title: TypeErroer: 'NoneType' is not iterable
body: <p>I'm trying to build an autonomous driving car with the Raspberry Pi - Therefore I try to learn from Udacity's Nanodegree examples. </p>
  
  <p>The following Code is from some GitHub repositories and I just changed the code to work with the PI-CAM. Because the Udacity example Codes work all with .mp4 videos.</p>
  
  <p>When I try to run the following code on the Raspberry PI with the Thonny IDE, sometimes it works for a few seconds or a minute and sometimes it won't even start running.</p>
  
  <p>You can see the whole program <a href="https://github.com/wgt-lanedetection/version-1/blob/master/version1_for_PI.py" rel="nofollow noreferrer">here</a>.</p>
  
  <pre><code>def draw_lines(img, lines, thickness=5):
      global rightSlope, leftSlope, rightIntercept, leftIntercept
      rightColor=[0,0,255]
      leftColor=[255,0,0]
  
      #this is used to filter out the outlying lines that can affect the average
      #We then use the slope we determined to find the y-intercept of the filtered lines by solving for b in y=mx+b
      for line in lines:
          for x1,y1,x2,y2 in line:
              slope = (y1-y2)/(x1-x2)
              if slope &gt; 0.3:
                  if x1 &gt; 500 :
                      yintercept = y2 - (slope*x2)                    
                      rightSlope.append(slope)
                      rightIntercept.append(yintercept)
                  else: None                
              elif slope &lt; -0.3:
                  if x1 &lt; 600:
                      yintercept = y2 - (slope*x2)                    
                      leftSlope.append(slope)
                      leftIntercept.append(yintercept)    
  
       ...               
  </code></pre>
  
  <p>lines are defined in this part:</p>
  
  <pre><code>def hough_lines(img, rho, theta, threshold, min_line_len, max_line_gap):
      """
      `img` should be the output of a Canny transform.
      """
      lines = cv2.HoughLinesP(img, rho, theta, threshold, np.array([]), minLineLength=min_line_len, maxLineGap=max_line_gap)
      line_img = np.zeros((img.shape[0], img.shape[1], 3), dtype=np.uint8)
      draw_lines(line_img, lines)
      return line_img
  
  def linedetect(img):
      return hough_lines(img, 1, np.pi/180, 10, 20, 100)
  </code></pre>
  
  <p>This is the error I get when I execute the code :</p>
  
  <pre><code>/usr/local/lib/python3.5/dist-packages/numpy/core/fromnumeric.py:3118: RuntimeWarning: Mean of empty slice.
    out=out, **kwargs)
  /usr/local/lib/python3.5/dist-packages/numpy/core/_methods.py:85: RuntimeWarning: invalid value encountered in double_scalars
    ret = ret.dtype.type(ret / rcount)
  version1_for_PI.py:160: RuntimeWarning: divide by zero encountered in int_scalars
    slope = (y1-y2)/(x1-x2)
  Traceback (most recent call last):
    File "/home/pi/Desktop/version-1/version1_for_PI.py", line 244, in &lt;module&gt;
      myline = hough_lines(canny, 1, np.pi/180, 10, 20, 5)
    File "/home/pi/Desktop/version-1/version1_for_PI.py", line 209, in hough_lines
      draw_lines(line_img, lines)
    File "/home/pi/Desktop/version-1/version1_for_PI.py", line 158, in draw_lines
      for line in lines:
  TypeError: 'NoneType' object is not iterable
  
  </code></pre>