Overview of Ophelia
Ophelia creates XHTML pages from templates written in TAL, the Zope Template Attribute Language. It is designed to reduce code repetition to zero.
The package contains both a WSGI application running Ophelia as well as a request handler for mod_python, the Python module for the Apache2 web server.
Documentation files cited below can be found inside the package directory, along with a number of doctests for the modules.
After you installed Ophelia and wrote some templates, how can you make it render web pages?
- Use Ophelia with Apache
- The Python package contains a module ophelia.modpython that provides a request handler for the mod_python Apache module.
- Use Ophelia as a WSGI application
Ophelia defines an application class compliant with the WSGI standard, PEP 333: ophelia.wsgi.Application. You can either try it by running Ophelia's own wsgiref-based HTTP server or run it by any WSGI server you might care to use.
The wsgiref-based server is installed as the ophelia-wsgiref executable if Ophelia is installed as an egg with the "wsgiref" extra enabled. Its script entry point is ophelia.wsgi.wsgiref_server.
- Dump single pages to stdout
- An executable which is always installed with the ophelia egg is ophelia-dump. This script has Ophelia render the response corresponding to the path you specify, and prints it to sys.stdout, optionally with HTTP headers. The script's entry point is ophelia.dump.dump.
Both scripts provide some usage instructions when called with the --help option. They read a configuration file; see CONFIGURATION.txt for details.
What kind of sites is Ophelia good for?
Consider Ophelia as SSI on drugs. It's not fundamentally different, just a lot friendlier and more capable.
Use Ophelia for sites where you basically write your HTML yourself, except that you need write the recurring stuff only once. Reducing repetition to zero comes at a price: your site must follow a pattern for Ophelia to combine your templates the right way.
Consider your site's layout to be hierarchical: there's a common look to all your pages, sections have certain characteristics, and each page has unique content. It's crucial to Ophelia that this hierarchy reflect in the file system organization of your documents; how templates combine is deduced from their places in the hierarchy of directories.
Ophelia makes the Python language available for including dynamic content. Each template file may include a Python script. Python scripts and templates contributing to a page share a common set of variables to modify and use.
Ophelia's content model is very simple and works best if each content object you publish is its own view: the page it is represented on. If you get content from external resources anyway (e.g. a database or a version control repository), it's still OK to use Ophelia even with multiple views per content object as long as an object's views don't depend on the object's type or even the object itself.
Trying to use Ophelia on a more complex site will lead to an ugly entanglement of logic and presentation. Don't use Ophelia for sites that are actually web interfaces to applications, content management systems and the like.
How Ophelia works
For each request, Ophelia looks for a number of template files. It takes one file named "__init__" from each directory on the path from the site root to the page, and a final one for the page itself. The request is served by Ophelia if that final template is found.
When building the page, the page's template is evaluated and its content stored in what is called the inner slot. Then each template on the way back from the page to the root is evaluated in turn and may include the current content of the inner slot. The result is stored in the inner slot after each step.
The result of processing the root template is served as the page.
Each template file may start with a Python script. In that case, the script is separated from the template by the first occurrence of an "<?xml?>" tag on a line of its own (except for whitespace left or right). If the template file contains only a Python script but not actually a template, put "<?xml?>" in its last line.
Python scripts are executed in order while traversing from the site root to the page. They are run in the same namespace of variables that is later used as the evaluation context of the templates. Variables that are set by a Python script may be used and modified by any scripts run later, as well as by TALES expressions used in the templates.
The namespace is initialized by Ophelia with a single variable, __request__, that references the request object. Thus, scripts have access to request details and traversal internals. In addition to setting variables, scripts may also import modules, define functions, access the file system, and generally do anything a Python program can do.
How Ophelia behaves
URL canonicalization and redirection
If Ophelia encounters a URL that corresponds to a directory it behaves similarly to Apache in its default configuration: If the URL doesn't end with a slash, it will redirect the browser to add the slash. If the slash is there, it will try to find a template named index.html by default, and render it as the directory "index".
Depending on configuration, explicit requests for directory index pages may be redirected to bare directory URLs without the final path segment. This would turn <http://www.example.com/index.html> into <http://www.example.com/>.
Additionally, Ophelia canonicalizes URLs containing path segments "." and ".." according to RFC 3986 on generic URI syntax, and removes empty path segments which are not at the end of the path. If the URL is changed by these rules, Ophelia redirects the browser accordingly.
The mod_python handler
Apache2 processes a request in phases, each of which can be handled by modules such as mod_python. Ophelia provides a mod_python handler for the content generation phase. If a requested URL is configured to be handled by Ophelia, the handler tries to find appropriate templates in the file system, and build a page from them.
Ophelia's mod_python handler never causes a File Not Found HTTP error. Instead, it passes control back to Apache and other modules if it finds it can't build a particular resource. Apache falls back to serving static content from disk in that case. Ophelia can thus be installed on top of a static site to handle just those requests for which templates exist in the template directory.
Languages and APIs used in templates and scripts
For the Python language, see <http://docs.python.org/>.
For Zope page templates, see <http://www.zope.org/Documentation/Books/ZopeBook/2_6Edition/AppendixC.stx>.
For WSGI, the web server gateway interface, see <http://www.python.org/dev/peps/pep-0333/>.
For the mod_python API, see <http://www.modpython.org/live/current/doc-html/>.
For the Ophelia API and predefined script and template variables, see API.txt.