Using IDLE to Write and Run Programs

Reading and Writing Python Files

About IDLE

What are the two main ways that we can use IDLE? →

Again, You Can…

• run your program all at once (using Run → Run Module)

• run commands one-step-at-a-time using the interactive Python shell

Lastly, if your program ever outputs anything, it should show up in the interactive Python Shell

Starting IDLE

Just like what we did in the previous slides, to open IDLE:

• OSX - ⌘-space → start typing idle → choose IDLE - Python 3.2
• Windows - Start → All Programs → Python 3.2 → IDLE

Starting IDLE Continued

When you first open IDLE, you should see a window titled "Python Shell"

• to reiterate, the Python Shell is for entering and running programs line-by-line
• sometimes the shell can be referred to as the Python interpreter, console or interactive shell
• you'll probably want to start out by creating a new file…

Managing Files

All file management activities can be found in the File menu:

• Create a new file: File → New Window
• Opening an existing file: File → Open
• Running a file: Run → Run Module

Shortcuts for creating, opening and running a file are ⌘-n, ⌘-o, and the F5 key respectively.

(Again, Feel Free to Follow Along!)

A First Program

"Hello world!" is traditionally the first program you write when learning a new language. It simply outputs "Hello world" (yeah, that's all). Follow these steps:

• create a new file
• type in …

print("Hello world!")

• run your program
  • you may be prompted you to save your file
  • if you haven't saved it yet, you'll be prompted for a file name

Modifying Your Program

Let's modify the program that we just wrote:

• if your file is closed, reopen it
• add a second line:

print("Hi again!")

• run your program

Making Mistakes

Let's purposely make a mistake, then fix it

• if your file is closed, reopen it
• add a third line, but leave off the last parentheses:

print("Hola"

• run your program
• you should see a pop-up and a red highlight where your syntax error occurred
• this occurs if your program is not syntactically correct
• fix the mistake (how?)

Making More Mistakes

Let's make a run-time error:

• if your file is closed, reopen it
• add a fourth line:

print("Howdy" + 2)

• run your program
• you should see the error in the console
• let's read it and try to interpret it
• fix the error
• note that the current line number is on the lower-right-hand corner of the window (prefixed with "Ln: ")

Errors

Notice that we looked at two different types of errors:

• syntax error - an error with the syntax/structure of the program; the program cannot run at all
• runtime error - the program is syntactically correct, but some sort of error occurs while the program is running

A Quick Note on Syntax Highlighting

• The different colors represent different syntactic elements
• Some examples include
  • strings - green
  • built-in functions - purple
  • keywords - orange
• strings - a primitive type of data in Python; a sequence of characters
• keywords - (reserved words) words that have special meaning in Python

The Output Window and Interactive Shell

The Interactive Shell

• the output window can also be used to run Python code interactively
• you can also go to Run → Python Shell to open an interactive session if it's closed
• you can enter commands and have them return immediate feedback
• the prompt, ">>>", means the interactive shell is waiting for input
• try re-typing our hello world in the interactive shell, one line at a time

Help!

Note that typing help in the interactive shell actually gives you help! Let's try it! →

>>> help

You can use help on specific things… for example: →

>>> help(print)

Additionally, you can check out the Python docs by going to Help → Python Docs in the menu. →

Let's Provide Some Structure to Our Programming Process