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Other than swapping order of paths to python folders in system variable 'PATH' - what are the ways to swap python versions?

I can change the order to python2,3 folders in the system PATH variable. But what are other ways to do this?
There should be more elegant way to change versions of python i want to run.
e.g. in console:
python file.py #will run python2
and after i change python command to use python3, it should be the same:
python file.py #will use python3

I suppose you are trying to run your script with the correct interpreter depending on which python version was used. On Unix/Linux this is done with a so called “shebang” which is written in the very first line of the file. E.g.:
#!/usr/bin/env python3.6
If your installation of Python3 is newer than Python 3.3 you can use the python launcher for windows, which should be able to launch the correct version of Python depending on the shebang, even on window.
Also see here for more informations on shebangs.

If your concern is what Python version is executed when calling python in a console, then an alias or a stub script are the two ways to go.
This post will explain you how you can do this on Windows.
The alias way, just like it would be on a Unix system, is to create an alias, either temporary to the session or permanent, so that python now means C:\Python27\python, or whatever version you want.
The script approach consists in putting a script named python in a directory referred to in your PATH, and have that script run the right version of Python.
I highly doubt that this will affect all the batch scripts that call python, but it will definitely fire the right Python when you'll type python in a console.
Now, if you're concerned about what version a script is executed with, you can specify an explicit version with a shebang line, or manually select it by right-clicking the .py file and clicking open with.

under what application does dev_appserver.py runs?

I'm not able to understand how
google-cloud-sdk/bin/dev_appserver.py helloworld runs automatically.
I mean to ask that it should run under python command as it is a python file.
is there something added to path??

Yes, dev_appserver.py is an executable python script. If you launch it by itself it will be executed under whichever python is the default one in your environment, see its 1st line:
#!/usr/bin/env python
If you want you can also execute it with a specific python version (it has to be a python 2.7 one, though, the only one supported by GAE standard environment):
/specific_path/python /path_to/google-cloud-sdk/bin/dev_appserver.py helloworld

Run a python script from bamboo

I'm trying to run a python script from bamboo. I created a script task and wrote inline "python myFile.py". Should I be listing the full path for python?
I changed the working directory to the location of myFile.py so that is not a problem. Is there anything else I need to do within the configuration plan to properly run this script? It isn't running but I know it should be running because the script works fine from terminal on my local machine. Thanks

I run a lot of python tasks from bamboo, so it is possible. Using the Script task is generally painless...
You should be able to use your script task to run the commands directly and have stdout written to the logs. Since this is true, you can run:
'which python' -- Output the path of which python that is being ran.
'pip list' -- Output a list of which modules are installed with pip.
You should verify that the output from the above commands matches the output when ran from the server. I'm guessing they won't match up and once that is addressed, everything will work fine.
If not, comment back and we can look at a few other things.
For the future, there are a handful of different ways you can package things with python which could assist with this problem (e.g. automatically installing missing modules, etc).

You can also use the Script Task directly with an inline Python script to run your myFile.py:
/usr/bin/python <<EOF
print "Hello, World!"
EOF
Check this page for a more complex example:
https://www.langhornweb.com/display/BAT/Run+Python+script+as+a+Bamboo+task?desktop=true&macroName=seo-metadata

Running a python program after setting the execute bit in Python 3

I was going through Google's Python instructions and it quotes:
The commands above are the simplest way to run python programs. If the
"execute bit" is set on a .py file, it can be run by name without
having to type "python" first. Set the execute bit with the "chmod"
command like this:
Where it proceeds to give me this code to run on my terminal:
~/google-python-exercises$ chmod +x hello.py
~/google-python-exercises$ ./hello.py ## now can run it as ./hello.py
Hello World
However when I type in ./hello.py, I get:
So is this something to do with me using Python 3 when this could be a command for Python 2?
EDIT
I just realised that this can only be run on a Linux/Mac. How does one go about doing this on a Windows?

You need to associate the file extension with Python properly. This should happen by default if you install system wide (which requires administrator privileges), but if you're using a third party distribution, it's not guaranteed, and it may require a reboot even so.
For more details running Python on Windows, see the Python on Windows docs. It looks like if you omitted the launcher from your Python install, it won't have registered the file extensions; it may be worth reinstalling properly to fix that.
Otherwise, if you can't get file extension associations working, just run:
python hello.py
or starting with 3.3 and later, you can use:
py hello.py
which lets you provide an argument to py.exe to change which installed version of Python to use (if desired).
You wouldn't use ./hello.py on Windows at all, because that's the wrong directory separator (Windows sometimes allows forward slashes, but in DOS prompts, that looks like trying to launch a program named . with the switch /hello.py). You'd use plain hello.py (Windows always searches the current working directory) or .\hello.py if you want to be explicit.

Do you have a shebang as the first line in hello.py?
something like:
#!/usr/bin/env python3

Python - running a script as a service with a name other than 'python'

I have a set of python scripts which I run as a daemon services. These all work great, but when all the scripts are running and I use top -u <USER>, I see all my scripts running as python.
I would really like to know which script is running under which process id. So is there any way to execute a python script as a different process name?
I'm stuck here, and I'm not ever sure what terms to Google. :-)
Note: I'm using Ubuntu Linux. Not sure if the OS matters or not.

Try using setproctitle. It should work fine on Linux.

Don't have a linux system here to test this on appropriately, but if the above doesn't work, you should be able to use the same trick they use for things like gzip etc.
The script has to tell what to run it at the top like this:
#!/usr/local/bin/python
Use a softlink like this:
ln -s /usr/local/bin/python ~/bin/myutil
Then just change your script to
#!~/bin/myutil
and it should show up that way instead. You may need to use a hard link instead of a soft link.
Launching a python script using the python script itself (and file associations and/or shell magic) is not very portable, but you can use similar methods on nearly any OS.

The easiest way to get this is using she bang. The first line of your python script should be:
#!/usr/bin/python
or
#!/usr/bin/python3
depending upon whether you use python or python3
and then assign executable permissions to the script as follows:
chmod +x <scriptname>
and then run the script as
./scriptname
this will show up as scriptname in top.