After you’ve installed Python, steps for launching it depend on which OS you are using:
To start Python on OS X you first need to launch a command line terminal. Do this by navigating to Applications, then Utilities, then double-click the “Terminal” program.
The command line Terminal is a tool for interacting with your computer. A window will open with a command line prompt message, something like this:
myname ~$
Note
Take note of the dollar sign $ at the end of the prompt. This tells you that you are using the system command line. We’ll be using two different interactive environments today, the system command line (with this prompt) and the Python interactive shell (which we’ll see shortly.)
To launch Python from the system command line type python and then press <Enter>
To start Python on Linux you first need to launch a command line terminal program. Exactly how to do this depend on what Linux desktop environment you are using, but try searching for Terminal in your computer’s program launcher interface.
The command line terminal is a tool for interacting with your computer. A window will open with a command line prompt message, something like this:
myname@mycomputer:~$
Note
Take note of the dollar sign $ at the end of the prompt. This tells you that you are using the system command line. We’ll be using two different interactive environments today, the system command line (with this prompt) and the Python interactive shell (which we’ll see shortly.)
To launch Python from the system command line type python and then press <Enter>
On Windows you can launch Python through the Start menu.
After Python opens, it will show you some contextual information similar to this:
Python 2.7.2 (default, Feb 1 2012, 00:28:57)
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
The prompt >>> on the last line indicates that you are now in an interactive Python session, also called the “Python shell”. It is waiting for your commands:
print("Hello world")
Press <Enter> and see what happens. You will now see the phrase “Hello world” appear and then Python will bring you back to the interactive prompt, where you could enter another command:
>>> print("Hello world")
Hello world
>>>
To leave the interactive shell, press Ctrl-Z and then Enter on Windows or Ctrl-D anywhere else.
But you don’t want to type everything into the Python shell every time. Instead you can save the commands to a file and pass a file name to the python executable. It will execute that file instead of launching the interactive interpreter.
Let’s try that. Create a file hello.py in your current directory with your favorite text editor and paste the print command from above. Now save that file.
On Mac OS X and Linux go back to the system command line (where the prompt ends with $ not >>>, use Ctrl-D to exit Python if it is running) and type:
python hello.py
Note
Not getting “Hello world” but some crazy error about “can’t open file” or “No such file or directory?” Probably your command line is not running in the directory you saved the file in; you can use the system command line to change your active directory with the cd command, which stands for “change directory”:
cd "c:\Users\myuser\Desktop\Python_Exercises"
This changes to the directory Python_Exercises on my desktop (yours might be somewhere a bit different). If you don’t know the location of the directory where you saved the file, you can simply drag the directory to the command line window. If you don’t know which directory your shell is currently running in use pwd, which stands for “print working directory”.
On Windows you can double-click the Python file to run it.
When pressing <Enter> now, the file is executed and you see the output as before. But this time, after Python finished executing all commands from that file it exits instead of going back to the interactive shell.
Tip
Wordpad, TextEdit, Notepad, and Word are not suitable text editors. If you are unsure whether you already have a usable editor, you might want to download and install Sublime Text. Sophisticated editors like this can also help you with formatting, running and debugging your code.
And now we are all set and can get started with turtle!
Important
When playing around with turtle, avoid naming your file turtle.py — rather use more appropriate names such as square.py or rectangle.py. Otherwise, whenever you refer to turtle, Python will pick up your file instead of the standard turtle.