"ValueError: sleep length must be non-negative" in a thread
<p>The code below is in a thread. The code is copied from an answer to a question asked here a couple of years ago (unfortenately I can't find the question at the moment). Though it looks sound to me, I get an error: "ValueError: sleep length must be non-negative". This happens only after about an hour or so. Could it be that it has to do something with the PC going to sleep (I think unlikely, but it's a laptop running Windows 10 ;-)). This is the code:</p>

<pre><code>def upd_variables():
next_call = time.time()
while True:
    measured_temp = 19.5 # te wijzigen in uitlezing van de sensor
    acttemp.configure(text = str(measured_temp))
    humi = '49.5' # te wijzigen in uitlezing van de sensor
    humidity.configure(text = humi)
    actdaytime = datetime.datetime.now()
    today = actdaytime.strftime("%A %d %B %Y     %H:%M:%S")
    datumtijd.configure(text=today)
    if (measured_temp &lt; desired_temp):
        #CV aan
        status.configure(text="CV aan")
    else:
        #CV uit
        status.configure(text="CV uit")
    next_call = next_call+1;
    time.sleep(next_call - time.time())
</code></pre>

<p>I hope this is readable, I'm not sure the code block will look like I want it to...
BTW: it is part of a thermostat program I'm working on, intended to run on a Raspberry Pi.</p>