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diff --git a/docs/narr/configuration.rst b/docs/narr/configuration.rst index 11de4f741..08ca49710 100644 --- a/docs/narr/configuration.rst +++ b/docs/narr/configuration.rst @@ -1,48 +1,19 @@ -.. index:: - single: frameworks vs. libraries - single: framework - .. _configuration_narr: -Creating Your First :mod:`repoze.bfg` Application -================================================= - -Most of the logic in a web application is completely -application-specific. For example, the content of a web page served -by one web application might be a representation of the contents of an -accounting ledger, while the content of of a web page served by -another might be a listing of songs. These applications probably -won't serve the same set of customers. However, both the -ledger-serving and song-serving applications can be written using -:mod:`repoze.bfg`, because :mod:`repoze.bfg` is a very general -*framework* which can be used to create all kinds of web applications. -As a framework, the primary job of :mod:`repoze.bfg` is to make it -easier for a developer to create an arbitrary web application. - -.. sidebar:: Frameworks vs. Libraries - - A *framework* differs from a *library* in one very important way: - library code is always *called* by code that you write, while a - framework always *calls* code that you write. Using a set of - libraries to create an application is usually easier than using a - framework initially, because you can choose to cede control to - library code you have not authored very selectively. But when you - use a framework, you are required to cede a greater portion of - control to code you have not authored: code that resides in the - framework itself. You needn't use a framework at all to create a - web application using Python. A rich set of libraries already - exists for the platform. In practice, however, using a framework - to create an application is often more practical than rolling your - own via a set of libraries if the framework provides a set of - facilities that fits your application requirements. +.. index:: + single: application configuration + +Application Configuration +========================= Each deployment of an application written using :mod:`repoze.bfg` implies a specific *configuration* of the framework itself. For -example, a song-serving application might plug code into the framework -that manages songs, while the ledger- serving application might plug -in code that manages accounting information. :mod:`repoze.bfg` refers -to the way in which code is plugged in to it for a specific -application as "configuration". +example, an application which serves up MP3s for user consumption +might plug code into the framework that manages songs, while an +application that application might plug in code that manages +accounting information. :mod:`repoze.bfg` refers to the way in which +code is plugged in to it for a specific application as +"configuration". Most people understand "configuration" as coarse knobs that inform the high-level operation of a specific application deployment. For @@ -55,24 +26,25 @@ code into the :mod:`repoze.bfg` framework, you are "configuring" :mod:`repoze.bfg` for the purpose of creating a particular application deployment. -There are a number of different mechanisms you may use to configure +There are two different mechanisms you may use to configure :mod:`repoze.bfg` to create an application: *imperative* configuration and *declarative* configuration. We'll examine both modes in the sections which follow. .. index:: - pair: imperative; helloworld + pair: imperative; configuration -.. _helloworld_imperative: +.. _imperative_configuration: -Hello World, Configured Imperatively ------------------------------------- +Imperative Configuration +------------------------ -Experienced Python programmers might find that "imperative" -configuration is easiest to use. This is the configuration mode in -which a developer cedes the least amount of control to the framework; -it's "imperative" because you express the configuration directly in -Python code. +Experienced Python programmers might find that performing +configuration "imperatively" fits their brain best. This is the +configuration mode in which a developer cedes the least amount of +control to the framework; it's "imperative" because you express the +configuration directly in Python code, and you have the full power of +Python at your disposal as you perform configuration statements. Here's one of the simplest :mod:`repoze.bfg` applications, configured imperatively: @@ -87,491 +59,45 @@ imperatively: def hello_world(request): return Response('Hello world!') - def goodbye_world(request): - return Response('Goodbye world!') - if __name__ == '__main__': config = Configurator() config.begin() config.add_view(hello_world) - config.add_view(goodbye_world, name='goodbye') config.end() app = config.make_wsgi_app() serve(app) -When this code is inserted into a Python script named -``helloworld.py`` and executed by a Python interpreter which has the -:mod:`repoze.bfg` software installed, an HTTP server is started on -port 8080. When port 8080 is visited by a user agent on the