why this code loop forever after a draw a rectangle

[MRAB]

On 2016-09-16 20:14, Gary Herron wrote:
> On 09/16/2016 04:24 AM, meInvent bbird wrote:
>> im = img.copy()
>> cntcounter = 0
>> for cnt in contours:
>>          epsilon = 0.1*cv2.arcLength(cnt,True)
>>          approx = cv2.approxPolyDP(cnt,epsilon,True)	
>>          #peri = cv2.arcLength(cnt, True)
>>          #approx = cv2.approxPolyDP(c, 0.5 * peri, True)
>>          #print("len(approx)="+str(len(approx)))
>>          if len(approx) == 4:
>>              print("approx=" + str(approx))
>>              cntcounter = cntcounter + 1
>>              print("here1")
>>              x,y,w,h = cv2.boundingRect(cnt)
>>              print("here2")
>>              while im is None:
>>                  time.sleep(1)
>>              if im is not None:
>>                  print("here3")
>>                  im = cv2.rectangle(im.copy(), (x,y), (x+w, y+h), (0,255,0), 2)
>>                  #im = cv2.line(im,(x,y),(x+w,y),(0,255,0),2)
>>                  #im = cv2.line(im,(x+w,y),(x+w,y+h),(0,255,0),2)
>>                  #im = cv2.line(im,(x,y+h),(x+w,y+h),(0,255,0),2)
>>                  #im = cv2.line(im,(x,y),(x,y+h),(0,255,0),2)
>>
>>
>> cv2.imwrite(r'C:\Users\tester\Documents\masda.png',im)
>
>
> These two lines:
>
>    while im is None:
>      time.sleep(1)
>
> are an infinite loop if im is None;
>
>
> Since you haven't told us what im (or img, contours, cv2) are, I can't
> tell how im might become None, but it does look like you (confusingly)
> use im for two different things:  an img.copy() and a cv2.rectangle,
> whatever those may be.
>
> Pure guesswork:  if cv2.rectangle draws a rectangle, what does it
> return?  If it doesn't return anything, the line
>      im = cv2.rectangle(...)
> is how im gets the value of None.
>
It looks like the OP is using OpenCV.

You're right about cv2.rectangle; it does return None.

The line:

     im = cv2.rectangle(im.copy(), (x,y), (x+w, y+h), (0,255,0), 2)

makes a copy of the image im, draws a rectangle on it, and then binds 
None to im.

The copied rectangle is discarded because there's no reference to it, so 
the entire line in pointless.

It basically does the same thing as:

     im = None

only slower!