[workshop] lesson 00: "hello %s!"

[Michal Wallace]

############################################################
## Hey all,
##
## I've got friends in town this week, so I'm off to a slow 
## start here. :) I'm new at this, so let me know if I'm
## moving too fast/slow/etc with this tutorial... 
##
## Anyway, here goes!  
##
## -Michal
############################################################

## lesson 00: "hello, %s!" #################################

* Hello, World!

The traditonal introduction to a computer language is the
"Hello, World!" program. In python, it's a one-liner:

###
print "hello, world!"
###

Easy, huh? If you load up a python interpreter and type
that line, it'll print "hello, world!" to the screen.

* The Response Object

Web apps don't print their output to a screen, but send it across the
internet. For this reason (and others I'll go into later on), we
redirect the output to a response object. In the framework that I use,
this object is called RES:

###
print >> RES, "hello, world!"
###


That's it! Our first web application! 

So how can we see it work?

To run this on a cornerhost server, put it in a text file with the
extension ".app". Call this one "hello.app" and upload it to your
account. 

Now load it in your browser as if it were a normal web page.  If all
goes well, you should see:

   hello, world!


If you get an error message, double check that you have the filename
correct and that the file contains no leading tabs or spaces - python
has very strict rules about whitespace.

* Hello, NAME!

Let's personalize our greeting a bit. Python lets us insert a variable
(a defined value) into our output by using the percent sign:

###
name = "Orville"
print >> RES, "hello, %s!" % name
###

If you upload this script and load it in the browser, the
result should be:


   hello, Orville!

* The Request Object

Of course, most of the people using your app won't be named
Orville. One way of passing a name into our app would be to include it
in the query string:


   http:// ... /hello.app?name=Gomer


We can read the query string with the REQ (Request) object:

###
name = REQ.get("name")
###

Better yet, we can provide a default:

###
name = REQ.get("name", "whoever you are")
print >> RES, "hello, %s!" % name
###

Try running this with and without a "?name=whatever" 
appended to the url.

* Adding a Form

But why settle for a default, when we can simply ask users what their
names are? This next example uses the REQ.has_key() method to tell if
a name has been passed in. If so, it says hello. If not, it shows an
HTML form:

###
if REQ.has_key("name"):
    print >> RES, "hello, %s!" % REQ.get("name")
else:
    # notice the single quotes, to prevent conflict with HTML:
    print >> RES, '<form action="hello.app" method="GET">'
    print >> RES, 'What is your name?'
    print >> RES, '<input type="text" name="name" value="">'
    print >> RES, '<input type="submit">'
    print >> RES, '</form>'
###

* Yuck!

There's nothing wrong with that code, but it sure isn't pretty.  If
you wanted to make the form look nicer, you'd have to add a whole lot
more of those "print >> RES" lines, and that would get old real fast.
Worse, it's hard to read and hard to maintain.

In the next lesson, we'll look at a way of solving these problems by
separating the logic of an app from its presentation.


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(c)2002 sabren enterprises inc
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