Run as long as Face is detected

I have the code for live face detection:

import cv2


class FaceRec:
    # Enable camera
    cap = cv2.VideoCapture(0)
    cap.set(3, 640)
    cap.set(4, 420)

    # import cascade file for facial recognition
    faceCascade = cv2.CascadeClassifier(cv2.data.haarcascades + "haarcascade_frontalface_default.xml")

    while True:
        success, img = cap.read()
        imgGray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        # Getting corners around the face
        faces = faceCascade.detectMultiScale(imgGray, 1.3, 5)  # 1.3 = scale factor, 5 = minimum neighbor
        # drawing bounding box around face
        for (x, y, w, h) in faces:
            img = cv2.rectangle(img, (x, y), (x + w, y + h), (0, 255, 0), 3)

        cv2.imshow('face_detect', img)
        if cv2.waitKey(10) & 0xFF == ord('q'):
            break
    cap.release()
    cv2.destroyWindow('face_detect')

This code uses the webcam to detect and draw a box around a face when it enters the frame. But I want it to run a code block once a face is detected. How would I go about doing so?

I tried to incorporate a while loop but the code in the loop ran before a face was detected when I want it to run ONLY when a face is detected.

The opencv documentation on cv2.CascadeClassifier.detectMultiScale states that the function :

Detects objects of different sizes in the input image. The detected objects are returned as a list of rectangles.

Thus when no faces are detected, the list of rectangles returned is empty.

Your code can be as simple as

while True:

    ...

    faces = faceCascade.detectMultiScale(imgGray, 1.3, 5)

    if faces : # check if faces is not empty
        ... # do stuff

Obviously keep in mind that the test will have to be executed each loop to assert if there are still faces in front of the webcam or not.