Real-time interrupts in Python

Question

I have a Python 2.7 program running an infinite while loop and I want to incorporate a timer interrupt. What I aim to do is to set off a timer at some point in the loop, and when 5 seconds have elapsed I want the code to branch to a specific part of the while loop.

What I have been doing so far is the following: in each iteration of the while loop I am checking how much time has elapsed when I reach that point in the code using

time.clock()

and if the difference exceeds 5 I run the chunk of code I mean to run

However that way 7 seconds might pass before I evaluate the time, it will be >5sec but I want to go there exactly when 5 seconds pass

Also, I need this to work for more than 1 counter (possibly up to a 100) but I do not want the interrupts to interrupt each other. Using Timer did not work either.

I know this can be done using timer interrupts in assembly but how can I do that in python?

Answer 1

If a single event is to be handled, then the easiest way is to use the signal framework which is a standard module of Python.

However, if we need a fully-fledged scheduler, then we have to resort to another module: sched. Here is a pointer to the official documentation. Please be aware, though, that in multi-threaded environments sched has limitations with respect to thread-safety.

Another option is the Advanced Python Scheduler, which is not part of the standard distribution.

Answer 2

You can't get real-time without special hardware/software support. You don't need it in most cases (do you need to control giant robots?).

How to delay several function calls by known numbers of seconds depends on your needs e.g., if a time it takes to run a function is negligible compared to the delay between the calls then you could run all functions in a single thread:

#!/usr/bin/env python
from __future__ import print_function
from Tkinter import Tk

root = Tk()
root.withdraw() # don't show the GUI window
root.after(1000, print, 'foo') # print foo in a second
root.after(0, print, 'bar') # print bar in a jiffy
root.after(2000, root.destroy) # exit mainloop in 2 seconds
root.mainloop()
print("done")

It implements yours "I do not want the interrupts to interrupt each other either" because the next callback is not called until the previous one is complete.