Threads not executing concurrently-CodePudding

I am working with a simple Python script that controls a sensor and reads measurements from this sensor. I want to take different measurement types concurrently, the function below can be used for each type of measurement:

def measure(measurement_type, num_iterations):
    file = open(measurement_type, 'w')
    writer = csv.writer(file)

    for i in range(num_iterations):
        pm2.5, pm10 = sensor.query()
        writer.writerow(pm2.5, pm10, curr_time())
        time.sleep(60)
    
    file.close()
    upload_data(file, measurement_type)

I attempt to invoke multiple calls to this function on separate threads in order to obtain files describing measurements in various contexts of times (hourly, daily, weekly, etc.):

if __name__ == '__main__':
    sensor = SDS011("/dev/ttyUSB0")  
    sensor.sleep(sleep=False) 
    print("Preparing sensor...")
    time.sleep(15)
    print("Sensor is now running:")

    try:
        while True:
            Thread(target=take_measurements('hourly', 60)).start()
            Thread(target=take_measurements('daily', 1440)).start()
            Thread(target=take_measurements('weekly', 10080)).start()
            Thread(target=take_measurements('monthly', 43800)).start()
    except KeyboardInterrupt:
        clean_exit()

Only one of these threads is ever running at a given time, and which one is executed appears random. It may be worth noting that this script is running on a RaspberryPi. My first thought was that multiple threads attempting to access the sensor could create a race condition, but I would not expect the script to continue running any threads if this occurred.

CodePudding user response：

when you call your function directly in the target operation, Python will first try to evaluate what your function returns and execute its code. There is a special way to indicate to the threading module that you want some arguments for your function and not call your function until the moment you start the thread. Hope the example below helps:

from time import sleep
from random import randint
from threading import Thread

def something(to_print):
    sleep(randint(1,3))
    print(to_print)

threadlist = []
threadlist.append(Thread(target=something, args=["A"]))
threadlist.append(Thread(target=something, args=["B"]))
threadlist.append(Thread(target=something, args=["C"]))

for thread in threadlist:
    thread.start()

This will return a different value each time:

(.venv) remzi in ~/Desktop/playground  > python test.py    
A
C
B
(.venv) remzi in ~/Desktop/playground  > python test.py
C
A
B