web2py

Components and plugins

Components and plugins are relatively new features of web2py, and there is some disagreement between developers about what they are and what they should be. Most of the confusion stems from the different uses of these terms in other software projects and from the fact that developers are still working to finalize the specifications.

However, plugin support is an important feature and we need to provide some definitions. These definitions are not meant to be final, just consistent with the programming patterns we want to discuss in this chapter.

We will try to address two issues here:

• How can we build modular applications that minimize server load and maximize code reuse?
• How can we distribute pieces of code in a more or less plugin-and-play fashion?

Components address the first issue; plugins address the second.

Components

A component is a functionally autonomous part of a web page.

A component may be composed of modules, controllers and views, but there is no strict requirement other than, when embedded in a web page, it must be localized within an html tag (for example a DIV, a SPAN, or an IFRAME) and it must perform its task independently of the rest of the page. We are specifically interested in components that are loaded in the page and communicate with the component controller function via Ajax.

An example of a component is a "comments component" that is contained into a DIV and shows users' comments and a post-new-comment form. When the form is submitted, it is sent to the server via Ajax, the list is updated, and the comment is stored server-side in the database. The DIV content is refreshed without reloading the rest of the page.

The web2py LOAD function makes this easy to do without explicit JavaScript/Ajax knowledge or programming.

Our goal is to be able to develop web applications by assembling components into page layouts.

Consider a simple web2py app "test" that extends the default scaffolding app with a custom model in file "models/db_comments.py":

db.define_table('comment_post',
   Field('body','text',label='Your comment'),
   auth.signature)

one action in "controllers/comments.py"

@auth.requires_login()
def post():
    return dict(form=SQLFORM(db.comment_post).process(),
                comments=db(db.comment_post).select())

and the corresponding "views/comments/post.html"

{{extend 'layout.html'}}
{{for post in comments:}}
<div class="post">
  On {{=post.created_on}} {{=post.created_by.first_name}}
  says <span class="post_body">{{=post.body}}</span>
</div>
{{pass}}
{{=form}}

You can access it as usual at:

http://127.0.0.1:8000/test/comments/post

So far there is nothing special in this action, but we can turn it into a component by defining a new view with extension ".load" that does not extend the layout.

Hence we create a "views/comments/post.load":

{{for post in comments:}}
<div class="post">
  On {{=post.created_on}} {{=post.created_by.first_name}}
  says <blockquote class="post_body">{{=post.body}}</blockquote>
</div>
{{pass}}
{{=form}}

We can access it at the URL

http://127.0.0.1:8000/test/comments/post.load

This is a component that we can embed into any other page by simply doing

{{=LOAD('comments','post.load',ajax=True)}}

For example in "controllers/default.py" we can edit

def index():
    return dict()

and in the corresponding view add the component:

{{extend 'layout.html'}}
{{=LOAD('comments','post.load',ajax=True)}}

Visiting the page

http://127.0.0.1:8000/test/default/index

will show the normal content and the comments component:

The {{=LOAD(...)}} component is rendered as follows:

<script type="text/javascript"><!--
web2py_component("/test/comment/post.load","c282718984176")
//--></script><div id="c282718984176">loading...</div>

(the actual generated code depends on the options passed to the LOAD function).

The web2py_component(url,id) function is defined in "web2py_ajax.html" and it performs all the magic: it calls the url via Ajax and embeds the response into the DIV with corresponding id; it traps every form submission into the DIV and submits those forms via Ajax. The Ajax target is always the DIV itself.

The full signature of the LOAD helper is the following:

LOAD(c=None, f='index', args=[], vars={},
     extension=None, target=None,
     ajax=False, ajax_trap=False,
     url=None,user_signature=False,
     timeout=None, times=1,
     content='loading...',**attr):

Here:

