Question

I’m working on a project using National Instruments boards to do data acquistion. I have functional C codes for doing the tasks, but would like to use Python, so the GUI programming is less painful. In my C-code, I use the API call setTimer, which raises a WM_TIMER event at regular intervals. Is there a similar, mechanism in a Tk loop? I tried using the following code.

def DAQ(self):
    if self.do_DAQ:
        result = self.myDAQ.getData()
        currTime = time.time() - self.start_time
        self.time_label.config(text="{:.1f} seconds".format(currTime))
        self.volt_label.config(text="{:.4f} volts".format(result))
        self.time_data[self.i] = currTime
        self.volt_data[self.i] = result
        self.i += 1
        self.after(1962, self.DAQ)

The magic “1962” in the after() was determined by trial and error to give about a 2 second delay, but the time slices drift depending on what else is in the queue. Is there a way I can do this so my time slices are more accurate? Specifically, can I force Tk to do my DAQ event before other things in the queue?

Answer 1

Here’s a sort of quickie example of what I’m talking about in my comment:

import Tkinter as tk
import threading
import random
import time
from Queue import Queue, Empty

root = tk.Tk()
time_label = tk.Label(root, text='<unknown> seconds')
volt_label = tk.Label(root, text='<unknown> volts')
time_label.pack()
volt_label.pack()

def DAQ(q):
    while True:
        q.put((time.time(), random.randrange(100)))
        time.sleep(2)

def update_data(queue, root):
    try:
        timestamp, volts = queue.get_nowait()
    except Empty:
        pass
    else:
        time_label.config(text='{:.1f} seconds'.format(timestamp))
        volt_label.config(text='{:.4f} volts'.format(volts))
    root.after(100, update_data, queue, root)

data_queue = Queue()
t = threading.Thread(target=DAQ, args=(data_queue,))
t.daemon = True
t.start()
update_data(data_queue, root)
root.mainloop()

Obviously the above DAQ() function is just a stand-in for the real thing. The point is, as @ballsdotballs suggested in their answer, you can sample at whatever rate you want in your DAQ thread, add the values to a queue, and then update the GUI at a more appropriate rate.