Compare:

$ ./python -c "import subprocess, signal, time; p = subprocess.Popen(['cat']); time.sleep(1); p.send_signal(signal.SIGINT); print(p.wait())"
-2

with:

$ ./python -c "import subprocess, signal, time; p = subprocess.Popen(['python', '-c', 'input()']); time.sleep(1); p.send_signal(signal.SIGINT); print(p.wait())"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
KeyboardInterrupt
1

Python's behaviour apparently breaks a common assumption towards Unix processes (see bug reported at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=652926). A solution would be to add a signal number attribute to KeyboardInterrupt, and use that value when computing the process exit code.