• the first two arguments c and f are the controller and the function that we want to call respectively.
• args and vars are the arguments and variables that we want to pass to the function. The former is a list, the latter is a dictionary.
• extension is an optional extension. Notice that the extension can also be passed as part of the function as in f='index.load'.
• target is the id of the target DIV. If it is not specified a random target id is generated.
• ajax should be set to True if the DIV has to be filled via Ajax and to False if the DIV has to be filled before the current page is returned (thus avoiding the Ajax call).
• ajax_trap=True means that any form submission in the DIV must be captured and submitted via Ajax, and the response must be rendered inside the DIV. ajax_trap=False indicates that forms must be submitted normally, thus reloading the entire page. ajax_trap is ignored and assumed to be True if ajax=True.
• url, if specified, overrides the values of c, f, args, vars, and extension and loads the component at the url. It is used to load as components pages served by other applications (which my or may not be created with web2py).
• user_signature defaults to False but, if you are logged in, it should be set to True. This will make sure the ajax callback is digitally signed. This is documented in chapter 4.
• times specifies how many times the component is to be requested. Use "infinity" to keep loading the component continuously. This option is useful for triggering regular routines for a given document request.
• timeout sets the time to wait in milliseconds before starting the request or the frequency if times is greater than 1.
• content is the content to be displayed while performing the ajax call. It can be a helper as in content=IMG(..).
• optional **attr (attributes) can be passed to the contained DIV.

If no .load view is specified, there is a generic.load that renders the dictionary returned by the action without layout. It works best if the dictionary contains a single item.

If you LOAD a component having the .load extension and the corresponding controller function redirects to another action (for example a login form), the .load extension propagates and the new url (the one to redirect too) is also loaded with a .load extension.

If you call an action via Ajax and you want the action to force a redirect of the parent page you can do it with

redirect(url,type='auto')

Because Ajax post does not support multipart forms, i.e. file uploads, upload fields will not work with the LOAD component. You could be fooled into thinking it would work because upload fields will function normally if POST is done from the individual component's .load view. Instead, uploads are done with ajax-compatible 3rd-party widgets and web2py manual upload store commands.

Client-Server component communications

When the action of a component is called via Ajax, web2py passes two HTTP headers with the request:

web2py-component-location
web2py-component-element

which can be accessed by the action via the variables:

request.env.http_web2py_component_location
request.env.http_web2py_component_element

The latter is also accessible via:

request.cid

The former contains the URL of the page that called the component action. The latter contains the id of the DIV that will contain the response.

The component action can also store data in two special HTTP response headers that will be interpreted by the full page upon response. They are:

web2py-component-flash
web2py-component-command

and they can be set via:

response.headers['web2py-component-flash']='....'
response.headers['web2py-component-command']='...'

or (if the action is called by a component) automatically via:

response.flash='...'
response.js='...'

The former contains text that you want to be flashed upon response. The latter contains JavaScript code that you want to be executed upon response. It cannot contain newlines.

As an example, let's define a contact form component in "controllers/contact/ask.py" that allows the user to ask a question. The component will email the question to the system administrator, flash a "thank you" message, and remove the component from the page:

def ask():
    form=SQLFORM.factory(
        Field('your_email',requires=IS_EMAIL()),
        Field('question',requires=IS_NOT_EMPTY()))
    if form.process().accepted:
        if mail.send(to='[email protected]',
                  subject='from %s' % form.vars.your_email,
                  message = form.vars.question):
            response.flash = 'Thank you'
            response.js = "jQuery('#%s').hide()" % request.cid
        else:
            form.errors.your_email = "Unable to send the email"
    return dict(form=form)

The first four lines define the form and accept it. The mail object used for sending is defined in the default scaffolding application. The last four lines implement all the component-specific logic by getting data from the HTTP request headers and setting the HTTP response headers.

Now you can embed this contact form in any page via

{{=LOAD('contact','ask.load',ajax=True)}}

Notice that we did not define a .load view for our ask component. We do not have to because it returns a single object (form) and therefore the "generic.load" will do just fine. Remember that generic views are a development tool. In production you should copy "views/generic.load" into "views/contact/ask.load".

{{=LOAD('contact','ask.load',ajax=True,user_signature=True)}}

which add a digital signature to the URL. The digital signature must then be validated using a decorator in the callback function:

@auth.requires_signature()
def ask(): ...

