There are two ways of using Python: either using its interactive
interpreter as you have just done, or by writing modules. The
interpreter is useful for small snippets of programs we
want to try out. In general, all the examples you see in this book can
be typed at the interactive prompt (
»>). You should get into the
habit of trying things out at the prompt: you can do no harm, and
it is a good way of experimenting.
However, working interactively has the serious drawback that you cannot save your work. When you exit the interactive interpreter everything you have done is lost. If you want to write a longer program you create a module. This is just a text file containing a list of Python instructions. When the module is run Python simply reads through it one line after another, as though it had been typed at the interactive prompt.
When you start up IDLE, you should see the Python interactive
interpreter. You can always recognise an interpreter window by the
»> prompt whereas new module windows are empty. IDLE
only ever creates one interpreter window: if you close it and need to
get the interpreter back, select Python shell from the
Run menu. You can have multiple module windows open
simultaneously: as described in Chapter 2 each one is
really an editor which allows
you to enter and modify your program code (because of this, they will
often be referred to in this handbook as editor windows).
To get a new empty module (editor) window select New window
in the File menu.
Here is an example of a complete Python module. Type it into an editor window and run it by choosing Run from the Run menu (or press the F5 key on your keyboard)3.1.
print "Please give a number: " a = input() print "And another: " b = input() print "The sum of these numbers is: " print a + b
If you get errors then check through your copy for small mistakes like
missing punctuation marks. Having run the program it should be apparent
how it works. The only thing which might not be obvious are the lines
input() is a function which allows
a user to type in a number and returns what they enter for use in the
rest of the program: in this case the inputs are stored in
If you are writing a module and you want to save your work, do so by
selecting Save from the File menu then type a name for
your program in the box. The name you choose should indicate what the
program does and consist only of letters, numbers, and ``_'' the
underscore character. The name must end with a
that it is recognised as a Python module, e.g.
Furthermore, do NOT use spaces in filenames or directory
Change the program so it subtracts the two numbers, rather than adds them up. Be sure to test that your program works as it should.