Hello World in Python 3 on Mac OS X

Your First Program in Python 3 on Mac OS X

This page tells you how to setup a Python programming environment for your Mac OS X computer and provides a step-by-step guide for creating and running a simple "Hello, world" Python program. All of the software is freely available on the Web. These instructions are for MacOS X 10.13 (High Sierra), but the instructions for other recent versions of Mac OS X are similar.

Overview

The Python programming environment required by this booksite consists of:

Python, that is, the Python compiler/interpreter.
The Python standard libraries.
IDLE, the Python Integrated DeveLopment Environment.
The Tkinter, NumPy, and Pygame libraries, which are used by the booksite programs that do graphics or audio processing.
The booksite library, that is, a set of modules that we developed specifically to support this booksite.
The Terminal application that is bundled with Mac OS X.

Downloading and Installing Python, IDLE, Tkinter, NumPy, and Pygame

Note: The Mac Finder application uses the term folder to mean a container of documents and, perhaps, other folders. This document instead uses the equivalent Unix term directory.

Python 2 is bundled with Mac OS X. However, Python 3 is not. We recommend that you download and install Python 3, and use Python 3 exclusively when running programs associated with this booksite.

Perform these steps to download and install Python, IDLE, Tkinter, NumPy, and PyGame:

Browse to the Python for Mac download page.
In the resulting page click on the Download Python 3.6.5 button. Your browser will download the file python-3.6.5-macosx10.6.pkg to your Downloads directory.
Using Mac Finder Manager, navigate to your Downloads directory, and double-click on the python-3.6.5-macosx10.6.pkg file to install Python, Tkinter, and IDLE. Use the default options. (It's OK to delete the python-3.6.5-macosx10.6.pkg file after the installation is complete.)
Open a Terminal window. To do that, click on the Spotlight Search icon on the right end of the menu bar. (It has the appearance of a magnifying glass.) In the resulting dialog box, type Terminal.app followed by the Enter key.
In the Terminal window issue these commands to install NumPy and PyGame:

python3 -m pip install -U numpy --user
python3 -m pip install -U pygame --user

To "issue a command" you should type the command followed by the Enter key.

Perform these steps to test your installation:

In the Terminal window issue the python3 command. You should see something like this:

Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 03:03:55) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

If you see that output, then your installation of Python, IDLE, and Tkinter was successful.
At the Python >>> prompt, type import numpy followed by the Enter key. If no error messages appear, then your installation of NumPy was successful.
At the Python >>> prompt, type import pygame followed by the Enter key. If no error messages appear, then your installation of PyGame was successful.
At the Python >>> prompt, type exit() followed by the Enter key to exit Python.
In the Terminal window issue the command idle3. If an IDLE window appears, then you have installed IDLE properly. Close the IDLE window.
Close the Command Prompt window.

IMPORTANT: When running the Python programs associated with this booksite, do not issue the python command. Instead issue the python3 command. The former command runs the Python 2 compiler/interpreter that is bundled with Mac OS X; the latter command runs the Python 3 compiler/interpreter that you installed by following the instructons on this page.

Downloading and Installing the Booksite Library

Perform these steps to download and install the booksite library:

Use your Web browser to download this introcs-1.0.zip file to your /Users/yourusername/Downloads directory.
In the Finder, double click on /Users/yourusername/Downloads/introcs-1.0.zip to unzip the file, thus creating a directory named /Users/yourusername/Downloads/introcs-1.0. (It's OK to delete the /Users/yourusername/Downloads/introcs-1.0.zip file after you have unzipped it.)
Open a Terminal window.
At any time the Terminal application has a working directory. Initially the working directory is /Users/yourusername. In the Terminal window issue the cd Downloads command to change your working directory to /Users/yourusername/Downloads, and then issue the cd introcs-1.0 command to change your working directory to /Users/yourusername/Downloads/introcs-1.0. (Incidentally, the cd .. command changes your working directory to the "parent" of the current working directory.)
Issue the ls command to display the names of all files in your working directory. Make sure that a file named setup.py is in your working directory.
Issue the python3 setup.py install --user command. The computer copies the files defining the booksite modules to a directory where Python can find them, and writes status messages to your Terminal window to indicate its progress.

Perform these steps to test your installation of the booksite library:

In the Terminal window issue the python3 command.
At the Python >>> prompt, type the statement import stdio followed by the Enter key. If Python generates no error messages, then you have installed the booksite library properly.
At the Python >>> prompt, type exit() followed by the Enter key to exit Python.
Close the Terminal window.

Configuring IDLE

So far you've downloaded and installed all of the software that you'll need. You should perform one more step before creating your first program: configure the IDLE programming environment. Follow these instructions:

Open a Terminal window.
In the Terminal window issue the command idle3 to launch IDLE.
Click on the Python → Preferences... menu item.
Click on the General tab.
Click on the Open Edit Window radio button.
Click on the Ok button.
Close the IDLE window.
Close the Terminal window.

