如何在Python中将Integer转换为Signed Word（用于PySerial）？

Question

I'm having some problems making Python talk to a hardware display using pyserial. Some of the display's functions require a signed word to be sent as arguments after commands (ie. X or Y on display screen).

I've been getting by with chr() previously, but that only works with numbers < 255.

I've tried the following for conversion but it's giving some weird results, placing things way off the set position:

def ByteIt(self,data):
    datastring = str()
    for each in tuple(str(data)):
        datastring = datastring + chr(int(each))
    return datastring

I may be way off myself here :)

Example of how i would use it:

x = 100
y = 350
serial.Write('\x01' + ByteIt(x) + ByteIt(y)) # command , xpos , ypos

The thing is, when i do this, the stuff is not placed at x100,y350, most times the display will crash :(

Any tips on how this can be done properly?

Answer 1

Read about the struct module.

http://docs.python.org/library/struct.html

Replace all of the "chr" and stuff with proper struct.pack() calls.

Specifically

bytes = struct.pack( 'h', some_data )

Should give you a "signed word".

Answer 2

You may want to revise again the documentation about "pack" and "unpack". Selecting the appropriate upper-case or lower-case allows you to specify the Endianness. So, based on the example above which didn't work perfectly on your device, I presume you need:

x = 100
y = 350
serial.Write('\x01' + 
             struct.pack('>hh', x) + 
             struct.pack('>hh', y)) # command , xpos , ypos