bits. If I do:

x = 0b1100 x = not x 

I should get 0b0011 but instead I just get 0b0. What am I doing wrong? Or is Python just missing this basic functionality?

I know that Python has a built-in Xor function but I've been using Python to test things for an HDL project/course where I need to build an Xor gate. I wanted to test this in Python but I can't without an equivalent to a Not gate.

Answer 1

The problem with using ~ in Python, is that it works with signed integers. This is also the only way that really makes sense unless you limit yourself to a particular number of bits. It will work ok with bitwise math, but it can make it hard to interpret the intermediate results.

For 4 bit logic, you should just subtract from 0b1111

0b1111 - 0b1100  # == 0b0011 

For 8 bit logic, subtract from 0b11111111 etc.

The general form is

def bit_not(n, numbits=8):     return (1 << numbits) - 1 - n 

Answer 2

Another way to achieve this, is to assign a mask like this (should be all 1's):

mask = 0b1111 

Then xor it with your number like this:

number = 0b1100 mask = 0b1111 print(bin(number ^ mask)) 

You can refer the xor truth table to know why it works.