gray.py

def grayscale(img):
    """Applies the Grayscale transform
    This will return an image with only one color channel
    but NOTE: to see the returned image as grayscale
    you should call plt.imshow(gray, cmap='gray')"""
    return cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

image = mpimg.imread('test_images/solidYellowCurve2.jpg')
# grayscale the image
grayscaled = grayscale(image)
plt.imshow(grayscaled, cmap='gray')