Running interacting functions simultaneously in python

Question

So I'm trying to build a robot that can drive autonomously. For that I need the robot to drive forward and check distance at the same time. And if distance is less than preferred distance, stop forward movement. By now I've written this code below, but it doesn't seem to run simultaneously and they also don't interact. How can I make it that these two functions do indeed interact. If there's anymore information needed I'm happy to supply you. Thanks!

from multiprocessing import Process
from TestS import distance
import Robot
import time

constant1 = True
min_distance = 15

def forward():
    global constant1:
    robot.forward(150)                       #forward movement, speed 150
    time.sleep(2)

def distance_check():
    global constant1
    while constant1:
        distance()                           #checking distance
        dist = distance()
        return dist
        time.sleep(0.3)

        if dist < min_distance:
            constant1 = False
            print 'Something in the way!'
            break

def autonomy():                              #autonomous movement
    while True:
        p1 = Process(target=forward)         
        p2 = Process(target=distance_check)
        p1.start()                           #start up 2 processes
        p2.start()
        p2.join()                            #wait for p2 to finish

Answer 1

So, there's some serious problems with the code you posted. First, you don't want the distance_check process to finish, because it's running a while loop. You should not do p2.join(), nor should you be starting new processes all the time in your while loop. You're mixing too many ways of doing things here - either the two children run forever, or they each run once, not a mix.

However, the main problem is that the original processes can't communicate with the original process, even via global (unless you do some more work). Threads are much more suited to this problem.

You also have a return inside your distance_check() function, so no code below that statement gets executed (including the sleep, and the setting of constant1 (which should really have a better name).

In summary, I think you want something like this:

from threading import Thread
from TestS import distance
import Robot
import time

can_move_forward = True
min_distance = 15


def move_forward():
    global can_move_forward
    while can_move_forward:
        robot.forward(150)
        time.sleep(2)
        print('Moving forward for two seconds!')


def check_distance():
    global can_move_forward
    while True:
        if distance() < min_distance:
            can_move_forward = False
            print('Something in the way! Checking again in 0.3 seconds!')
        time.sleep(0.3)


def move_forward_and_check_distance():
    p1 = Thread(target = move_forward)
    p2 = Thread(target = check_distance)
    p1.start()
    p2.start()

Since you specified python-3.x in your tags, I've also corrected your print.

Obviously I can't check that this will work as you want it to because I don't have your robot, but I hope that this is at least somewhat helpful.