Programming Python, Second Edition

Signals

For lack of a better analogy, signals are a way to poke a stick at a process. Programs generate signals to trigger a handler for that signal in another process. The operating system pokes too -- some signals are generated on unusual system events and may kill the program if not handled. If this sounds a little like raising exceptions in Python it should; signals are software-generated events, and the cross-process analog of exceptions. Unlike exceptions, though, signals are identified by number, are not stacked, and are really an asynchronous event mechanism controlled by the operating system, outside the scope of the Python interpreter.

In order to make signals available to scripts, Python provides a signal module that allows Python programs to register Python functions as handlers for signal events. This module is available on both Unix-like platforms and Windows (though the Windows version defines fewer kinds of signals to be caught). To illustrate the basic signal interface, the script in Example 3-20 installs a Python handler function for the signal number passed in as a command-line argument.

Example 3-20. PP2E\System\Processes\signal1.py

########################################################## # catch signals in Python; pass signal number N as a # command-line arg, use a "kill -N pid" shell command # to send this process a signal; most signal handlers # restored by Python after caught (see network scripting # chapter for SIGCHLD details); signal module avaiable ...