status 0913.2005: accurate. weblib hasn't changed at all. however: platonic+wsgi+rest would be a much better first lesson

lesson 00: "hello %s!"

Michal Wallace

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.