Use xdotool to send special characters

I am trying to control the volume of omxplayer while the track is playing. I can use the keyboards + and – keys. However I want to control the volume using a python script. The following script works to pause and unpause omxplayer, by sending p using xdotool.

xdotool does not accept special characters. Is there a work around for this?

When I replace the p with + or –, I get the following error:

Error: Invalid key sequence '-'
Failure converting key sequence '-' to keycodes
Error: Invalid key sequence '-'
Failure converting key sequence '-' to keycodes
xdo_send_keysequence_window reported an error for string '-'

Here is the script that I am using:

threading.Thread(target=play_clip).start()
print"startng tone"
time.sleep(1)
while a==1:
    print 'ttt'
    time.sleep(1)
    os.system('xdotool key p')
    time.sleep(1)
    os.system('xdotool key p')

xdotool key minus plus

will simulate hitting -and +.