Translating an integer into two-byte hexadecimal with Python

Question

I need to take an integer (for example, -200) and change it into a hexadecimal (FF 38), then into an integer 0-255 to send via serial. The example I was given was:

-200 = hex FF38 = [hex FF] [hex 38] = [255] [56]

I've tried using struct.pack('>h', -200) but that returned the incorrect value. I also tried hex() but that returned a negative hex value that was incorrect as well.

I don't know what else I should be trying.

Answer 1

>>> hex(struct.unpack('>H', struct.pack('>h', -200))[0])
'0xff38'

First, you pack the -200 as a signed 2-byte short int . Then you unpack that data as an unsigned 2-byte short int . This gives you 65336, of which hex is ff38

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If you need to send 0-255 integers, this might work better:

>>> struct.unpack('>BB', struct.pack('>h', -200))
(255, 56)
>>> struct.unpack('>BB', struct.pack('>h', 200))
(0, 200)

Where you're unpacking into two unsigned chars

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Requisite documentation on struct

Answer 2

Two's complement arithmetic and some string slicing:

hex(200 - (1 << 16)) #2 bytes -> 16 bits
Out[26]: '-0xff38'

int(hex(200 - (1 << 16))[-4:-2], 16)
Out[28]: 255

int(hex(200 - (1 << 16))[-2:], 16)
Out[29]: 56

Note that you'll have to take care to correctly handle non-negative numbers (ie don't take the two's complement) and when the length of the string returned by hex is of varying length.