Can't run file with Python 2.7

Hi- When I try to run my .py file, it says "python 3.4...." and then I get an error message, because the app uses a module for Python 2.7 only. How can I test files by running them with just Python 2.7?

You can use a thing called a "hashbang" to mark what version of Python a file is for. More info here