Python3 import error

Your problem is that by running mad.py from the module_test directory, you guessed that Python would use your current directory as base for finding module b. To prove that it doesn't work, do the following:

1. Edit your mad.py script and add the following to the beggining of the script:

   import sys
   print(sys.path)
   exit()
   

   This will just print the search path Python uses to find modules you try to import and then exit.

2. Open the terminal, go to folder /module_test and run the following:

   > python lib/mad.py
   

   Now you should see the path dumped to the terminal. Note that the first entry in the list is the folder 'module_test/lib' not 'module_test' as you would have thought.

So, how do you solve the problem?

Simple: you just have to tell Python the correct directory it should be using to look for your modules. To do that, you must set the PYTHONPATH enviroment variable:

> export PYTHONPATH=$PYTHONPATH:/path/to/module_test

Now if you try to run your script, it should import a.hello as expected!

PS: You should read the official Python tutorial, specially chapter 6 which explains the concept of modules and packages:

http://docs.python.org/3/tutorial/modules.html

That should give you a basic understading and help you solve your problem.