Now you will need to implement the prison logic using these turtle

functions, a ``while`` loop and a bit of conditional logic. It's a bit

of a stretch but keep at it! Don't be afraid to talk it out with a

coach or another student.

# Set the angle we want to use for our square

angle = 20

Comments

--------

In the solution above, the line that starts with a ``#`` is called a

comment. In Python, anything that goes on a line after ``#`` is ignored

by the computer. Use comments to explain what your program does,

without changing the behaviour for the computer.

Comments can also go at the end of a line, like this:

::

angle = 20 # Set the angle we want to use for our square

After you've installed Python, steps for launching it depend on which

Starting Python on Mac OS X

===========================

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 :program:`python` and

then press :kbd:`<Enter>`

Starting Python on Linux

========================

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 :program:`python` and

then press :kbd:`<Enter>`

Starting Python on Windows

==========================

On Windows you can launch Python through the Start menu.

Interacting With Python

=======================

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::

On Mac OS X and Linux go back to the system command line (where the

prompt ends with ``$`` not ``>>>``, use :kbd:`Ctrl-D` to exit Python if it is

running) and type:

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

:command:`cd` command, which stands for "change directory"::

directory. If you don't know which directory you are currently in then

running in then use :command:`pwd`,which stands for "print working directory".

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

<http://www.sublimetext.com/>`_. Sophisticated editors like this

can also help you with formatting, running and debugging your code.

.. note::

Python is a programming language where horizontal indenting of text is important. We'll learn all about this in the Functions chapter later on, but for now just keep in mind that stray spaces or tabs before any line of Python code will cause an unexpected error.

.. tip::

Are you seeing an error like this::

NameError: name 'turtle' is not defined``

when trying to view help? In Python you have to import names before you can refer to them, so in a new Python interactive shell you'll need to ``import turtle`` before ``help(turtle.color)`` will work.

Another way to find out about functions is to browse the `online documentation`__.

turtle.left(20)

turtle.left(20)

turtle.left(20)

... and so on

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