Python bitwise ~ operator invert all bits of integer but we can’t see native result because all integers in Python has signed representation.

Indirectly we can examine that:

>>> a = 65
>>> a ^ ~a
-1

Or the same:

>>> a + ~a
-1

Ther result -1 means all bits are set. But the minus sign ahead don’t allow us to directly examine this fact:

>>> bin(-1)
'-0b1'

The solution is simple: we must use unsigned integers.
First way is to import numpy or ctypes modules wich both support unsigned integers. But numpy more simplest using than ctypes (at least for me):

import numpy as np
a = np.uint8(0b1100)
y = ~x

Check result:

>>> bin(x)
'0b1100'
>>> bin(y)
'0b11110011'

And finally check:

>>> x + y
255

Unsigned integer ‘255’ for 8-bits integers (bytes) mean the same as ‘-1’ becouse has all bits set to 1. Make sure:

>>> np.uint8(-1)
255

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And another simplest solution, not quite right, but if you want to include additional modules, you can invert all bits with XOR operation, where second argument has all bits are set to 1:

a = 0b1100
b = a ^ 0xFF

This operation will also drop most significant bit of signed integer and we can see result like this:

>>> print('{:>08b}'.format(a))
00001100
>>> print('{:>08b}'.format(b))
11110011

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Finally solution contains one more operation and therefore is not optimal:

>>> b = ~a & 0xFF
>>> print('{:>08b}'.format(b))
11110011