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# Pyblosxom Architecture

## Summary

Pyblosxom uses the file system for data storage allowing you to use the text-based tools that you use for other parts of your workflow for your blog.

Pyblosxom has a plugin system allowing users to augment and extend Pyblosxom’s behavior to meet their specific needs.

This chapter covers Pyblosxom’s architecture.

The code is fairly well documented and you should always consider the code to be the authority when the code and this manual are in disagreement.

## Parts

Pyblosxom is composed of several parts:

1. `pyblosxom.cgi` - This is the CGI script that is executed by your web server, pulls in configuration variables from `config.py` and then instantiates Pyblosxom objects to handle the request.

2. `PyblosxomWSGIApp` - This is the WSGI application for Pyblosxom.

3. `Pyblosxom` package - This is the Python package that holds the Pyblosxom objects and utility functions that handle the request.

   1. the `entries` package - Handles the abstraction allowing Pyblosxom to use entries other than those solely found on the file system.

   2. the `renderers` package - Pyblosxom can handle different renderers. The renderer gets a list of entries to be rendered and can render them using whatever means it so desires: blosxom templates, htmltmpl templates, Cheetah templates, hard-coded RSS 2.0 markup, ...

      Pyblosxom comes with two renderers: blosxom and debug.

   3. the `cache` package - Pyblosxom allows for entry-level caching. This helps in cases where your entries are stored in a format that requires a lot of processing to convert to HTML.

Pyblosxom’s behavior and output is then augmented by:

1. plugins - Plugins allow you to augment Pyblosxom’s default behavior. These you can get from the plugin registry or write yourself.
2. flavour templates - Flavour templates allow you to create the look and feel of your blog. These you can get from the flavour registry or write yourself.

## Lifecycle of a Pyblosxom request

This is the life cycle of a single Pyblosxom CGI request. It involves the following “entities”:

- `pyblosxom.cgi` - A script found in the `web/` directory. This is the CGI script that handles Pyblosxom requests.
- `config.py` - The configuration file that defines the behavior and properties of your blog.
- `Pyblosxom.pyblosxom` - The pyblosxom module holds the default Pyblosxom behavior functions. It also defines the Request class and the Pyblosxom class.
- `Pyblosxom.pyblosxom.Request` - The Request object holds the state of the Pyblosxom request at any given time throughout the lifecycle of the request. The Request object is passed to most callbacks in the args dict as `request`.
- `Pyblosxom.pyblosxom.Pyblosxom` - The Pyblosxom object holds a list of registered plugins, what callbacks they’re registered to, and the methods that handle the the actual request.

The Pyblosxom request lifecycle starts with the web server executing `pyblosxom.cgi`.

1. `pyblosxom.cgi` loads `config.py`

2. `pyblosxom.cgi` instantiates a Request object

3. `pyblosxom.cgi` instantiates a `Pyblosxom.pyblosxom.Pyblosxom` object passing it the Request object

4. `pyblosxom.cgi` calls `run()` on the Pyblosxom object

   1. Pyblosxom instance, run method: calls `initialize`

      > 1. Pyblosxom instance, `initialize` method: calls the entry parser callback to get a map of all the entry types Pyblosxom can handle

   2. Pyblosxom instance, `run` method: calls the start callback to allow plugins to do any initialization they need to do

   3. Pyblosxom instance, `run` method: calls the handle callback allowing plugins to handle the request

      If a plugin handles the request, the plugin should return a `1` signifying it has handled the request and Pyblosxom should stop. FINISHED.

      If no plugin handles the request, then we continue using the `blosxom_handler`.

   4. Pyblosxom instance, `run` method: calls the end callback to allow plugins to do any cleanup they need to do.

FIXME - add lifecycle for long-running processes through WSGI—it’s slightly different.

## Lifecycle of the blosxom_handler

This describes what the `blosxom_handler` does. This is the default handler for Pyblosxom. It’s called by the Pyblosxom instance in the run method if none of the plugins have handled the request already.

1. Calls the `renderer` callback to get a renderer instance.

   If none of the plugins return a `Renderer` instance, then Pyblosxom checks to see if the `renderer` property is set in `config.py`.

