How to wait for a thread to finish before continuing

Question

I need to run a function in another thread, and get the return value of that function to save it to a variable in the main thread. Basically, my code calls the function, which communicates with a balance through a serial port, waits for and gets the response, parses the response, and returns the response as a float. I need to capture this float so I can save it. This is my code:

from multiprocessing.pool import ThreadPool

pool = ThreadPool(processes=2)
result = pool.apply_async(manual_short_command, (self.status_signal2, self.conn, command, status_string, 2))
self.readout = result.get()

def manual_short_command(signal, conn, command, status_string, string_arg='', timeout=60):
""" Send specified command (expects a balance response)to balance through "conn" and emit status signal """
print "Command: " + command.strip('\r\n')
emit_status(signal, status_string[0], string_arg)
if not conn.isOpen():
    conn.open()
# Wait until nothing can be read at the balance port
while not timeout and conn.readlines():
    time.sleep(1)
    timeout -= 1
# Write the command to the balance port and wait for response
while timeout:
    time.sleep(1)
    conn.write(command)
    resp = conn.readlines()

    try:
        resp = float(resp[0])
    except ValueError:
        pattern = r'(.?\d+\.\d+)'
        match = re.findall(pattern, resp[0])
        resp = float(match[0])
    except IndexError:
        continue

    print resp
    print 'timeout: %s' % timeout
    if resp:
        emit_status(signal, status_string[1], str(resp))
        print resp, 'here'
        return resp

    timeout -= 1
conn.close()
print resp
return resp

I start the function manual_short_command in another thread, and this function sends a command through a serial port and waits for a response. It then returns this response and writes it on the status browser (I'm using PyQt4 for the GUI but I think that is irrelevant).

I get an error when I try to assign self.readout = result.get() , saying IndexError: list index out of range , which means that the function hasn't completed in the other thread yet. How do I wait for the thread to finish before I assign the result? My program hangs otherwise. I looked at Threading pool similar to the multiprocessing Pool? for some guidance, but I couldn't find how to synchronize the two threads. Any suggestions? Sorry for the huge block of code.

Answer 1

You can use join() . This will make the program to wait until the thread finish, and then will move next to show the result.