Capturing video from two cameras in OpenCV at once

Question

How do you capture video from two or more cameras at once (or nearly) with OpenCV, using the Python API?

I have three webcams, all capable of video streaming, locate

Answer 1

Adding a little to what @TheoreticallyNick posted earlier:

import cv2
import threading

class camThread(threading.Thread):
    def __init__(self, previewName, camID):
        threading.Thread.__init__(self)
        self.previewName = previewName
        self.camID = camID
    def run(self):
        print("Starting " + self.previewName)
        camPreview(self.previewName, self.camID)

def camPreview(previewName, camID):
    cv2.namedWindow(previewName)
    cam = cv2.VideoCapture(camID)
    if cam.isOpened():
        rval, frame = cam.read()
    else:
        rval = False

    while rval:
        cv2.imshow(previewName, frame)
        rval, frame = cam.read()
        key = cv2.waitKey(20)
        if key == 27:  # exit on ESC
            break
    cv2.destroyWindow(previewName)

# Create threads as follows
thread1 = camThread("Camera 1", 0)
thread2 = camThread("Camera 2", 1)
thread3 = camThread("Camera 3", 2)

thread1.start()
thread2.start()
thread3.start()
print()
print("Active threads", threading.activeCount())

This will open up a new thread for each webcam you have. In my case, I wanted to open up three different feeds. Tested on Python 3.6. Let me know if you have any issues, also thanks to TheoreticallyNick for the readable/functioning code!