import os, subprocess
import tee

long_string = "0123456789" * 30 # just to have it over 255 chars (the OS limit is 8192 !)

# this works, the only difference is that the called program is not included in quotes :p
cmd_a = r'C:\Windows\System32\help.exe "%s"' % (long_string)

# no in real life you may need to quote the program been executed because the path (may) contain spaces
cmd_b = r'"C:\Windows\System32\help.exe" "%s"' % (long_string)

# if we add additional quotes it will run just fine
cmd_c = r'""C:\Windows\System32\help.exe" "%s""' % (long_string)

cmd = cmd_b

print("-"*80)
os.system(cmd)

print("-"*80)
subprocess.Popen(cmd, shell=True)