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author	akr <akr@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>	2008-01-04 09:41:44 +0000
committer	akr <akr@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>	2008-01-04 09:41:44 +0000
commit	b3e2ea9fa29e114e12affd9402f335aeacca1c3c (patch)
tree	b239f1e3c0937bee463ac02e4a31a6a9224f0e27 /bin/testrb
parent	768afa284479d7d586263d58273355c58aee224d (diff)

parenthesize macro arguments.

git-svn-id: http://svn.ruby-lang.org/repos/ruby/trunk@14882 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Diffstat (limited to 'bin/testrb')

0 files changed, 0 insertions, 0 deletions

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# Authors:
#   Jason Gerard DeRose <jderose@redhat.com>
#
# Copyright (C) 2008  Red Hat
# see file 'COPYING' for use and warranty information
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation; version 2 only
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA


'''
Package containing the core library.

=============================
 Tutorial for Plugin Authors
=============================

This tutorial will introduce you to writing plugins for freeIPA v2. It does
not cover every detail, but it provides enough to get you started and is
heavily cross-referenced with further documentation that (hopefully) fills
in the missing details.

In addition to this tutorial, the many built-in plugins in `ipalib.plugins`
and `ipaserver.plugins` provide real-life examples of how to write good
plugins.


----------------------------
How this tutorial is written
----------------------------

The code examples in this tutorial are presented as if entered into a Python
interactive interpreter session.  As such, when you create a real plugin in
a source file, a few details will be different (in addition to the fact that
you will never include the ``>>>`` nor ``...`` that the interpreter places at
the beginning of each line of code).

The tutorial examples all have this pattern:

    ::

        >>> from ipalib import Command, create_api
        >>> api = create_api()
        >>> class my_command(Command):
        ...     pass
        ...
        >>> api.register(my_command)
        >>> api.finalize()

In the tutorial we call `create_api()` to create an *example* instance
of `plugable.API` to work with.  But a real plugin will simply use
``ipalib.api``, the standard run-time instance of `plugable.API`.

A real plugin will have this pattern:

    ::

        from ipalib import Command, api

        class my_command(Command):
            pass
        api.register(my_command)

As seen above, also note that in a real plugin you will *not* call
`plugable.API.finalize()`.  When in doubt, look at some of the built-in
plugins for guidance, like those in `ipalib.plugins`.

If you don't know what the Python *interactive interpreter* is, or are
confused about what this *Python* is in the first place, then you probably
should start with the Python tutorial:

    http://docs.python.org/tutorial/index.html


------------------------------------
First steps: A simple command plugin
------------------------------------

Our first example will create the most basic command plugin possible.  This
command will be seen in the list of command plugins, but it wont be capable
of actually doing anything yet.

A command plugin simultaneously adds a new command that can be called through
the command-line ``ipa`` script *and* adds a new XML-RPC method... the two are
one in the same, simply invoked in different ways.

A freeIPA plugin is a Python class, and when you create a plugin, you register
this class itself (instead of an instance of the class).  To be a command
plugin, your plugin must subclass from `frontend.Command` (or from a subclass
thereof).  Here is our first example:

>>> from ipalib import Command, create_api
>>> api = create_api()
>>> class my_command(Command): # Step 1, define class
...     """My example plugin."""
...
>>> api.register(my_command) # Step 2, register class

Notice that we are registering the ``my_command`` class itself, not an
instance of ``my_command``.

Until `plugable.API.finalize()` is called, your plugin class has not been
instantiated nor does the ``Command`` namespace yet exist.  For example:

>>> hasattr(api, 'Command')
False
>>> api.finalize() # plugable.API.finalize()
>>> hasattr(api.Command, 'my_command')
True
>>> api.Command.my_command.doc
'My example plugin.'

Notice that your plugin instance is accessed through an attribute named
``my_command``, the same name as your plugin class name.


------------------------------
Make your command do something
------------------------------

This simplest way to make your example command plugin do something is to
implement a ``run()`` method, like this:

>>> class my_command(Command):
...     """My example plugin with run()."""
...
...     def run(self):
...         return 'My run() method was called!'
...
>>> api = create_api()
>>> api.register(my_command)
>>> api.finalize()
>>> api.Command.my_command() # Call your command
'My run() method was called!'

When `frontend.Command.__call__()` is called, it first validates any arguments
and options your command plugin takes (if any) and then calls its ``run()``
method.


------------------------
Forwarding vs. execution
------------------------

However, unlike the example above, a typical command plugin will implement an
``execute()`` method instead of a ``run()`` method.  Your command plugin can
be loaded in two distinct contexts:

    1. In a *client* context - Your command plugin is only used to validate
       any arguments and options it takes, and then ``self.forward()`` is
       called, which forwards the call over XML-RPC to an IPA server where
       the actual work is done.

    2. In a *server* context - Your same command plugin validates any
       arguments and options it takes, and then ``self.execute()`` is called,
       which you should implement to perform whatever work your plugin does.

The base `frontend.Command.run()` method simply dispatches the call to
``self.execute()`` if ``self.env.in_server`` is True, or otherwise
dispatches the call to ``self.forward()``.

For example, say you have a command plugin like this:

>>> class my_command(Command):
...     """Forwarding vs. execution."""
...
...     def forward(self):
...         return 'in_server=%r; forward() was called.' % self.env.in_server
...
...     def execute(self):
...         return 'in_server=%r; execute() was called.' % self.env.in_server
...

