if your file is in /usr/bin and the file starts with
#!/usr/bin/python
the above method assumes you have python install in /usr/bin or better yet
#!/usr/bin/env python
this method will run it no matter where you installed python
you just need to make the file executable with the command
chmod +x /usr/bin/hello.py
now you can just run it by entering its command hello.py. This will work because /usr/bin is in your normal path. If you had the program somewhere else like your home directory. you would need to type the whole path, /home/fallen/hello.py
It is not probably a good habit to be putting test programs in /usr/bin. It should not cause problems, but will make a mess of that directory. I myself have a python directory in my home directory
you also could ignore everything I said above and do it this way. type the command python then the location of the file
the above method will work if you are in the same directory as the program. else do it like this
python /full/path/to/hello.py