root URL -(``/``), the server will simply serve up the text "Hello world!" with -the HTTP response values ``200 OK`` as a response code and a -``Content-Type`` header value of ``text/plain``. But for reasons -we'll better understand shortly, when visited by a user agent on the -URL ``/goodbye``, the server will serve up "Goodbye world!". - -Let's examine this program piece-by-piece. - -Imports -~~~~~~~ - -The above script defines the following set of imports: - -.. code-block:: python - :linenos: - - from webob import Response - from paste.httpserver import serve - from repoze.bfg.configuration import Configurator - -:mod:`repoze.bfg` uses the :term:`WebOb` library as the basis for its -:term:`request` and :term:`response` objects. The script uses the -:class:`webob.Response` class later in the script to create a -:term:`response` object. - -Like many other Python web frameworks, :mod:`repoze.bfg` uses the -:term:`WSGI` protocol to connect an application and a web server -together. The :mod:`paste.httpserver` server is used in this example -as a WSGI server for convenience, as ``Paste`` is a dependency of -:mod:`repoze.bfg` itself. However, :mod:`repoze.bfg` applications can -be served by any WSGI server. - -The script also imports the ``Configurator`` class from the -``repoze.bfg.configuration`` module. This class is used to configure -:mod:`repoze.bfg` for a particular application. An instance of this -class provides methods which help configure various parts of -:mod:`repoze.bfg` for a given application deployment. - -View Declaration -~~~~~~~~~~~~~~~~ - -The above script, beneath its set of imports, defines two functions: -one named ``hello_world`` and one named ``goodbye_world``. - -.. code-block:: python - :linenos: - - def hello_world(request): - return Response('Hello world!') - - def goodbye_world(request): - return Response('Goodbye world!') - -Each function accepts a single argument (``request``) and returns an -instance of the :class:`webob.Response` class. In the ``hello_world`` -function, the string ``'Hello world!'`` is passed to the ``Response`` -constructor as the *body* of the response. In the ``goodbye_world`` -function, the string ``'Goodbye world!'`` is passed. - -Each of these functions is known as a :term:`view callable`. View -callables in a "real" :mod:`repoze.bfg` application are often -functions which accept a :term:`request` and return a -:term:`response`. A view callable can be represented via another type -of object, like a class or an instance, but for our purposes here, a -function serves us well. - -A view callable is called with a :term:`request` object, which is a -representation of an HTTP request sent by a remote user agent. A view -callable is required to return a :term:`response` object because a -response object has all the information necessary to formulate an -actual HTTP response; this object is then converted to text and sent -back to the requesting user agent. - -The ``hello_world`` view callable defined by the script does nothing -but return a response with the body ``Hello world!``; the -``goodbye_world`` view callable returns a response with the body -``Goodbye world!``. - -.. index:: - pair: traversal; introduction - -.. _traversal_intro: - -An Introduction to Traversal -~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -If you've run the code listed in :ref`helloworld_imperative` already, -you've unwittingly configured :mod:`repoze.bfg` to serve an -application that relies on :term:`traversal`. A full explanation of -how :mod:`repoze.bfg` locates "the right" :term:`view callable` for a -given request requires some explanation of :term:`traversal`. - -Traversal is part of a mechanism used by :mod:`repoze.bfg` to map the -URL of some request to a particular :term:`view callable`. It is not -the only mechanism made available by :mod:`repoze.bfg` that allows the -mapping a URL to a view callable. Another distinct mode known as -:term:`URL dispatch` can alternately be used to find a view callable -based on a URL. However, our sample application uses only -:term:`traversal`. - -In :mod:`repoze.bfg` terms, :term:`traversal` is the act of walking -over an object graph starting from a :term:`root` object in order to -find a :term:`context` object and a :term:`view name`. Once a context -and a view name are found, these two bits of information, plus other -information from the request are used to look up a :term:`view -callable`. :mod:`repoze.bfg` bothers to do traversal only because the -information returned from traversal allows a view callable to be -found. - -The individual path segments of the "path info" portion of a URL (the -data following the hostname and port number, but before any query -string elements or fragments, for example the ``/a/b/c`` portion of -the URL ``http://example.com/a/b/c?foo=1``) are used as "steps" during -traversal. - -.. note:: A useful analogy of how :mod:`repoze.bfg` :term:`traversal` - works is available within the chapter section entitled - :ref:`traversal_behavior`. - -The results of a :term:`traversal` include a :term:`context` and a -:term:`view name`. The :term:`view name` is the *first* URL path -segment in the set of path segments "left over" in