Composing Your First Program

Having installed Python, the Python standard libraries, IDLE, Tkinter, NumPy, Pygame, and the booksite libraries, and having configured IDLE, you are ready to compose your first Python program. Perform these instructions:

Using the Mac Finder, create a directory named /Users/yourusername/hello.
Open a Terminal window.
Issue the cd hello command to make the /Users/yourusername/hello directory your working directory.
Issue the command idle3 & to launch IDLE. Note the trailing ampersand. The trailing ampersand tells the computer to run the idle3 program in the background, thereby leaving your Terminal application free to handle additional commands while IDLE is running.
In IDLE, type the four-line Python program helloworld.py exactly as it appears below. Use the arrow keys, mouse, or touchpad to move within the text that you have typed. Use the Delete key to delete text. Be careful; the smallest typing mistake might cause the program to fail.

import stdio

# Write 'Hello, World' to standard output.
stdio.writeln('Hello, World')

When you are finished typing, in IDLE click on the File → Save... menu item to save the Python program. Save it in a file named helloworld.py in the directory /Users/yourusername/hello. The file name is case sensitive, so make sure you use all lowercase letters.

Running Your First Program

The final step is to run your program. It is possible to run some Python programs from within IDLE, but you should run the programs associated with this booksite directly from a Terminal window. To do that, perform these steps:

Within the same Terminal window, issue the ls command to display the names of all files in the working directory. Confirm that the working directory contains your helloworld.py file.
Issue the python3 helloworld.py command to run your program. If the computer writes "Hello, World" to the Terminal window, then the execution of your helloworld.py program was successful. If the computer instead writes error messages, then use IDLE to correct your program, and issue the python3 helloworld.py command again. Repeat until your program runs successfully. If your program runs successfully the first time you try, then intentionally introduce an error into your program, just so you get some experience with correcting errors.
Close the IDLE window.
Close the Terminal window.

You now have installed and configured a reasonable Python environment, and have used it to compose and run a Python program. Congratulations! You are a Python programmer!

Downloading the Booksite Example Programs (optional)

We recommend that you download the booksite example programs, that is, the example Python programs that are presented incrementally throughout the booksite. Having done so, you can run those programs to help you learn about them. Perform these instructions:

Use your Web browser to download this introcs-python.zip file to your /Users/yourusername/Downloads directory.
In the Mac Finder, double-click on /Users/yourusername/Downloads/introcs-python.zip to unzip the file, thus creating the /Users/yourusername/Downloads/introcs-python directory containing the booksite example programs. (It's OK to delete the /Users/yourusername/Downloads/introcs-python.zip file after you have unzipped it.)

Then perform these steps to test your download of the booksite example programs:

Open a Terminal window.
Issue the cd Downloads and cd introcs-python commands to make /Users/yourusername/Downloads/introcs-python your working directory.
Issue the ls command. Confirm that the working directory contains a file named bouncingball.py.
Issue the python3 bouncingball.py command. If Python launches a stddraw window showing an animated bouncing ball, then your download of the booksite example programs was successful.
Close the stddraw window.
Close the Terminal window.

Downloading the Booksite Example Data (optional)

We recommend that you download the booksite example data, that is, the data files used by the booksite example programs that are presented incrementally throughout the booksite. Perform these instructions:

Use your Web browser to download this introcs-data.zip file to your /Users/yourusername/Downloads directory.
In the Finder, double-click on /Users/yourusername/Downloads/introcs-data.zip to unzip the file, thus creating the /Users/yourusername/Downloads/introcs-data directory containing the booksite example data files. (It's OK to delete the /Users/yourusername/Downloads/introcs-data.zip file after you have unzipped it.)

Downloading the Booksite Library: Part 2 (optional)

Previously on this page we described how to download and install the booksite library so Python can find it. Now we describe how to download the booksite library so you can find it — for the sake of studying the code that implements it, should you so desire. Perform these instructions:

Use your Web browser to download this stdlib-python.zip file to your /Users/yourusername/Downloads directory.
In the Mac Finder, double-click on /Users/yourusername/Downloads/stdlib-python.zip to unzip the file, thus creating the /Users/yourusername/Downloads/stdlib-python directory containing the booksite library. (It's OK to delete the /Users/yourusername/Downloads/stdlib-python.zip file after you have unzipped it.)

We invite you to study the code that implements the booksite library. But don't be concerned if some of the code is cryptic. The code that implements the booksite library uses some features of Python that are beyond the scope of the textbook and this booksite.

Q & A

Q. Why do I get the error ImportError: No module named stdio when I issue the command python program_that_uses_stdio.py?

A. You must issue the command python3 program_that_uses_stdio.py, as described above.

Q. I downloaded files using my browser, but can't find them. Where are they?

A. Many browsers by default place downloaded files in the directory /Users/yourusername/Downloads.

Q. How do I break out of an infinite loop when running my program from the Terminal application?

A. Type Ctrl-c. That is, while pressing the Ctrl key, type the c key.

Q. Must I use IDLE to create my Python programs? Can I use some other text editor?

A. You need not use IDLE to create your Python programs; it is fine to use some other text editor. For example, it is reasonable to use the TextEdit editor that is bundled with Mac OS X. However if you do use some other text editor, then make sure you change its settings so it (1) uses a four-space indentation scheme, and (2) indents using spaces instead of tabs. The Wikipedia Comparison of text editors page provides summary descriptions of many text editors.