   If there `renderer` is specified, Pyblosxom instantiates that.

   If there `renderer` is not specified, Pyblosxom uses the `blosxom` renderer in the `renderer` package.

2. Calls the `pathinfo` callback which allows all plugins to help figure out what to do with the HTTP URI/QUERYSTRING that’s been requested.

3. Calls the `filelist` callback which returns a list of entries to render based on what the pathinfo is.

4. Calls the `prepare` callback which allows plugins to transform the entries and any other data in the `Request` object prior to rendering.

5. Renders the entries.

## Lifecycle of the blosxom renderer

The blosxom renderer renders the entries in a similar fashion to what Blosxom does. The blosxom renderer uses flavour templates and template variables. It also has a series of callbacks allowing plugins to modify templates and entry data at the time of rendering that specific piece.

1. Renders the `content_type` template.
2. Calls the `head` callback and then renders the `head` template.
3. Calls the `date_head` callback and renders the `date_head` template.
4. For each entry:
   1. If the date of this entry’s mtime is different than the last entry, call the `date_foot` callback and render the `date_foot` template. Then call the `date_head` callback and render the `date_head` template.
   2. Call the `story` callback and render the `story` template.
5. Call the `date_foot` callback and render the `date_foot` template.
6. Call the `foot` callback and render the `foot` template.

## About callbacks

Callbacks allow plugins to override behavior in Pyblosxom or provide additional behavior. The callback mechanism actually encompasses a series of different functions. Callbacks can act as handlers, as notifiers, and also as modifiers.

### Types of callbacks

In the case of handler callbacks, Pyblosxom will query each plugin implementing the callback until one of the plugins returns that it has handled the callback. At that point, execution of handling code stops. If none of the plugins handle the callback, then Pyblosxom will run its default behavior code.

In the case of notifier callbacks, Pyblosxom will notify each plugin implementing the callback regardless of return values.

In the case of modifier callbacks, Pyblosxom will query each plugin implementing the callback passing in some input. It takes the output from the callback function and passes that in as input to the next callback function. In this way, each plugin has a chance to modify and transform the data.

There’s no reason you can’t implement a handler-type callback and use it for notification purposes—that’s fine. You should know that in the case of handler callbacks and modifier callbacks, the return value that your plugin gives will affect Pyblosxom’s execution.

### Callbacks that have blosxom equivalents

There are a series of callbacks in Pyblosxom that have equivalents in blosxom 2.0. The names are sometimes different and in most cases the arguments the Pyblosxom versions take are different than the blosxom 2.0 versions. Even so, the Pyblosxom versions serve the same purpose as the blosxom 2.0 versions.

This isn’t very interesting unless you’re trying to implement the functionality of a blosxom 2.0 plugin in Python for Pyblosxom.

The available blosxom renderer callbacks are:

- cb_head - corresponds to blosxom 2.0 head
- cb_date_head - corresponds to blosxom 2.0 date
- cb_story - corresponds to blosxom 2.0 story
- cb_foot - corresponds to blosoxm 2.0 foot

Additionally, we have these lifecycle callbacks available:

- the blosxom 2.0 entries callback is handled by cb_filelist
- the blosxom 2.0 filter callback is handled by cb_prepare
- the blosxom 2.0 sort callback can sort of be handled by cb_prepare depending on what you’re trying to do

## Callbacks

### cb_prepare

The prepare callback is called in the default blosxom handler after we’ve figured out what we’re rendering and before we actually go to the renderer.

Plugins should implement `cb_prepare` to modify the data dict which is in the Request. Inside the data dict is `entry_list` (amongst other things) which holds the list of entries to be renderered (in the order they will be rendered).

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback can return whatever they want—it doesn’t affect the callback chain.