the results of -:term:`traversal`. This will either be the empty string (``''``) or a -non-empty string (one of the path segment strings). The empty string -represents the :term:`default view` of a context object. - -The :term:`default view` is found when all path elements in the URL -are exhausted before :term:`traversal` returns a :term:`context` -object, causing the :term:`view name` to be ``''`` (the empty string). -When no path segments are "left over" after traversal, the -:term:`default view` for the context found is invoked. - -If traversal returns a non-empty :term:`view name`, it means that -traversal "ran out" of nodes in the graph before it finished -exhausting all the path segments implied by the path info of the URL: -no segments are "left over". In this case, because the :term:`view -name` is non-empty, a *non-default* view callable will be invoked. - -The combination of the :term:`context` object and the :term:`view -name` (and, in more complex configurations, other :term:`predicate` -values) are used to find "the right" :term:`view callable`, which will -be invoked after traversal. - -The object graph of our hello world application is very simple: -there's exactly one object in our graph; the default :term:`root` -object. - -Relating Traversal to the Hello World Application -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - -Our application's :term:`root` object is the *default* root object -used when one isn't otherwise specified in application configuration. -This root object does not have a ``__getitem__`` method, thus it has -no children. Although in a more complex system there can be many -contexts which URLs resolve to in our application, effectively there -is only ever one context: the root object. - -We have only a single default view registered (the registration for -the ``hello_world`` view callable). Due to this set of circumstances, -you can consider the sole possible URL that will resolve to a -:term:`default view` in this application the root URL ``'/'``. It is -the only URL that will resolve to the :term:`view name` of ``''`` (the -empty string). - -We have only a single view registered for the :term:`view name` -``goodbye`` (the registration for the ``goodbye_world`` view -callable). Due to this set of circumstances, you can consider the -sole possible URL that will resolve to the ``goodbye_world`` in this -application the URL ``'/goodbye'`` because it is the only URL that -will resolve to the :term:`view name` of ``goodbye``. - -.. index:: - pair: imperative; configuration - single: Configurator - -.. _helloworld_imperative_appconfig: - -Application Configuration -~~~~~~~~~~~~~~~~~~~~~~~~~ - -In the above script, the following code, representing the -*configuration* of an application which uses the previously defined -imports and function definitions is placed within the confines of an -``if`` statement: - -.. code-block:: python - :linenos: - - if __name__ == '__main__': - config = Configurator() - config.begin() - config.add_view(hello_world) - config.add_view(goodbye_world, name='goodbye') - config.end() - app = config.make_wsgi_app() - simple_server.make_server('', 8080, app).serve_forever() - -Let's break this down this piece-by-piece. - -Configurator Construction -~~~~~~~~~~~~~~~~~~~~~~~~~ - -.. code-block:: python - :linenos: - - if __name__ == '__main__': - config = Configurator() - -The ``if __name__ == '__main__':`` line above represents a Python -idiom: the code inside this if clause is not invoked unless the script -is run directly from the command line via, for example, ``python -helloworld.py`` where the file named ``helloworld.py`` contains the -entire script body. ``helloworld.py`` in this case is a Python -*module*. Using the ``if`` clause is necessary (or at least "best -practice") because code in any Python module may be imported by -another Python module. By using this idiom, the script is indicating -that it does not want the code within the ``if`` statement to execute -if this module is imported; the code within the ``if`` block should -only be run during a direct script execution. - -The ``config = Configurator()`` line above creates an instance of the -:class:`repoze.bfg.configuration.Configurator` class. The resulting -``config`` object represents an API which the script uses to configure -this particular :mod:`repoze.bfg` application. - -.. note:: - - An instance of the :class:`repoze.bfg.configuration.Configurator` - class is a *wrapper* object which mutates an :term:`application - registry` as its methods are called. An application registry - represents the configuration state of a :mod:`repoze.bfg` - application. The ``Configurator`` is not itself an - :term:`application registry`, it is a mechanism used to configure - an application registry. The underlying application registry - object being configured by a ``Configurator`` is available as its - ``registry`` attribute. - -Beginning Configuration -~~~~~~~~~~~~~~~~~~~~~~~ - -.. ignore-next-block -.. code-block:: python - - config.begin() - -The :meth:`repoze.bfg.configuration.Configurator.begin` method tells -the the system that application configuration has begun. In -particular, this causes the :term:`application