cv2.imread: checking if image is being read

If you're sure that the value of img is None in your case, you can simply use if not img is None, or, equivalently, if img is not None. You don't need to check the type explicitly.

Note that None and False are not the same value. However, bool(None)==False, which is why if None fails.

The documentation for imread, both for OpenCV 2 and 3, states, however, that a empty matrix should be returned on error. You can check for that using if img.size ==0

from documentation, we may use

retval  =   cv.haveImageReader (filename)

source https://docs.opencv.org/master/d4/da8/group__imgcodecs.html

If you want to write the contents as soon as the image file is being generated then you can use os.path.isfile() which return a bool value depending upon the presence of a file in the given directory.

import cv2 
import os.path

while not os.path.isfile("myImage.jpg"):
    #ignore if no such file is present.
    pass

img = cv2.imread("myImage.jpg", 0)

cv2.imwrite("result.jpg", img)

You can also refer to docs for detailed implementation of each method and basic image operations.