Example of a `cb_prepare` function in a plugin:

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    def cb_prepare(args):
        """
        This plugin shows the number of entries we are going to render and
        place the result in $entrycount
        """
        request = args['request']
        data = request.get_data()
        config = request.get_configuration()

        # Can anyone say Ternary? :)
        IF = lambda a, b, c: (a() and [b()] or [c()])[0]

        num_entry = config['num_entries']
        entries = len(data['entry_list'])

        data['entrycount'] = IF(num_entry > entries, num_entry, entries)

### cb_logrequest

The logrequest callback is used to notify plugins of the current Pyblosxom request for the purposes of logging.

Functions that implement this callback will get an args dict containing:

`filename`
  a filename; typically a base filename
`return_code`
  an HTTP error code (e.g. 200, 404, 304, ...)
`request`
  a Request object

Functions that implement this callback can return whatever they want—it doesn’t affect the callback chain.

`cb_logrequest` is called after rendering and will contain all the modifications to the Request object made by the plugins.

An example input args dict is like this:

    {'filename': filename, 'return_code': '200', 'request': Request()}

### cb_filelist

The filelist callback allows plugins to generate the list of entries to be rendered. Entries should be EntryBase derivatives—either by instantiating EntryBase, FileEntry, or creating your own EntryBase subclass.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback should return `None` if they don’t plan on generating the entry list or a list of entries if they do. When a function returns `None`, the callback will continue to the next function to see if it will return a list of entries. When a function returns a list of entries, the callback will stop.

### cb_sortlist

The sortlist callback allows plugins to implement their own sorting of entries. This callback gets called by filelist handlers.

Functions that implement this callback will get an args dict containing:

`request`
  A Request object
`entry_list`
  The list of entries to be sorted.

Return `None` if the function doesn’t sort the list. Return the sorted list if the function does sort the list.

Example of a `cb_sortlist` function:

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    def cb_sortlist(args):
        """Sorts the list from oldest (beginning) to newest (end)
        for a site that's less like a blog and more like a
        journal.
        """
        entrylist = args["entry_list"]

        entrylist = [(e._mtime, e) for e in entrylist]
        entrylist.sort()
        entrylist = [e[1] for e in entrylist]

        return entrylist

### cb_truncatelist

The truncatelist callback allows plugins to implement their own truncation rules. This callback gets called by filelist handlers.

Functions that implement this callback will get an args dict containing:

`request`
  A Request object
`entry_list`
  The list of entries to be truncated.

Return `None` if the function doesn’t truncate the list. Return the new list if the function does truncate the list.

Example of a `cb_truncatelist` function:

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    def cb_truncatelist(args):
        request = args["request"]
        entrylist = args["entry_list"]

        data = request.data
        config = request.config

        num_entries = config.get("num_entries", 5)
        truncate = data.get("truncate", 0)
        if num_entries and truncate:
            entrylist = entrylist[:num_entries]
            return entrylist

        return None

### cb_filestat

The filestat callback allows plugins to provide mtimes for entries. Plugins may use this to override the mtime stored in the filesystem. For example, one of the contributed plugins uses this to set the mtime to the time specified in the entry’s filename.

Plugins may also use this to provide a cheaper alternative to filesystem stat calls—a notorious performance drag. The hardcodedates plugin, for example, stores mtimes in a file: it reads the file once at startup then returns mtimes from its in-memory database.

Functions that implement this callback will get an args dict containing:

`filename`
  the filename of the entry
`mtime`
  the result of an `os.stat` on the filename of the entry

Functions that implement this callback must return the input args dict whether or not they adjust anything in it. The callback chain will stop as soon as a callback modifies mtime. If no plugin handles the callback, Pyblosxom will fall back to calling `os.stat()`.

### cb_pathinfo

The pathinfo callback allows plugins to parse the HTTP `PATH_INFO` item. This item is stored in the http dict of the Request object. Functions would parse this as they desire, then set the following variables in the data dict of the Request object:

`bl_type`
  `dir` or `file`
`pi_bl`
  typically the same as `PATHINFO`
`pi_yr`
  the year in yyyy format
`pi_mo`
  the month in mm or mmm format (e.g. 02, Jan, Feb, ...)
`pi_da`
  the day of the month in dd format
`root_datadir`
  full path to the entry folder or entry file on the file system
`flavour`
  the flavour gathered from this URL

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback should make the modifications to the data dict in place—no need to return anything.

