Multiple timed functions on a single thread

Question

I am reading the docs on python threading and the timer subclass and I do not quite understand how it would work if I want to run two (or more) timed class methods at different rates on the same thread.

For example, I have an I/O serial device that I want to periodically read (if any data), write any input messages from outside modules, and periodically write a predefined device specific heartbeat message. This is all wrapped in a custom class. I/O messages are stored in two separate class queue objects.

Do I need to create three threading.Timer objects for each function, or can I somehow use one thread that switches?

import serial, threading, Queue
# the exact syntax below may be incorrect

class MySerialDevice:

    def __init__():
         # Some other modules will be appending to this (through a class method) 
         self.write_queue = Queue()
         self.read_queue = Queue()
         self.port = serial.Serial()
         # check for incoming data every 20ms
         self.thread1 = threading.Timer(0.02, self.read)
         # check for outgoing data every 20ms
         self.thread2 = threading.Timer(0.02, self.write)
         # Send the heaertbeat every 1 second
         self.thread3 = threading.Timer(1, self.heartbeat)

         # what do I do here???
         # can I make all three tasks on a single thread, that just continuously loops 
         # and "if incoming data, read, if outgoing data, write, heartbeat???

    def read(self):
        # checks for actual data, appending to queue are ommited
        self.port.read()
    def write(self):
        # checks for data in queue ommitted
        self.port.write(self.write_queue[0])
    def heartbeat(self):
        self.port.write("Device Heartbeat message)"

Answer 1

Timer is dead-simple; there's nothing more to it than you see. It runs a single timer, one time, taking up a whole thread to do it.

In fact, it's really there more as sample code than anything else. The second sentence in its documentation is " Timer is a subclass of Thread and as such also functions as an example of creating custom threads." And the threading docs link to the source , and you can see how simple it is.

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It's not that hard to build something more complicated out of that sample code.

There used to be some good examples in the ActiveState recipe collection. I don't know how to search their new repo, but you can start off with a tag lookup .

Alternatively, there are a bunch of more powerful schedulers ready to use on PyPI.

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Or you can rewrite your code around a tick method. If you tick every 20ms, read and write run on every tick, and heartbeat runs every 50 ticks, right?

def __init__(self):
    self.tickcount = 0
    self.timer = threading.Timer(0.02, self.tick)

def tick(self):
    self.read()
    self.write()
    self.tickcount += 20
    if not self.tickcount % 1000:
        self.heartbeat()
    self.timer = threading.Timer(0.02, self.tick)

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However, at this point, it's almost as simple to just write a function that loops every 20ms instead of using a timer. And that's much easier to extend if, eg, you need to worry about not accumulating lag. For example:

def ticker(self):
    last = datetime.datetime.now()
    while True:
        wake = last + datetime.timedelta(milliseconds=20)
        now = datetime.datetime.now()
        while wake > now:
            time.sleep((wake - now).total_seconds())
        self.tick()
        last = wake

Now you can just run self.ticker in a thread.

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Or you can just use multiple Timer objects. You're talking about a handful of objects and three timers per object, right? It may not seem the most elegant solution, but it's not like it's going to overburden the scheduler.