output.avi file created but its size is 0k

edit

I have faced the same issue, the reason is the application(.exe) folder should contain the ffmpeg dll depending on your OpenCV version in order to write the videofile. If the dll is not present there, the video file written will be of 0KB. Kindly check whether your application folder contains that dll.

For example, opencv_ffmpeg310.dll for OpenCV 3.1.0

From this answer, frame size used while initializing videoWriter and the size of the frame being written to the file should be the same.

Resize "frame" before writing it

import numpy
import cv2

cap = cv2.VideoCapture(0)
fourcc = cv2.cv.CV_FOURCC(*'MJPG')
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (640,480))

while(cap.isOpened()):
    ret, frameTmp = cap.read()
    if ret==True:
        frame = cv2.resize(frameTmp, (640, 480))
        frame = cv2.flip(frame,1)

        # write the flipped frame
        out.write(frame)
        cv2.imshow('frame',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

cap.release()
out.release()
cv2.destroyAllWindows()

Get frame size from videoCapture to initialize videoWriter.

import numpy
import cv2

cap = cv2.VideoCapture(0)
width = cap.get(cv2.cv.CV_CAP_PROP_FRAME_WIDTH)
height = cap.get(cv2.cv.CV_CAP_PROP_FRAME_HEIGHT)
fourcc = cv2.cv.CV_FOURCC(*'MJPG')
out = cv2.VideoWriter('output.avi', fourcc, 20.0, (int(width),int(height)))

while(cap.isOpened()):
    ret, frame = cap.read()
    if ret==True:
        frame = cv2.flip(frame,1)

        # write the flipped frame
        out.write(frame)
        cv2.imshow('frame',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
    else:
        break

cap.release()
out.release()
cv2.destroyAllWindows()

Update 1: Added code snippets