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

You would probably also want this:

import sys
print(sys.path)

Or as a one liner from the terminal:

python -c "import sys; print('\n'.join(sys.path))"

Caveat: If you have multiple versions of Python installed you should use a corresponding command python2 or python3.

You can type which python on the ubuntu terminal and it will give the Python installed location path.

Typically, the environment variable $PYTHONPATH is empty (try echo $PYTHONPATH). The actual list of folders python searches for libraries can be found with (in python):

import sys
print(sys.path)

This will consist (in search order) of the current directory, any directories in your $PYTHONPATH, and finally the default library directories, set by site.py. The main default locations are (where X.Y is the python version, eg 2.7):

/usr/lib/pythonX.Y (python system libraries, eg re, urllib)
/usr/lib/pythonX.Y/dist-packages (python libraries installed with deb packages)

If you use pip install --user or similar to install libraries as yourself, it will also contain the user library directory:

/home/USERNAME/.local/lib/pythonX.Y/site-packages

You can manipulate the PYTHONPATH by either setting the environment variable before you launch python (PYTHONPATH=$PYTHONPATH:/foo/bar), or by editing sys.path once you've started python (import sys; sys.path = ["/foo/bar"] + sys.path).

However, if you want to play with your own python libraries, a good idea is to create a virtualenv. This is a directory in which you can play around with your own versions of python libraries without any risk of messing up the python libraries used by the system. See How to set up and use a virtual python environment in Ubuntu? for information about creating and using a virtualenv.

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.

Try appending to PYTHONPATH instead of overwriting it completely.

export PYTHONPATH=$PYTHONPATH:/home/dev/python-files

References:

According to the the Python documentation on PYTHONPATH

Augment the default search path for module files. [...]

The default search path is installation dependent, but generally begins with prefix/lib/pythonversion (see PYTHONHOME above). It is always appended to PYTHONPATH.

meaning that some values exist in PYTHONPATH and the default search path is also only appended.

Additionally, this blog post (Archive.org link) also explains clearly why you need to append to PYTHONPATH and not overwrite it. Scrolling down to the section - Special cases and examining the search path explains it clearly (unfortunately no relative URL to that link so you'll have to scroll). Although the user gives the examples on a mac they are very much relevant to any platform