How to get the latest frame from capture device (camera) in opencv

I think the solution mentioned in the question, namely having a separate thread that clears the buffer, is the easiest non-brittle solution for this. Here reasonably nice (I think) code for this:

import cv2, queue, threading, time

# bufferless VideoCapture
class VideoCapture:

  def __init__(self, name):
    self.cap = cv2.VideoCapture(name)
    self.q = queue.Queue()
    t = threading.Thread(target=self._reader)
    t.daemon = True
    t.start()

  # read frames as soon as they are available, keeping only most recent one
  def _reader(self):
    while True:
      ret, frame = self.cap.read()
      if not ret:
        break
      if not self.q.empty():
        try:
          self.q.get_nowait()   # discard previous (unprocessed) frame
        except queue.Empty:
          pass
      self.q.put(frame)

  def read(self):
    return self.q.get()

cap = VideoCapture(0)
while True:
  time.sleep(.5)   # simulate time between events
  frame = cap.read()
  cv2.imshow("frame", frame)
  if chr(cv2.waitKey(1)&255) == 'q':
    break

The frame reader thread is encapsulated inside the custom VideoCapture class, and communication with the main thread is via a queue.

I posted very similar code for a node.js question, where a JavaScript solution would have been better. My comments on another answer to that question give details why a non-brittle solution without separate thread seems difficult.

An alternative solution that is easier but supported only for some OpenCV backends is using CAP_PROP_BUFFERSIZE. The 2.4 docs state it is “only supported by DC1394 [Firewire] v 2.x backend currently.” For Linux backend V4L, according to a comment in the 3.4.5 code, support was added on 9 Mar 2018, but I got VIDEOIO ERROR: V4L: Property <unknown property string>(38) not supported by device for exactly this backend. It may be worth a try first; the code is as easy as this:

cap.set(cv2.CAP_PROP_BUFFERSIZE, 0)