Why do Python modules sometimes not import their s

问题:

I noticed something weird today I would like explained. I wasn't 100% sure how to even phrase this as a question, so google is out of the question. The logging module does not have access to the module logging.handlers for some odd reason. Try it yourself if you don't believe me:

>>> import logging
>>> logging.handlers
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'handlers'
>>> import logging.handlers
>>> logging.handlers
<module 'logging.handlers' from '/usr/lib/python2.6/logging/handlers.pyc'>

Can anyone explain why this happens?

回答1:

In Python, modules need to be imported before they're accessible. import logging imports just the logging module. It so happens that logging is a package with submodules, but those submodules are still not automatically loaded. So, you need to explicitly import logging.handlers before you can access it.

If you're wondering why it looks like sometimes you don't need those extra imports: some packages import some or all of their submodules when they are imported -- simply by doing those imports in their __init__.py files. In other cases it might be that something else that you import, also imported logging.handlers. It doesn't matter which piece of code does the import; as long as something in your process imports logging.handlers before you access it, it'll be there. And sometimes a module that looks like a package really isn't one, like os and os.path. os isn't a package, it just imports the correct other module (for your platform) and calls it path, just so you can access it as os.path.

回答2:

I'm also new to python and after having lots of practice now I can differentiate between, package (folder) , module(.py) , classes,variables...etc...

if you want any of your folder to be python package - It must contain __init__.py file even empty file will do !!!

and as Thomas said, you can import extra module in __init__.py if you want !!! but modules/packages are accessible only after importing it...

if you want to import everything from a module you can use

from logging import *

rest you can access the handlers module like below too,

from logging import handlers
print dir(handlers)

回答3:

I've faced recently the same odd situation. So, I bet you've removed some third-party lib import. That removed lib contained from logging import handlers or from logging import * and provided you handlers. And in other script you've had something like import logging and just used logging.handlers and you've thought that is a way things work as I did.