如何组合线程？

Question

I am trying to set up a program on Raspberry Pi that would log temperature data to a csv as a main process and a parallel process would send e-mail notifications with time intervals if the temperature is out of margins. I read that it could be done with Python's Threading module, so I am trying to set - up a test code and will optimize it for the sensors later. I am using the random.randrange module for virtual readings, however the temperature monitoring thread cannot read the temp variable to be activated. How can the "if" statement read the variable correctly?

I have tried setting up temp outside of the while True loop and activating the thread after the loop

import time
import csv
import random
import smtplib
import threading

path = 'C:/Users/saint/.PyCharmCE2019.1/config/scratches/log.csv'
templogfile = open(path,'a')

sender = 'sender@gmail.com'
receivers = ['receiver@hotmail.com']

def send_email():
    while True: 
        print("mail module")
        if temp <= 20:
            try:
                smtpObj = smtplib.SMTP('smtp.gmail.com', 587)
                smtpObj.starttls()
                smtpObj.login('sender@gmail.com', 'senderpassword')
                smtpObj.sendmail(sender, receivers, message)
                print("Successfully sent email")
                time.sleep(15)
            except smtplib.SMTPException:
                print("Error: unable to send email")
                time.sleep(15)
Threads = []
for i in range(1):
    t = threading.Thread(target=send_email)
    t.daemon = True
    t.start()

with open(path,'a') as csvfile:
    tempwriter = csv.writer(csvfile)
    tempwriter.writerow(['Time', 'Temperature C'])
    while True:
        time_now = time.asctime()
        temp = random.randrange(-10,20)
        tempwriter.writerow([time_now, temp])
        templogfile.close()
        print('Time:', time_now, 'Temperature:', temp)
        csvfile.flush()
        time.sleep(5)

Answer 1

I'm not sure that a separate thread is needed to send emails here. Why not just:

def send_email(temp):
    print("mail module")
    if temp <= 20:
        try:
            smtpObj = smtplib.SMTP('smtp.gmail.com', 587)
            smtpObj.starttls()
            smtpObj.login('sender@gmail.com', 'senderpassword')
            smtpObj.sendmail(sender, receivers, message)
            print("Successfully sent email")
        except smtplib.SMTPException:
            print("Error: unable to send email")

with open(path,'a') as csvfile:
    tempwriter = csv.writer(csvfile)
    tempwriter.writerow(['Time', 'Temperature C'])
    while True:
        time_now = time.asctime()
        temp = random.randrange(-10,20)
        send_email(temp)
        tempwriter.writerow([time_now, temp])
        templogfile.close()
        print('Time:', time_now, 'Temperature:', temp)
        csvfile.flush()
        time.sleep(5)

Answer 2

If you are getting an exception about temp not being defined in your email-sending thread, I suspect it is due to the timing of the two threads you're running. If the email thread runs before the main thread makes its first (simulated) temperature reading, the temp variable won't exist yet. This may or may not happen, it depends on the exact details of the thread timings, which are not always predictable (though it may be very common for the child thread to run for a while before its parent is resumed).

There are a few ways to work around this. One easy fix would be to add a delay into the start of the send_email function so that the thread running it will pause for long enough for the temperature readings in the main thread to have been started. Just add a line like sleep(1) or similar before the start of the while True loop.

Another option would be to initialize temp to some dummy value before you start the email thread. That way there would be a variable to read, even if it didn't actually represent a measured (or simulated) temperature. Something like temp = 21 placed somewhere before the thread starting loop would do the trick.

But I'd note that it's not really clear what the purpose of using threads is in this situation. The email thread is more or less sampling every third temperature reading, but you could do that more explicitly if that's what you want. You could also inspect every reading, and send emails only at some more limited rate, if you don't want to spam them out at the full speed of the sensor loop. I'd consider something like this:

email_flag = False
count = 0
while True:
    time_now = time.asctime()
    temp = random.randrange(-10,20)
    tempwriter.writerow([time_now, temp])

    if temp <= 20:
        email_flag = True

    count += 1
    if email_flag and count >= 3:
        send_email() # unconditionally sends the message
        email_flag = False
        count = 0

Answer 3

I think the problem is because one of the threads is trying to access the global temp variable before it's created. A very simple solution would be to create it near the top of the module with:

temp = None

and then add a short preamble to the thread function that makes it wait until the variable been set to a value:

def send_email():
   # Wait for global to have data (ADDED)
   while temp is None:
       time.sleep(.5)
   while True:
       print("mail module")
       if temp <= 20:
           try:
               smtpObj = smtplib.SMTP('smtp.gmail.com', 587)
               smtpObj.starttls()
               smtpObj.login('sender@gmail.com', 'senderpassword')
               smtpObj.sendmail(sender, receivers, message)
               print("Successfully sent email")
               time.sleep(15)
           except smtplib.SMTPException:
               print("Error: unable to send email")
               time.sleep(15)

Another alternative would be to just set it to some numeric value (that won't be logged) which wouldn't require the preamble to be added since the variable will always exist.