As far as I know, correct me if I am wrong.
First, import a.b must import a module(a file) or a package(a directory contains __init__.py).
For example, you can import tornado.web as web but you cannot import flask.Flask as Flask as Flask is an object in package flask.
Second, import a.b also import the namespace a which from a import b won't. You can check it by globals().
So what's the influence? For example:
import tornado.web as web
Now you have access to namespace tornado, but you cannot access tornado.options even though tornado has this module. But as python's global package management, if you from tornado import options, you will not only get access to options but also add it to namespace tornado. So now you can also access options by tornado.options.
