You can use python -V (et al.) to show you the version of Python that the python command resolves to. If that's all you need, you're done. But to see every version of python in your system takes a bit more.

In Ubuntu we can check the resolution with readlink -f $(which python). In default cases in 14.04 this will simply point to /usr/bin/python2.7.

We can chain this in to show the version of that version of Python:

$ readlink -f $(which python) | xargs -I % sh -c 'echo -n "%: "; % -V'
/usr/bin/python2.7: Python 2.7.6

But this is still only telling us what our current python resolution is. If we were in a Virtualenv (a common Python stack management system) python might resolve to a different version:

$ readlink -f $(which python) | xargs -I % sh -c 'echo -n "%: "; % -V'
/home/oli/venv/bin/python: Python 2.7.4

This is real output.

The fact is there could be hundreds of different versions of Python secreted around your system, either on paths that are contextually added, or living under different binary names (like python3).

If we assume that a Python binary is always going to be called python<something> and be a binary file, we can just search the entire system for files that match those criteria:

$ sudo find / -type f -executable -iname 'python*' -exec file -i '{}' \; | awk -F: '/x-executable; charset=binary/ {print $1}' | xargs readlink -f | sort -u | xargs -I % sh -c 'echo -n "%: "; % -V'
/home/oli/venv/bin/python: Python 2.7.4
/media/ned/websites/venvold/bin/python: Python 2.7.4
/srv/chroot/precise_i386/usr/bin/python2.7: Python 2.7.3
/srv/chroot/trusty_i386/usr/bin/python2.7: Python 2.7.6
/srv/chroot/trusty_i386/usr/bin/python3.4: Python 3.4.0
/srv/chroot/trusty_i386/usr/bin/python3.4m: Python 3.4.0
/usr/bin/python2.7: Python 2.7.6
/usr/bin/python2.7-dbg: Python 2.7.6
/usr/bin/python3.4: Python 3.4.0
/usr/bin/python3.4dm: Python 3.4.0
/usr/bin/python3.4m: Python 3.4.0
/web/venvold/bin/python: Python 2.7.4

It's obviously a pretty hideous command but this is again real output and it seems to have done a fairly thorough job.

To check the version of one's Python's Software version, one should use the following code in command prompt:

python -V

Reference: http://docs.python.org/using/cmdline.html#generic-options

This information is available in the sys.version string in the sys module:

>>> import sys

Human readable:

>>> print(sys.version)  # parentheses necessary in python 3.       
2.5.2 (r252:60911, Jul 31 2008, 17:28:52) 
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)]

For further processing, use sys.version_info or sys.hexversion:

>>> sys.version_info
(2, 5, 2, 'final', 0)
# or
>>> sys.hexversion
34014192

To ensure a script runs with a minimal version requirement of the Python interpreter add this to your code:

assert sys.version_info >= (2, 5)

This compares major and minor version information. Add micro (=0, 1, etc) and even releaselevel (='alpha','final', etc) to the tuple as you like. Note however, that it is almost always better to "duck" check if a certain feature is there, and if not, workaround (or bail out). Sometimes features go away in newer releases, being replaced by others.

The more easy way its by executing the next command:

ls -ls /usr/bin/python*

Output look like this:

/usr/bin/python           /usr/bin/python2.7        /usr/bin/pythonw
/usr/bin/python-config    /usr/bin/python2.7-config /usr/bin/pythonw2.7