How to print a signed integer as hexadecimal number in two's complement with python?

Question

I have a negative integer (4 bytes) of which I would like to have the hexadecimal form of its two's complement representation.

>>> i = int("-312367")
>>> "{0}".format(i)
'-312367'
>>> "{0:x}".format(i)
'-4c42f'

But I would like to see "FF..."

Solution 1

Here's a way (for 16 bit numbers):

>>> x=-123
>>> hex(((abs(x) ^ 0xffff) + 1) & 0xffff)
'0xff85'

(Might not be the most elegant way, though)

Solution 2

>>> x = -123
>>> bits = 16
>>> hex((1 << bits) + x)
'0xff85'
>>> bits = 32
>>> hex((1 << bits) + x)
'0xffffff85'

Solution 3

Using the bitstring module:

>>> bitstring.BitArray('int:32=-312367').hex
'0xfffb3bd1'

Solution 4

Simple

>>> hex((-4) & 0xFF)
'0xfc'

Solution 5

To treat an integer as a binary value, you bitwise-and it with a mask of the desired bit-length.

For example, for a 4-byte value (32-bit) we mask with 0xffffffff:

>>> format(-1 & 0xffffffff, "08X")
'FFFFFFFF'
>>> format(1 & 0xffffffff, "08X")
'00000001'
>>> format(123 & 0xffffffff, "08X")
'0000007B'
>>> format(-312367 & 0xffffffff, "08X")
'FFFB3BD1'

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Comments

[dcoles]

This doesn't work for positive numbers. If x=1 you get 0xffff (-1 in 16-bit two's complement), instead of 0x0001.

[dcoles]

For 123, isn't the 16-bit 2's complement representation 0x007b? 0xff85 would be for -123.

[John La Rooy]

@dcoles, no the sum of 123 and it's 2's complement should be 0 (the carry/overflow bit is discarded) It doesn't matter whether you want to treat 0xff85 as signed or unsigned.

[dcoles]

I agree that the 2's complement (the operation on binary numbers) of 0x007b is 0xff85, but I believe the question was about the 2's complement signed number representation.

[John La Rooy]

@dcoles, ah. I think I get what you are saying. It's proably clearer if I make x = -123

[dcoles]

The example given was a negative integer, but the question was "How to print a signed integer... in two's complement [representation]". It could be confusing if this answer only works for negative signed integers.