What happens when import modules in python?

问题:

I want to know really what happens when we import a module file in python.I mean it's process, in other words what things by python will be run or check?! like __init__.py or sys.modules and etc. for example i know __init__.py are necessary files in every package,i want to know python what does with these files on import time? please light this for me.

回答1:

Read the tutorial section about modules, the documentation of the import statement, the imp module (particularly the examples) and maybe the docs for the __import__ builtin. That should get you a long way. If you still want to know more, I'd suggest to ask a specific question, this one is a bit on the broad side.

Edit: After reading your question once more, there is a specific part to your question, about what __init__.py does in packages. It basically can be empty or contain initialization code that will be executed when that package is imported. See the section about packages for details.

In an __init__.py you could also set __all__, which defines what symbols get imported when you do from yourpackage import *. This is explained in detail in importing * from a package.