### cb_commandline

The commandline callback allows plugins to implement additional `pyblosxom-cmd` commands. This allows a plugin to expose maintenance and setup functionality to the user at the command line or through cron.

For example. if you wrote a plugin that built an map of tags to entries that used that tag, you’d probably want to write a command that updates the index which the user could create a cron job for.

The `cb_commandline` function takes a single `args` argument which is a map of command -> tuple of handler and help text. It then returns the args dict.

For example:

    def cb_commandline(args):
        args["printargs"] = (printargs, "prints command line arguments")

See Writing a plugin that adds a commandline command for more details.

### cb_renderer

The renderer callback allows plugins to specify a renderer to use by returning a renderer instance to use. If no renderer is specified, we use the default blosxom renderer.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback should return `None` if they don’t want to specify a renderer or the renderer object instanct if they do. When a function returns a renderer instance, processing stops.

### cb_entryparser

The entryparser callback allows plugins to register the entryparsers they have. Entry parsers are linked with a filename extension. For example, the default blosxom text entry parser will be used for any file ending in `.txt`.

Functions that implement this callback will get the entryparser dict consisting of file extension -> entry parsing function pairs.

Functions that implement this callback should return the entryparser dict after modifying it.

Example:

    def cb_entryparser(entryparsingdict):
        entryparsingdict["txtl"] = txtl_parse
        return entryparsingdict

Then the plugin would define `txtl_parse` which takes a filename and a Request and returns an entrydata dict with `title` and `body` (or whatever the templates need to render this entry).

See Writing an entryparser.

### cb_preformat

The preformat callback acts in conjunction with the entryparser that handled the entry to do a two-pass formatting of the entry.

Functions that implement `cb_preformat` are text transformation tools. Once one of them returns a transformed entry, then we stop processing.

Functions that implement this callback will get an args dict containing:

`parser`
  a string that indicates whether a preformatter should run
`story`
  the list of lines of the blog post with `\n` included
`request`
  a Request object

Functions that implement this callback should return None if they didn’t modify the story or a single story string.

See Writing a preformatter plugin.

### cb_postformat

The postformat callback allows plugins to make further modifications to entry text. It typically gets called after a preformatter by the entryparser. It can also be used to add additional properties to entries. The changes from postformat functions are saved in the cache (if the user has caching enabled). As such, this shouldn’t be used for dynamic data like comment counts.

Examples of usage:

- adding a word count property to the entry
- using a macro replacement plugin (Radio Userland glossary)
- acronym expansion
- a ‘more’ text processor
- ...

Functions that implement this callback will get an args dict containing:

`entry_data`
  a dict that minimally contains `title` and `story`
`request`
  a Request object

Functions that implement this callback don’t need to return anything—modifications to the `entry_data` dict are done in place.

See Writing a postformatter plugin.

### cb_start

The start callback allows plugins to execute startup/initialization code. Use this callback for any setup code that your plugin needs, like:

- reading saved data from a file
- checking to make sure configuration variables are set
- allocating resources

Note

`cb_start` is different in Pyblosxom than in blosxom.

The `cb_start` callback is slightly different than in blosxom in that `cb_start` is called for every Pyblosxom request regardless of whether it’s handled by the default blosxom handler. In general, it’s better to delay allocating resources until you absolutely know you are going to use them.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback don’t need to return anything.

### cb_end

The start callback allows plugins to execute teardown/cleanup code, save any data that hasn’t been saved, clean up temporary files, and otherwise return the system to a normal state.

Examples of usage:

- save data to a file
- clean up any temporary files
- ...

Functions that implement this callback will get an args dict containing:

`request`
  a Request object

Functions that implement this callback don’t need to return anything.

Note

`cb_end` is different in Pyblosxom than in blosxom

The `cb_end` callback is called for every Pyblosxom request regardless of whether it’s handled by the default blosxom handler or not. This is slightly different than blosxom.

### cb_staticrender_filelist

Gives plugins a chance to modify the list of (url, query) tuples that are about to be rendered statically. Plugins can add additional tuples, remove tuples, modify tuples, ...

