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I am using OpenCV 4.0, using raspberry pi 3. Do not put outside of while condition block. And do not used this else: break. Because it will disappeared window. The correct code like this:

import cv2

cap = cv2.VideoCapture("C:/Users/user/Desktop/Foot_Detection/ball_tracking_example.mp4")
def rescale_frame(frame, percent=30):
    width = int(frame.shape[1] * percent/ 100)
    height = int(frame.shape[0] * percent/ 100)
    dim = (width, height)
    return cv2.resize(frame, dim, interpolation =cv2.INTER_AREA)
if (cap.isOpened() == False):
    print("Error opening video stream or file")
while (cap.isOpened()):
    # Capture frame-by-frame
    ret, frame = cap.read()
    frame = rescale_frame(frame, percent=30)
    if ret == True:
        cv2.imshow('Frame', frame)
        if cv2.waitKey(25) & 0xFF == ord('q'):
            break


cap.release()
cv2.destroyAllWindows()

I am using OpenCV 4.0, 4.0.1, using raspberry pi 3. Do not put outside of while condition block. And do not used this else: break. Because it will disappeared window. The correct code like this:

import cv2

cap = cv2.VideoCapture("C:/Users/user/Desktop/Foot_Detection/ball_tracking_example.mp4")
def rescale_frame(frame, percent=30):
    width = int(frame.shape[1] * percent/ 100)
    height = int(frame.shape[0] * percent/ 100)
    dim = (width, height)
    return cv2.resize(frame, dim, interpolation =cv2.INTER_AREA)
if (cap.isOpened() == False):
    print("Error opening video stream or file")
while (cap.isOpened()):
    # Capture frame-by-frame
    ret, frame = cap.read()
    frame = rescale_frame(frame, percent=30)
    if ret == True:
        cv2.imshow('Frame', frame)
        if cv2.waitKey(25) & 0xFF == ord('q'):
            break


cap.release()
cv2.destroyAllWindows()