Python binary value of integer of certain byte size

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I know python probably isn't the best tool for this, but let's say I have a value that I would like to display as an unsigned char with values between -128 and 127. For example:

# ok for positive number
>>> f'0b{1:>08b}' 
'0b00000001'

# how to do it for negative number?
>>> f'0b{-1:>08b}' # should be 0b11111111
'0b000000-1'

# how to do it for 2's complement?
>>> f'0b{~1:>08b}' # should be 0b11111110
'0b00000-10'

How could I do this display in python?

bit-manipulation python-3.x binary python
2 Answers

Use modulo 256.

# positive number is the same
>>> f'0b{1 % 0x100:>08b}' 
'0b00000001'

# correct bit pattern, were you to notate -1 in a signed int8
# same as notating 255 in unsigned int8, which is what -1 % 255 is
>>> f'0b{-1 % 0x100:>08b}'
'0b11111111'

# flipped bits from 1, truncated to only the least significant 8 digits
>>> f'0b{~1 % 0x100:>08b}'
'0b11111110'

Essentially this is just 'convert your signed char into an unsigned char, and print the bit pattern' - the benefit of using the modulo operator is you always get a positive number, and if your modulo is a power of two, the bit pattern for every bit less than that modulus is left exactly the same.

user_26488110

You could try manually setting the bits like this knowing that 2^n - 1 sets the first n bits:

>>> negative   =lambda value, bits: bin(2**bits-1-value+1)
>>> complement =lambda value, bits: bin(2**bits-1-value)

# to verify (note, python doesn't 'know' its 1 byte so will
# equal 256 unless we do the 1-byte mask with &0xFF
>>> (1+int(complement(1,8),2))&0xFF
0
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