Webware for Python

Version:	1.0.2
Released:	06/07/09

Contents

Synopsis
Installation
Setting Up Your Environment
- Creating a Working Directory
- Changing Webware Configuration
Creating and Understanding the Servlet
A Brief Introduction to the Servlet
A Photo Album
- Iteration 1: Displaying Files
- Iteration 2: Uploading Files

Synopsis

We present a tutorial on making a Webware script, and some guidance on turning that into a web application.

Installation

This document does not cover the basic installation. See the Install Guide -- you should be able to open up the WebKit Examples context in you browser before you continue. For the beginning, you can use the built-in HTTP server, but in a production environment you should use a real web server like Apache.

Setting Up Your Environment

Creating a Working Directory

We'll first set up a directory dedicated to your application. Run this command:

$ cd ~
$ python /path/to/Webware/bin/MakeAppWorkDir.py -c Context -l Lib \
    --cvsignore WebwareTest

You'll now have a directory WebwareTest in your home directory (or of course you can put it in some other location). Inside this directory will be several subdirectories and a couple files. The only file we'll worry about is AppServer (or AppServer.bat if you're on Windows). The directories of interest are Context (that you specified with -C context) where you'll be putting your servlets; Configs that holds some configuration files; and Lib where you can put your non-servlet code.

For more information about the working directory and setting up the file structure for your application, see Application Development.

Changing Webware Configuration

For the most part the configuration is fine, but we'll make a couple changes to make it easier to develop. For more information on configuration see the Configuration Guide.

In the file AppServer.config, change this line:

# Original (default setting):
    AutoReload = False
# To:
    AutoReload = True

This will cause the AppServer to restart if any loaded files are changed -- without this you may edit a file and your application won't see the updated version until you manually restart the AppServer.

The other change you may want to make to allow you to use more interesting URLs. In Application.config:

# Original (default setting):
    ExtraPathInfo = False
# To:
    ExtraPathInfo = True

Otherwise the settings should be appropriate for development. (There are several setting you would want to change before deploying the application in a production environment).

Creating and Understanding the Servlet

Webware's core concept for serving pages is the servlet. This is a class that creates a response given a request.

The core classes to understanding the servlet are Servlet, HTTPServlet, and Page. Also of interest would be the request (Request and HTTPRequest) and the response (Response and HTTPResponse) -- the HTTP- versions of these classes are more interesting. There is also a Transaction object, which is solely a container for the request and response.

While there are several levels you can work on while creating your servlet, in this tutorial we will work solely with subclassing the Page class. This class defines a more high-level interface, appropriate for generating HTML (though it can be used with any content type). It also provides a number of convenience methods.

A Brief Introduction to the Servlet

Each servlet is a plain Python class. There is no Webware magic (except perhaps for the level one import module based on URL spell). PSP has more magic, but that's a topic for another day.

An extremely simple servlet might look like:

from WebKit.Page import Page

class MyServlet(Page):

    def title(self):
        return 'My Sample Servlet'

    def writeContent(self):
        self.write('Hello world!')

This would be placed in MyServlet.py. Webware will create a pool of MyServlet instances, which will be reused. Servlets "write" the text of the response (like self.write("some text")). Webware calls the servlet like this:

An unused servlet is taken from the pool, or another servlet is created.
awake(transaction) is called. This is a good place to set up data for your servlet. You can put information in instance variables for use later on. But be warned -- those instance variables will hang around potentially for a long time if you don't delete them later (in sleep).
Several low-level methods are called, which Page isolates you from. We will ignore these.
writeHTML() is called. Page implements this just fine, but you can override it if you want total control, or if you want to output something other than HTML.
writeDocType() would write something like <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
The <head> section of the page is written. title() gives the title, and you probably want to override it.
writeStyleSheet() is called, if you want to write that or anything else in the <head> section.
The <body> tag is written. Have htBodyArgs() return anything you want in the <body> tag (like onLoad="loadImages()").
writeBodyParts() is called, which you may want to override if you want to create a template for other servlets.
writeContent() should write the main content for the page. This is where you do most of your display work.
The response is packaged up, the headers put on the front, cookies handled, and it's sent to the browser. This is all done for you.
sleep(transaction) is called. This is where you should clean up anything you might have set up earlier -- open files, open database connections, etc. Often it's empty. Note that sleep() is called even if an exception was raised at any point in the servlet processing, so it should (if necessary) check that each resource was in fact acquired before trying to release it.
The servlet is placed back into the pool, to be used again. This only happens after the transaction is complete -- the servlet won't get reused any earlier.

You only have to override the portions that you want to. It is not uncommon to only override the writeContent() method in a servlet, for instance.

You'll notice a file context/Main.py in your working directory. You can look at it to get a feel for what a servlet might look like. (As an aside, a servlet called Main or index will be used analogous to the index.html file). You can look at it for a place to start experimenting, but here we'll work on developing an entire (small) application, introducing the other concepts as we go along.

A Photo Album

If you look online, you'll see a huge number of web applications available for an online photo album. The world needs one more!

You will need the Python Imaging Library (PIL) installed for this example. First we'll use it to find the sizes of the images, and later we will use it to create thumbnails.

We'll develop the application in two iterations.

Iteration 1: Displaying Files

For simplicity, we will store image files in a subdirectory Pics of the default context directory WebwareTest/Context and let the AppServer deliver the files. In a production environment, you would place the Pics directory outside of the context and let the web server deliver the files directly.

For the first iteration, we'll display files that you upload by hand to the Pics directory.

