When i run this command " cascPath = sys.argv[1] " i get error IndexError: list index out of range
<p>I am using a Raspberry Pi 3 Model B, with Raspbian, opencv 2.x and Python 3 installed.</p>

<p>I want to access my USB Webcam and take a picture with it. 
I've found tons of code but none are of any use. I found one which is better but when I run the command</p>

<pre><code>cascPath = sys.argv[1]
</code></pre>

<p>I get the error </p>

<blockquote>
  <p>Traceback (most recent call last):</p>
  
  <p>File "/home/pi/test.py", line 4, in </p>

<pre><code>cascPath = sys.argv[1]
</code></pre>
  
  <p>IndexError: list index out of range</p>
</blockquote>

<p>I simply need to access my webcam to take a picture.</p>

<hr>

<pre><code>import cv2

import sys

cascPath = sys.argv[1]

faceCascade = cv2.CascadeClassifier(cascPath)

video_capture = cv2.VideoCapture(0)

while True:

    # Capture frame-by-frame
    ret, frame = video_capture.read()

    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    faces = faceCascade.detectMultiScale(
        gray,
        scaleFactor=1.1,
        minNeighbors=5,
        minSize=(30, 30),
        flags=cv2.cv.CV_HAAR_SCALE_IMAGE
    )

    # Draw a rectangle around the faces
    for (x, y, w, h) in faces:
        cv2.rectangle(frame, (x, y), (x+w, y+h), (0, 255, 0), 2)

    # Display the resulting frame
    cv2.imshow('Video', frame)

    if cv2.waitKey(1) &amp; 0xFF == ord('q'):
        break

#When everything is done, release the capture
video_capture.release()

cv2.destroyAllWindows()
</code></pre>

<p>This is the code</p>

<p><code>sys.argv[1]</code> is the first CLI parameter and if you don't provide any CLI parameter, <code>len(sys.argv)=1</code> so you can only access sys.argv[0] which is the script name.</p>

<p>What you need to do is provide a CLI parameter which is for <code>cascPath</code>.</p>

<p>The Cascade is a xml file contains OpenCV data to detect objects which might comes with the tutorial you read / the code snippet you downloaded.</p>

python opencv webcam raspberry-pi3 getpicture