Michael Mossey wrote: > On Jul 6, 2:47 pm, Philip Semanchuk <phi...@semanchuk.com> wrote: >> On Jul 6, 2009, at 5:37 PM, Michael Mossey wrote: >> >>> What is required in a python program to make sure it catches a >>> control- >>> c on the command-line? Do some i/o? The OS here is Linux. >> You can use a try/except to catch a KeyboardInterrupt exception, or >> you can trap it using the signal >> module:http://docs.python.org/library/signal.html >> >> You want to trap SIGINT. >> >> HTH >> Philip > > Thanks to both of you. However, my question is also about whether I > need to be doing i/o or some similar operation for my program to > notice in any shape or form that Control-C has been pressed. In the > past, I've written Python programs that go about their business > ignoring Ctrl-C. Other programs respond to it immediately by exiting. > I think the difference is that the latter programs are doing i/o. But > I want to understand better what the "secret" is to responding to a > ctrl-C in any shape or form. > > For example, does trapping SIGINT always work, regardless of what my > process is doing? > > Thanks, > Mike
Are you asking: "when would the python interpreter process KeyboardInterrupt?" In a multi threaded python program (where KeyboardInterrupt doesn't always work), only the main thread can process KeyboardInterrupt; thus the main thread must regain control before the interrupt is raised. Normally, python will context switch (i.e. thread switch) every 100 interpreter "ticks", but when the interpreter received a SIGINT, the interpreter will try to context switch as fast as it can (every tick) to allow the main thread to regain control. So the answer is in multithreaded python program: "when the main thread regains control" In single threaded python program, the currently running thread is always the main thread (which can handle KeyboardInterrupt). I believe SIGINT is checked at every ticks. But SIGINT cannot interrupt atomic operations (i.e. it cannot interrupt long operations that takes a single tick). An example, where python have difficulties processing KeyboardInterrupt: >>> print 'foo'*10000000 ...foofoofoo... because printing a string is an atomic operation I believe a tick in python is equivalent to a single bytecode, but please correct me if I'm wrong. -- http://mail.python.org/mailman/listinfo/python-list
