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

I want to connect to a camera, and only capture a frame when an event happens (e.g. keypress). A simplified version of what I'd like to do is this:

cap = cv2.VideoCapture(device_id)

while True:
    if event:
        img = cap.read()
        preprocess(img)

    process(img)
    cv.Waitkey(10)

However, cap.read seems to only capture the next frame in the queue, and not the latest. I did a lot of searching online, and there seems to be a lot of questions on this but no definitive answer. Only some dirty hacks which involve opening and closing the capture device just before and after grabbing (which won't work for me as my event might be triggered multiple times per second); or assuming a fixed framerate and reading a fixed-n times on each event (which won't work for me as my event is unpredictable and could happen at any interval).

A nice solution would be:

while True:
    if event:
        while capture_has_frames:
            img = cap.read()
        preprocess(img)

    process(img)
    cv.Waitkey(10)

But what is capture_has_frames? Is it possible to get that info? I tried looking into CV_CAP_PROP_POS_FRAMES but it's always -1.

For now I have a separate thread where the capture is running at full fps, and on my event I'm grabbing the latest image from that thread, but this seems overkill.

(I'm on Ubuntu 16.04 btw, but I guess it shouldn't matter. I'm also using pyqtgraph for display)

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)