Functions that implement this callback will get an args dict containing:

`request`
  a request object
`filelist`
  list of (url, query) tuples of all urls to be rendered
`flavours`
  list of flavours to be rendered
`incremental`
  whether (true) or not (false) this is an incremental static rendering run

Functions that implement this callback can modify the filelist in-place and don’t have to return anything.

Example in which the plugin adds the search page url so that the search page gets rendered:

    def cb_staticrender_filelist(args):
        filelist = args["filelist"]
        filelist.append(("/search", ""))

### cb_head

The head callback is called before a head flavour template is rendered.

`cb_head` is called before the variables in the entry are substituted into the template. This is the place to modify the head template based on the entry content. You can also set variables on the entry that will be used by the `cb_story` or `cb_foot` templates. You have access to all the content variables via entry.

Blosxom 2.0 calls this callback `head`.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object
`renderer`
  the `BlosxomRenderer` instance that called the callback
`entry`
  the entry to be rendered
`template`
  a string containing the flavour template to be processed

Functions that implement this callback must return the input args dict whether or not they adjust anything in it.

Example in which we add the number of entries being rendered to the `$blog_title` variable:

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    def cb_head(args):
        request = args["request"]
        config = request.get_configuration()
        data = request.get_data()

        num_entries = len(data.get("entry_list", []))
        bt = config.get("blog_title", "")
        config["blog_title"] = bt + ": %d entries" % num_entries

        return args

### cb_date_head

The `date_head` callback is called before a `date_head` flavour template is rendered.

`cb_date_head` is called before the variables in the entry are substituted into the template. This is the place to modify the `date_head` template based on the entry content. You have access to all the content variables via entry.

Blosxom 2.0 calls this callback `date`.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object
`renderer`
  the `BlosxomRenderer` instance that called the callback
`entry`
  the entry to be rendered
`template`
  a string containing the flavour template to be processed

Functions that implement this callback must return the input args dict whether or not they adjust anything in it.

### cb_story

The `story` callback gets called before the entry is rendered.

The template used is typically the `story` template, but we allow entries to override this if they have a `template` property. If they have the `template` property, then we’ll use the template of that name instead.

`cb_story` is called before the variables in the entry are substituted into the template. This is the place to modify the `story` template based on the entry content. You have access to all the content variables via entry.

Blosxom 2.0 calls this callback `story`.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object
`renderer`
  the `BlosxomRenderer` that called the callback
`entry`
  the entry to be rendered
`template`
  a string containing the flavour template to be processed

Functions that implement this callback must return the input args dict whether or not they adjust anything in it.

Example which adds a `guid` variable to the entry which is available to use in the story template. The `guid` can be manually set by the user in the entry metadata and if it’s not there, then it defaults to the `file_path` value:

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    def cb_story(args):
        # grab the entry from the args dict.
        entry = args["entry"]

        # set the "guid" variable in the entry.  it's either the "guid"
        # value (if the user set one in the entry metadata) or it's
        # whatever the value of "file_path" is.
        entry["guid"] = entry.get("guid", entry.get("file_path"))

        # the cb_story function _must_ return the args dict.
        return args

### cb_story_end

The `story_end` callback is is called after the variables in the entry are substituted into the template. You have access to all the content variables via entry.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object
`renderer`
  the `BlosxomRenderer` instance that called the callback
`entry`
  the entry object to be rendered
`template`
  a string containing the flavour template to be processed

Functions that implement this callback must return the input args dict whether or not they adjust anything in it.

### cb_foot

The `foot` callback is called before the variables in the entry are substituted into the foot template. This is the place to modify the `foot` template based on the entry content. You have access to all the content variables via entry.

Blosxom 2.0 calls this callback `foot`.

Functions that implement this callback will get an args dict containing:

`request`
  a Request object
`renderer`
  the `BlosxomRenderer` instance that called the callback
`entry`
  the entry to be rendered
`template`
  a string containing the flavour template to be processed

Functions that implement this callback must return the input args dict whether or not they adjust anything in it.

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