We do this with two servlets -- one servlet Main.py to show the entire album, and another View.py for individual pictures. Place these two servlets in the default context directory. First, Main.py:

from WebKit.Page import Page # the base class for web pages
import os
import Image # the Python Imaging Library
from urllib import quote as urlEncode
dir = os.path.join(os.path.dirname(__file__), 'Pics')

class Main(Page):

    def title(self):
        # It's nice to give a real title, otherwise "Main" would be used.
        return 'Photo Album'

    def writeContent(self):
        # We'll format these simpy, one thumbnail per line:
        for filename in os.listdir(dir):
            im = Image.open(os.path.join(dir, filename))
            x, y = im.size
            # Here we figure out the scaled-down size of the image,
            # so that we preserve the aspect ratio. We'll use fake
            # thumbnails, where the image is scaled down by the browser.
            x, y = x * 100 / y, 100
            # Note that we are just using % substitution to generate
            # the HTML. There's other ways, but this works well enough.
            # We're linking to the View servlet which we'll show later.
            # Notice we use urlEncode -- otherwise we'll encounter bugs if
            # there's a file with an embedded space or other character in it.
            url = urlEncode(filename)
            self.writeln('<p><a href="View?filename=%s">'
                '<img src="Pics/%s" width="%i" height="%i"></a></p>'
                % (url, url, x, y))

The servlet View takes one URL parameter of filename. You can get the value of a URL parameter like self.request().field('filename') or, if you want a default value, you can use self.request().field('filename', defaultValue). In the likely case you don't want to write self.request() before retrieving each value, do:

req = self.request()
self.write(req.field('username'))

# Even more compactly:
field = self.request().field
self.write(field('username'))

So here is our complete View servlet:

from WebKit.Page import Page
import os
import Image
from urllib import quote as urlEncode
dir = os.path.join(os.path.dirname(__file__), 'Pics')

class View(Page):

    def title(self):
        return 'View: %s' \
               % self.htmlEncode(self.request().field('filename'))

    def writeContent(self):
        filename = self.request().field('filename')
        im = Image.open(os.path.join(dir, filename))
        self.writeln('<div style="text-align:center">')
        self.writeln('<h4>%s</h4>' % filename)
        self.writeln('<img src="Pics/%s" width="%i" height="%i">'
                   % (self.urlEncode(filename), im.size[0], im.size[1]))
        self.writeln('<p><a href="Main">Return to Index</a></p>')
        self.writeln('</div>')

Iteration 2: Uploading Files

That was fairly simple -- but usually you want to upload files, potentially through a web interface. Along the way we'll add thumbnail generation using PIL, and slighly improve the image index.

We'll generate thumbnails kind of on demand, so you can still upload files manually -- thumbnails will be put in the directory Thumbs and have -tn appended to the name just to avoid confusion:

from WebKit.Page import Page # the base page class
import os
import Image # the Python Imaging Library
from urllib import quote as urlEncode
baseDir = os.path.dirname(__file__)
picsDir = os.path.join(baseDir, 'Pics')
thumbsDir = os.path.join(baseDir, 'Thumbs')

class Main(Page):

    def title(self):
        return 'Photo Album'

    def writeContent(self):
        # The heading:
        self.writeln('<h1 style="text-align:center">%s</h1>' % self.title())
        # We'll format these in a table, two columns wide
        self.writeln('<table width="100%">')
        col = 0 # will be 0 for the left and 1 for the right column
        filenames = os.listdir(picsDir)
        # We'll sort the files, case-insensitive
        filenames.sort(lambda a, b: cmp(a.lower(), b.lower()))
        for filename in filenames:
            if not col: # left column
                self.write('<tr style="text-align:center">')
            thumbFilename = os.path.splitext(filename)
            thumbFilename = '%s-tn%s' % thumbFilename
            if not os.path.exists(os.path.join(thumbsDir, thumbFilename)):
                # No thumbnail exists -- we have to generate one
                if not os.path.exists(thumbsDir):
                    # Not even the Thumbs directory exists -- make it
                    os.mkdir(thumbsDir)
                im = Image.open(os.path.join(picsDir, filename))
                im.thumbnail((250, 100))
                im.save(os.path.join(thumbsDir, thumbFilename))
            else:
                im = Image.open(os.path.join(thumbsDir, thumbFilename))
            url = urlEncode(filename)
            self.writeln('<td><p><a href="View?filename=%s">'
                '<img src="Pics/%s" width="%i" height="%i"></a></p>'
                '<p>Filename: %s<br>Size: %i Bytes</p>'
                % (url, url, im.size[0], im.size[1], filename,
                os.stat(os.path.join(picsDir, filename)).st_size))
            if col: # right column
                self.writeln('</tr>')
            col = not col
        self.write('</table>')
        self.write('<p style="text-align:center">'
            '<a href="Upload">Upload an image</a></p>')

The View servlet we'll leave just like it was.

We'll add an Upload servlet. Notice we use enctype="multipart/form-data" in the <form> tag -- this is an HTMLism for file uploading (otherwise you'll just get the filename and not the file contents). Finally, when the form is finished and we have uploaded the image, we redirect them to the viewing page by using self.response().sendRedirect(url):

from WebKit.Page import Page
import os
from urllib import quote as urlEncode
dir = os.path.join(os.path.dirname(__file__), 'Pics')

class Upload(Page):

    def writeContent(self):
        if self.request().hasField('imageFile'):
            self.doUpload()
            return

        self.writeln('''
        <h3>Upload your image:</h3>
        <form action="Upload" method="post" enctype="multipart/form-data">
        <input type="file" name="imageFile">
        <input type="submit" value="Upload">
        </form>''')

    def doUpload(self):
        file = self.request().field('imageFile')
        # Because it's a file upload, we don't get a string back.
        # So to get the value we do this:
        filename, contents = file.filename, file.value
        open(os.path.join(dir, filename), 'wb').write(contents)
        url = 'View?filename=' + urlEncode(filename)
        self.response().sendRedirect(url)

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