Question

I have something like this:

 # a.py  
 import os
 class A:
   ...

 # b.py
 import a
 class B(A):
   ...

In class B (b.py) I’d like to be able to use the modules imported in a.py (os in this case). Is it possible to achieve this behavior in Python or should I import the modules in both files?

Edit: I’m not worried about the import times, my problem is the visual clutter that the block of imports puts on the files. I end up having stuff like this in every controller (RequestHandler):

 from django.utils import simplejson
 from google.appengine.ext import webapp
 from google.appengine.ext.webapp import template
 from google.appengine.ext import db

That’s what I’d like to avoid.

Answer 1

Yes you can use the imports from the other file by going a.os.

However, the pythonic way is to just import the exact modules you need without making a chain out of it (which can lead to circular references).

When you import a module, the code is compiled and inserted into a dictionary of names -> module objects. The dictionary is located at sys.modules.

import sys
sys.modules

>>> pprint.pprint(sys.modules)
{'UserDict': <module 'UserDict' from 'C:\python26\lib\UserDict.pyc'>,
 '__builtin__': <module '__builtin__' (built-in)>,
 '__main__': <module '__main__' (built-in)>,
 '_abcoll': <module '_abcoll' from 'C:\python26\lib\_abcoll.pyc'>,
# the rest omitted for brevity

When you try to import the module again, Python will check the dictionary to see if its already there. If it is, it will return the already compiled module object to you. Otherwise, it will compile the code, and insert it in sys.modules.

Since dictionaries are implemented as hash tables, this lookup is very quick and takes up negligible time compared to the risk of creating circular references.

Edit: I’m not worried about the import
times, my problem is the visual
clutter that the block of imports puts
on the files.

If you only have about 4 or 5 imports like that, its not too cluttery. Remember, “Explicit is better than implicit”. However if it really bothers you that much, do this:

<importheaders.py>
from django.utils import simplejson
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template
from google.appengine.ext import db


<mycontroller.py>
from importheaders import *