How do I increase the contrast of an image in Python OpenCV

I am new to Python OpenCV. I have read some documents and answers here but I am unable to figure out what the following code means:

if (self.array_alpha is None):
    self.array_alpha = np.array([1.25])
    self.array_beta = np.array([-100.0])

# add a beta value to every pixel 
cv2.add(new_img, self.array_beta, new_img)                    

# multiply every pixel value by alpha
cv2.multiply(new_img, self.array_alpha, new_img)  

I have come to know that Basically, every pixel can be transformed as X = aY + b where a and b are scalars.. Basically, I have understood this. However, I did not understand the code and how to increase contrast with this.

Till now, I have managed to simply read the image using img = cv2.imread('image.jpg',0)

Thanks for your help

from How do I increase the contrast of an image in Python OpenCV