Trapped Ajax links

Normally a link is not trapped, and by clicking in a link inside a component, the entire linked page is loaded. Sometimes you want the linked page to be loaded inside the component. This can be achieved using the A helper:

{{=A('linked page',_href='http://example.com',cid=request.cid)}}

If cid is specified, the linked page is loaded via Ajax. The cid is the id of the html element where to place the loaded page content. In this case we set it to request.cid, i.e. the id of the component that generates the link. The linked page can be and usually is an internal URL generated using the URL command.

Plugins

A plugin is any subset of the files of an application.

and we really mean any:

• A plugin is not a module, is not a model, it is not a controller, is not a view, yet it may contain modules, models, controllers and/or views.
• A plugin does not need to be functionally autonomous and it may depend on other plugins or specific user code.
• A plugin is not a plugins system and therefore has no concept of registration nor isolation, although we will give rules to try to achieve some isolation.
• We are talking about a plugin for your app, not a plugin for web2py.

So why is it called a plugin? Because it provides a mechanism for packing a subset of an app and unpacking it over another app (i.e. plug-in). Under this definition, any file in your app can be treated as a plugin.

When the app is distributed, its plugins are packed and distributed with it.

In practice, the admin provides an interface for packing and unpacking plugins separately from your app. Files and folder of your application that have names with the prefix plugin_name can be packed together into a file called:

web2py.plugin.name.w2p

and distributed together.

The files that compose a plugin are not treated by web2py any differently than other files except that admin understands from their names that they are meant to be distributed together, and it displays them in a separate page:

Yet as a matter of fact, by the definition above, these plugins are more general than those recognized as such by admin.

In practice we will only be concerned with two types of plugins:

• Component Plugins. These are plugins that contain components as defined in the previous section. A component plugin can contain one or more components. We can think for example of a plugin_comments that contains the comments component proposed above. Another example could be plugin_tagging that contains a tagging component and a tag-cloud component that share some database tables also defined by the plugin.
• Layout Plugins. These are plugins that contain a layout view and the static files required by such layout. When the plugin is applied it gives the app a new look and feel.

By the above definitions, the components created in the previous section, for example "controllers/contact.py", are already plugins. We can move them from one app to another and use the components they define. Yet they are not recognized as such by admin because there is nothing that labels them as plugins. So there are two problems we need to solve:

• Name the plugin files using a convention, so that admin can recognize them as belonging to the same plugin
• If the plugin has model files, establish a convention so that the objects it defines do not pollute the namespace and do not conflict with each other.

Let's assume a plugin is called name. Here are the rules that should be followed:

Rule 1: Plugin models and controllers should be called, respectively

• models/plugin_name.py
• controllers/plugin_name.py

and plugin views, modules, static, and private files should be in folders called, respectively:

• views/plugin_name/
• modules/plugin_name/
• static/plugin_name/
• private/plugin_name/

Rule 2: Plugin models can only define objects with names that start with

• plugin_name
• PluginName
• _

Rule 3: Plugin models can only define session variables with names that start with

• session.plugin_name
• session.PluginName

Rule 4: Plugins should include license and documentation. These should be placed in:

• static/plugin_name/license.html
• static/plugin_name/about.html

Rule 5: The plugin can only rely on the existence of the global objects defined in scaffolding "db.py", i.e.

• a database connection called db
• an Auth instance called auth
• a Crud instance called crud
• a Service instance called service

Some plugins may be more sophisticated and have a configuration parameter in case more than one db instance exists.

Rule 6: If a plugin needs configuration parameters, these should be set via a PluginManager as described below.

By following the above rules we can make sure that:

• admin recognizes all the plugin_name files and folder as part of a single entity.
• plugins do not interfere with each other.

The rules above do not solve the problem of plugin versions and dependencies. That is beyond our scope.

Component plugins

Component plugins are plugins that define components. Components usually access the database and define with their own models.

Here we turn the previous comments component into a comments_plugin by using the same code we wrote before, but following all of the previous rules.