If ``my_command`` is loaded in a *client* context, ``forward()`` will be
called:

>>> api = create_api()
>>> api.env.in_server = False # run() will dispatch to forward()
>>> api.register(my_command)
>>> api.finalize()
>>> api.Command.my_command() # Call your command plugin
'in_server=False; forward() was called.'

On the other hand, if ``my_command`` is loaded in a *server* context,
``execute()`` will be called:

>>> api = create_api()
>>> api.env.in_server = True # run() will dispatch to execute()
>>> api.register(my_command)
>>> api.finalize()
>>> api.Command.my_command() # Call your command plugin
'in_server=True; execute() was called.'

Normally there should be no reason to override `frontend.Command.forward()`,
but, as above, it can be done for demonstration purposes.  In contrast, there
*is* a reason you might want to override `frontend.Command.run()`: if it only
makes sense to execute your command locally, if it should never be forwarded
to the server.  In this case, you should implement your *do-stuff* in the
``run()`` method instead of in the ``execute()`` method.

For example, the ``ipa`` command line script has a ``help`` command
(`ipalib.cli.help`) that is specific to the command-line-interface and should
never be forwarded to the server.


---------------
Backend plugins
---------------

There are two types of plugins:

    1. *Frontend plugins* - These are loaded in both the *client* and *server*
       contexts.  These need to be installed with any application built atop
       the `ipalib` library.  The built-in frontend plugins can be found in
       `ipalib.plugins`.  The ``my_command`` example above is a frontend
       plugin.

    2. *Backend plugins* - These are only loaded in a *server* context and
       only need to be installed on the IPA server.  The built-in backend
       plugins can be found in `ipaserver.plugins`.

Backend plugins should provide a set of methods that standardize how IPA
interacts with some external system or library.  For example, all interaction
with LDAP is done through the ``ldap`` backend plugin defined in
`ipaserver.plugins.b_ldap`.  As a good rule of thumb, anytime you need to
import some package that is not part of the Python standard library, you
should probably interact with that package via a corresponding backend
plugin you implement.

Backend plugins are much more free-form than command plugins.  Aside from a
few reserved attribute names, you can define arbitrary public methods on your
backend plugin (in contrast, frontend plugins get wrapped in a
`plugable.PluginProxy`, which allow access to only specific attributes on the
frontend plugin).

Here is a simple example:

>>> from ipalib import Backend
>>> class my_backend(Backend):
...     """My example backend plugin."""
...
...     def do_stuff(self):
...         """Part of your API."""
...         return 'Stuff got done.'
...
>>> api = create_api()
>>> api.register(my_backend)
>>> api.finalize()
>>> api.Backend.my_backend.do_stuff()
'Stuff got done.'


-------------------------------
How your command should do work
-------------------------------

We now return to our ``my_command`` plugin example.

Plugins are separated into frontend and backend plugins so that there are not
unnecessary dependencies required by an application that only uses `ipalib` and
its built-in frontend plugins (and then forwards over XML-RPC for execution).

But how do we avoid introducing additional dependencies?  For example, the
``user_add`` command needs to talk to LDAP to add the user, yet we want to
somehow load the ``user_add`` plugin on client machines without requiring the
``python-ldap`` package (Python bindings to openldap) to be installed.  To
answer that, we consult our golden rule:

  **The golden rule:** A command plugin should implement its ``execute()``
  method strictly via calls to methods on one or more backend plugins.

So the module containing the ``user_add`` command does not itself import the
Python LDAP bindings, only the module containing the ``ldap`` backend plugin
does that, and the backend plugins are only installed on the server.  The
``user_add.execute()`` method, which is only called when in a server context,
is implemented as a series of calls to methods on the ``ldap`` backend plugin.

When `plugable.Plugin.set_api()` is called, each plugin stores a reference to
the `plugable.API` instance it has been loaded into.  So your plugin can
access the ``my_backend`` plugin as ``self.api.Backend.my_backend``.

Additionally, convenience attributes are set for each namespace, so your
plugin can also access the ``my_backend`` plugin as simply
``self.Backend.my_backend``.

This next example will tie everything together.  First we create our backend
plugin:

>>> api = create_api()
>>> api.env.in_server = True # We want to execute, not forward
>>> class my_backend(Backend):
...     """My example backend plugin."""
...
...     def do_stuff(self):
...         """my_command.execute() calls this."""
...         return 'my_backend.do_stuff() indeed did do stuff!'
...
>>> api.register(my_backend)

Second, we have our frontend plugin, the command:

>>> class my_command(Command):
...     """My example command plugin."""
...
...     def execute(self):
...         """Implemented against Backend.my_backend"""
...         return self.Backend.my_backend.do_stuff()
...
>>> api.register(my_command)

Lastly, we call ``api.finalize()`` and see what happens when we call
``my_command()``:

>>> api.finalize()
>>> api.Command.my_command()
'my_backend.do_stuff() indeed did do stuff!'

When not in a server context, ``my_command.execute()`` never gets called, so
it never tries to access the non-existent backend plugin at
``self.Backend.my_backend.``  To emphasize this point, here is one last
example:

>>> api = create_api()
>>> api.env.in_server = False # We want to forward, not execute
>>> class my_command(Command):


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