import download_file

from PIL import Image

def main():

url = "http://www.pythonchallenge.com/pc/ring/bell.png"

file_path = "ring/bell.png"

username = "repeat"

password = "switch"

# first_step(url, file_path, username, password)

second_step(file_path)

def first_step(url, file_path, username, password):

download_file.download_with_auth(url, file_path, username, password)

def second_step(file_path):

im = Image.open(file_path)

green_channel = 1

green_data = im.getdata(green_channel)

datas = list(green_data)

datas_len = len(datas)

assert datas_len % 2 == 0

message = ""

for i in range(int(datas_len / 2)):

index = 2 * i

left = datas[index]

right = datas[index + 1]

abs_left_min_right = abs(left - right)

if abs_left_min_right != 42:

message += chr(abs_left_min_right)

print(message)

if __name__ == "__main__":

main()

result: http://www.pythonchallenge.com/pc/ring/guido.html

- use image.split() to split RGB channels

- im.bands() return number of im's channels

- ring means "green" if you visit "green.html" will get response "yes"

- whodunnit is an word means "who is the murderer"