Check if image is black

Question

I'm running a Python script under Windows which deals with 480x800 PNG images with 32-bit depth. I need to check if the given image is fully black or not. After some searching I've found that ImageMagick could help me to achieve this but unfortunately there's no manual for such task.

So a more general question is how to check if the image consists only of one color?

Edit:

My apologies for not providing all the information about the environment from beginning. The python script is executed using Android's monkeyrunner. Since it uses it's own instance of Jython (version 2.5) it's not possible to use any modules from external libraries. Inside there's a MonkeyImage class to work with screenshots taken from the device. So I adopted @eumiro's answer for it.

Answer 1

import Image
im = Image.load("image.png")
diff_colors = list(set(im.getdata()))
if len(diff_colors) == 1 and diff_colors[0] == (0, 0, 0):
    print "all black"

EDIT as @JonClements proposes, this will be faster and stop as soon as anything else than black is found:

import Image
im = Image.load("image.png")
if all(rgb == (0,0,0) for rgb in im.getdata()):
    print "all black"