为什么没有保存图片？

Question

import cv2
cam = cv2.VideoCapture(0)
detector = cv2.CascadeClassifier('/usr/local/Cellar/opencv/3.4.2/share/OpenCV/haarcascades/haarcascade_frontalface_default.xml')

Id = input('enter your id')
sampleNum = 0
while(True):
    ret, img = cam.read()
    gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
    faces = detector.detectMultiScale(gray, 1.3, 5)
    for (x, y, w, h) in faces:
        cv2.rectangle(img, (x, y), (x + w, y + h), (255, 0, 0), 2)

        # incrementing sample number
        sampleNum = sampleNum + 1
        # saving the captured face in the dataset folder
        cv2.imwrite("dataSet/User." + Id + '.' + str(sampleNum) + ".jpg", gray[y:y + h, x:x + w])

        cv2.imshow('frame', img)
    # wait for 100 miliseconds
    if cv2.waitKey(100) & 0xFF == ord('q'):
        break
    # break if the sample number is morethan 20
    elif sampleNum > 20:
        break
cam.release()
cv2.destroyAllWindows()

cv2.imwrite应该将捕获的图像保存在名为dataSet的文件夹中，但不会捕获任何内容。这里有什么问题？谢谢