Python Signal Processing

Welcome to python signal processing tutorial. This is going to be a very basic lesson on how to handle signals with python. A computer receives and transmits various commands with the help of signals. An operating system has got at least one signal listed for each type of command it is programmed to receive or generate. And now, we will look at the basic signal processing in python 3.

What is Signal

A signal is like a notification of an event. When an event occurs in the system, a signal is generated to notify other programs about the event. For example, if you press Ctrl + c, a signal called SIGINT is generated and any program can actually be aware of that incident by simply reading that signal. Signals are identified by integers.

Python signal processing

Python signal module is required for almost all the basic signal handling operations in python.

To understand how python signal processing works, we need to know about ‘signal handler’. Signal handler is a task or program, which is executed when a particular signal is detected. A handler takes two arguments, namely, the signal number and a frame.

You may just print a line in the handler or initiate some other task in response to the signal. signal.signal() function assigns handlers to a signal. Now without further discussion, let’s proceed to an example.

Python Signal Example

The following example shows how to program the system to respond to a SIGINT signal.

import signal  
import time  
  
  
def handler(a, b):  # define the handler  
    print("Signal Number:", a, " Frame: ", b)  
  
signal.signal(signal.SIGINT, handler)  # assign the handler to the signal SIGINT  
  
while 1:  
    print("Press ctrl + c")  # wait for SIGINT  
    time.sleep(10) 

Lines 1 and 2 imports the signal and time module to be used in the program.

Signal handler is defined in lines 5 to 6. It prints the integer value of the signal and the frame it receives along with the signal.

Line 8 uses signal.signal() function to assign the handler to the signal SIGINT. This means every time the CPU picks up a ‘ctrl + c’ signal, the function handler is called.

Line 10 to 12 is written to keep the program running indefinitely.

To execute this program, save the code as python_signal.py and open command window at the same folder. Then run the command python python_signal.py in the command window. The program should be running by then. Now press ctrl + c to obtain the following result. You may terminate the program by simply closing the command window or pressing ctrl + z.

Below image shows the output produced by above python signal processing example.

Python Signal Alarm

Let’s see another example to demonstrate the use of SIGALARM signal.

import signal  
import time  
  
  
def alarm_handler(signum, stack):  
    print('Alarm at:', time.ctime())  
  
  
signal.signal(signal.SIGALRM, alarm_handler)  # assign alarm_handler to SIGALARM  
signal.alarm(4)  # set alarm after 4 seconds  
print('Current time:', time.ctime())  
time.sleep(6)  # make sufficient delay for the alarm to happen 

Above python alarm signal program produces following output.

In this example, it is notable that a delay is made at line 12 so that the program does not terminate before the alarm time. The alarm is set after 4 seconds (Line 10) and alarm_handler does what the alarm has to do.

Python Signal Processing Summary

It has to be remembered that signals are not same for every operating systems. Some of the signals work in all the operating systems while others don’t. And while working with threads, only the main thread of a process can receive signals. So that’s it for python signal processing. Hope to come up with more tutorials very soon.

Reference: Official Documentation