Python: frame parameter of signal handler

Question

I was looking through the Python documentation for signal and in the example code:

def handler(signum, frame):
    print 'Signal handler called with signal', signum
    raise IOError("Couldn't open device!")

The 'frame' parameter is not used in the actual function. I noticed this in a lot of code on stackoverflow/online regarding signal handlers. What is the 'frame' parameter? Why is it kept in the function header?

Thank you

Answer 1

The frame argument is the stack frame, also known as execution frame. It point to the frame that was interrupted by the signal. The parameter is required because any thread might be interrupted by a signal, but the signal is only received in the main thread.

Example:

import signal
import os
import traceback

def handler(signum, frame):
    print signum, frame
    print "print stack frames:"
    traceback.print_stack(frame)

def demo(n):
    if n == 3:
        os.kill(os.getpid(), signal.SIGUSR1)
        return
    demo(n+1)

signal.signal(signal.SIGUSR1, handler)
demo(1)

Output:

$ python t.py
10 <frame object at 0x1e00520>
print stack frames:
  File "t.py", line 17, in <module>
    demo(1)
  File "t.py", line 14, in demo
    demo(n+1)
  File "t.py", line 14, in demo
    demo(n+1)
  File "t.py", line 12, in demo
    os.kill(os.getpid(), signal.SIGUSR1)