CS 111
Summer 1 2020

Installation instructions for OS X

Installing Python 3.7

The Python installation also includes IDLE, the program we will use to write and execute our Python programs.


You must follow these instructions to install version 3 of Python. You cannot use the built-in Python version that comes with OS X, since that is version 2 and not compatible with the Python you will learn in this course.

  1. Go to the download page for Python 3.
  2. Download the latest version of Python 3 by clicking the appropriate Download button. (At the time these instructions were created, the latest version of Python 3 was Python 3.7.x. If you see a later version number, that’s okay, too. Just make sure to select the button for the version number that begins with Python 3 and not Python 2.)
  3. Double-click on the downloaded file in your Downloads folder (the name of the file should begin with python and end with .pkg). This should start an automatic installer.
  4. You should be able to use all of the default settings that the installer gives you. Wait for the installer to finish and then close it. You can now delete the installer by dragging the .pkg file to the Trash.
  5. In your Applications folder, you should now be able to find a Python 3 folder. Under that folder, the IDLE application should be present.

The newest version of Python is “signed” as a safe application, so OS X should not have a problem installing it. If OS X prevents you from installing Python because it does not pass a security check, you should be sure you are downloading the most recent Python version.

Lowering security settings

OS X may stop you from downloading or running applications downloaded from the Web (as opposed to through the Mac App Store) unless you lower your security settings. Here’s how to adjust them so that you may install and run applications from anywhere:

  1. Open System Preferences, located under the Apple menu in the top left corner.
  2. In the Security & Privacy preference pane, click the lock icon and authenticate as an administrator user.
  3. Make sure the radio button under Allow applications downloaded from: is set to Anywhere.
  4. Re-download or re-open the desired application.