First, we create a model called "models/plugin_comments.py":

db.define_table('plugin_comments_comment',
   Field('body','text', label='Your comment'),
   auth.signature)

def plugin_comments():
    return LOAD('plugin_comments','post',ajax=True)

(notice the last two lines define a function that will simplify the embedding of the plugin)

Second, we define a "controllers/plugin_comments.py"

def post():
    if not auth.user:
        return A('login to comment',_href=URL('default','user/login'))
    comment = db.plugin_comments_comment
    return dict(form=SQLFORM(comment).process(),
                comments=db(comment).select())

Third, we create a view called "views/plugin_comments/post.load":

{{for comment in comments:}}
<div class="comment">
  on {{=comment.created_on}} {{=comment.created_by.first_name}}
  says <span class="comment_body">{{=comment.body}}</span>
</div>
{{pass}}
{{=form}}

Now we can use admin to pack the plugin for distribution. Admin will save this plugin as:

web2py.plugin.comments.w2p

We can use the plugin in any view by simply installing the plugin via the edit page in admin and adding this to our own views

{{=plugin_comments()}}

Of course we can make the plugin more sophisticated by having components that take parameters and configuration options. The more complex the components, the more difficult it becomes to avoid name collisions. The Plugin Manager described below is designed to avoid this problem.

Plugin manager

The PluginManager is a class defined in gluon.tools. Before we explain how it works inside, we will explain how to use it.

Here we consider the previous comments_plugin and we make it better. We want to be able to customize:

db.plugin_comments_comment.body.label

without having to edit the plugin code itself.

Here is how we can do it:

First, rewrite the plugin "models/plugin_comments.py" in this way:

def _():
    from gluon.tools import PluginManager
    plugins = PluginManager('comments', body_label='Your comment')

    db.define_table('plugin_comments_comment',
        Field('body','text',label=plugins.comments.body_label),
        auth.signature)
    return lambda: LOAD('plugin_comments','post.load',ajax=True)
plugin_comments = _()

Notice how all the code except the table definition is encapsulated in a single function called _ so that it does not pollute the global namespace. Also notice how the function creates an instance of a PluginManager.

Now in any other model in your app, for example in "models/db.py", you can configure this plugin as follows:

from gluon.tools import PluginManager
plugins = PluginManager()
plugins.comments.body_label = T('Post a comment')

The plugins object is already instantiated in the default scaffolding app in "models/db.py"

The PluginManager object is a thread-level singleton Storage object of Storage objects. That means you can instantiate as many as you like within the same application but (whether they have the same name or not) they act as if there were a single PluginManager instance.

In particular each plugin file can make its own PluginManager object and register itself and its default parameters with it:

plugins = PluginManager('name', param1='value', param2='value')

You can override these parameters elsewhere (for example in "models/db.py") with the code:

plugins = PluginManager()
plugins.name.param1 = 'other value'

You can configure multiple plugins in one place.

plugins = PluginManager()
plugins.name.param1 = '...'
plugins.name.param2 = '...'
plugins.name1.param3 = '...'
plugins.name2.param4 = '...'
plugins.name3.param5 = '...'

When the plugin is defined, the PluginManager must take arguments: the plugin name and optional named arguments which are default parameters. However, when the plugins are configured, the PluginManager constructor must take no arguments. The configuration must precede the definition of the plugin (i.e. it must be in a model file that comes first alphabetically).

Layout plugins

Layout plugins are simpler than component plugins because usually they do not contain code, but only views and static files. Yet you should still follow good practice:

First, create a folder called "static/plugin_layout_name/" (where name is the name of your layout) and place all your static files there.

Second, create a layout file called "views/plugin_layout_name/layout.html" that contains your layout and links the images, CSS and JavaScript files in "static/plugin_layout_name/"

Third, modify the "views/layout.html" so that it simply reads:

{{extend 'plugin_layout_name/layout.html'}}
{{include}}

The benefit of this design is that users of this plugin can install multiple layouts and choose which one to apply simply by editing "views/layout.html". Moreover, "views/layout.html" will not be packed by admin together with the plugin, so there is no risk that the plugin will override the user's code in the previously installed layout.

Plugins repositories

While there is no single repository of web2py plugins you can find many of them at one of the following to URLs:

http://dev.s-cubism.com/web2py_plugins
http://web2py.com/plugins
http://web2py.com/layouts

Here is a screenshot from the s-cubism repository: