Beginner Tutorial
++++++++++++++++++

Webware for Python

:Version: 1.2.2
:Released: 08/04/19

.. contents:: Contents

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.

__ InstallGuide.html

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 \
        --gitignore 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`__.

__ ApplicationDevelopment.html

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`__.

__ Configuration.html

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.

__ ../../PSP/Docs/index.html

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">`` or simply ``<!DOCTYPE html>``.
* 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.

__ http://www.pythonware.com/products/pil/

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(key=lambda filename: filename.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)
