Very small issue about grammar.

[semi-nothing]

$print "XXX"$ and $print("XXX")$ are all used in the program. I think it is better to keep consistent in python 2.7 grammar or python 3.5 grammar.

[lanwuwei]

Thanks for your suggestions :) I am shifting from python 2 to python 3 currently and making this kind of inconsistent problem. I will pay attention to it in the future!