1. PYTHONPATH is an environment variable which you can set to add additional directories where python will look for modules and packages. e.g.:

   # make python look in the foo subdirectory of your home directory for   
   # modules and packages 
   export PYTHONPATH=${PYTHONPATH}:${HOME}/foo 
   

   Here I use the sh syntax. For other shells (e.g. csh,tcsh), the syntax would be slightly different. To make it permanent, set the variable in your shell's init file (usually ~/.bashrc).

2. Ubuntu comes with python already installed. There may be reasons for installing other (independent) python versions, but I've found that to be rarely necessary.

3. The folder where your modules live is dependent on PYTHONPATH and where the directories were set up when python was installed. For the most part, the installed stuff you shouldn't care about where it lives -- Python knows where it is and it can find the modules. Sort of like issuing the command ls -- where does ls live? /usr/bin? /bin? 99% of the time, you don't need to care -- Just use ls and be happy that it lives somewhere on your PATH so the shell can find it.

4. I'm not sure I understand the question. 3rd party modules usually come with install instructions. If you follow the instructions, python should be able to find the module and you shouldn't have to care about where it got installed.

5. Configure PYTHONPATH to include the directory where your module resides and python will be able to find your module.

I'm assuming that when you installed anaconda 2, you manually set the PYTHONPATH environment variable, by putting something like

PYTHONPATH=/home/user/anaconda2/python
export PYTHONPATH

in your .bash_profile or .bash_rc.

But since you deleted the /home/user/anacanda2/ directory, that path no longer exists.

Thus you want to change PYTHONPATH to point to the executable in /usr/lib, by changing the above to

PYTHONPATH=/usr/lib/my_python_distribution
export PYTHON

Rather than hardcoding paths in your Python script we should make use of the path operation from the module os.

os.path.expanduser(path) expands the path to the user's home directory
os.path.join(path1,*path2*,...) joins path elements with the appropriate separator
os.sep gives the OS dependent path separator (/ for Linux/Unix, \ for Windows)
os.getcwd() gives the current working directory
os.path.abspath(path) gives the OS dependent absolute path of a given path

Example:

>>>import os
>>>path = os.path.join(os.path.expanduser('~'), 'documents', 'python', 'file.txt')
>>>print (path)

Result

/home/user/documents/python/file.txt ## when on Ubuntu
C:\Users\user\documents\python\file.txt ## when running Windows

First question:

which python though its usually /usr/bin/python for the 2.7

Second question:

From a terminal & python2.7: python2.7 yourfile.py.
Simailarly for 3.2: python3.2 yourfile.py though 3.2 isn't installed by default. (You can apt-get install python3.2.)

What python yourfile.py will do depends on which alternative is used for your python interpreter. You can change that by issuing update-alternatives python as root (or by using su).

Third question:

Environment variables are shell dependent, though you can write them out with echo $variable and set them with variable=value (from bash). The search path is simply called PATH and you can get yours by typing echo $PATH.

I hope this was helpful.

The environment variable PYTHONPATH is actually only added to the list of locations Python searches for modules. You can print out the full list in the terminal like this:

python -c "import sys; print(sys.path)"

Or if want the output in the UNIX directory list style (separated by :) you can do this:

python -c "import sys; print(':'.join(x for x in sys.path if x))"

Which will output something like this:

/usr/local/lib/python2.7/dist-packages/feedparser-5.1.3-py2.7.egg:/usr/local/lib/
python2.7/dist-packages/stripogram-1.5-py2.7.egg:/home/qiime/lib:/home/debian:/us
r/lib/python2.7:/usr/lib/python2.7/plat-linux2:/usr/lib/python2.7/lib-tk:/usr/lib
/python2.7/lib-old:/usr/lib/python2.7/lib- dynload:/usr/local/lib/python2.7/dist-
packages:/usr/lib/python2.7/dist-packages:/usr/lib/python2.7/dist-packages/PIL:/u
sr/lib/python2.7/dist-packages/gst-0.10:/usr/lib/python2.7/dist-packages/gtk-2.0:
/usr/lib/pymodules/python2.7

This

import glob, os
glob.glob(os.path.expanduser('~/.mozilla/firefox/*.default'))

will give you a list of all files ending in ".default" in the current user's ~/.mozilla/firefox directory using os.path.expanduser to expand the ~ in the path and glob.glob to match the *.default file pattern.