registry` associated -with this configurator to become the "current" application registry, -meaning that code which attempts to use the application registry -:term:`thread local` will obtain the registry associated with the -configurator. This is an explicit step because it's sometimes -convenient to use a configurator without causing the registry -associated with the configurator to become "current". - -Adding Configuration -~~~~~~~~~~~~~~~~~~~~ - -.. ignore-next-block -.. code-block:: python - :linenos: - - config.add_view(hello_world) - config.add_view(goodbye_world, name='goodbye') - -Each of these lines calls the -:meth:`repoze.bfg.configuration.Configurator.add_view` method. The -``add_view`` method of a configurator registers a :term:`view -configuration` within the :term:`application registry`. A :term:`view -configuration` represents a :term:`view callable` which must be -invoked when a set of circumstances related to the :term:`request` is -true. This "set of circumstances" is provided as one or more keyword -arguments to the ``add_view`` method, otherwise known as -:term:`predicate` arguments. - -The line ``config.add_view(hello_world)`` registers the -``hello_world`` function as a view callable. The ``add_view`` method -of a Configurator must be called with a view callable object as its -first argument, so the first argument passed is ``hello_world`` -function we'd like to use as a :term:`view callable`. However, this -line calls ``add_view`` with a single default :term:`predicate` -argument, the ``name`` predicate with a value of ``''``, meaning that -we'd like :mod:`repoze.bfg` to invoke the ``hello_world`` view -callable for any request for the :term:`default view` of an object. - -Our ``hello_world`` :term:`view callable` returns a Response instance -with a body of ``Hello world!`` in the configuration implied by this -script. It is configured as a :term:`default view`. Therefore, a -user agent contacting a server running this application will receive -the greeting ``Hello world!`` when any :term:`default view` is -invoked. - -.. sidebar:: View Dispatch and Ordering - - When :term:`traversal` is used, :mod:`repoze.bfg` chooses the most - specific view callable based *only* on view :term:`predicate` - applicability. This is unlike :term:`URL dispatch`, another - dispatch mode of :mod:`repoze.bfg` (and other frameworks, like - :term:`Pylons` and :term:`Django`) which first uses an ordered - routing lookup to resolve the request to a view callable by running - it through a relatively-ordered series of URL path matches. We're - not really concerned about the finer details of :term:`URL - dispatch` right now. It's just useful to use for demonstrative - purposes: the ordering of calls to - :meth:`repoze.bfg.configuration.Configurator.add_view`` is never - very important. We can register ``goodbye_world`` first and - ``hello_world`` second; :mod:`repoze.bfg` will still give us the - most specific callable when a request is dispatched to it. - -The line ``config.add_view(goodbye_world, name='goodbye')`` registers -the ``goodbye_world`` function as a view callable. The line calls -``add_view`` with the view callable as the first required positional -argument, and a :term:`predicate` keyword argument ``name`` with the -value ``'goodbye'``. This :term:`view configuration` implies that a -request with a :term:`view name` of ``goodbye`` should cause the -``goodbye_world`` view callable to be invoked. For the purposes of -this discussion, the :term:`view name` can be considered the first -non-empty path segment in the URL: in particular, this view -configuration will match when the URL is ``/goodbye``. - -Our ``goodbye_world`` :term:`view callable` returns a Response -instance with a body of ``Goodbye world!`` in the configuration -implied by this script. It is configured as with a :term:`view name` -predicate of ``goodbye``. Therefore, a user agent contacting a server -running this application will receive the greeting ``Goodbye world!`` -when the path info part of the request is ``/goodbye``. - -Each invocation of the ``add_view`` method implies a :term:`view -configuration` registration. Each :term:`predicate` provided as a -keyword argument to the ``add_view`` method narrows the set of -circumstances which would cause the view configuration's callable to -be invoked. In general, a greater number of predicates supplied along -with a view configuration will more strictly limit the applicability -of its associated view callable. When :mod:`repoze.bfg` processes a -request, however, the view callable with the *most specific* view -configuration (the view configuration that matches the largest number -of predicates) is always invoked. - -Earlier we explained that the server would return ``Hello world!`` if -you visited the *root* (``/``) URL. However, actually, because the -view configuration registration for the ``hello_world`` view callable -has no :term:`predicate` arguments, the ``hello_world`` view callable -is applicable for the :term:`default view` of any :term:`context` -resulting from a request. This isn't all that interesting in this -application, because we always only have *one* potential context (the -root object): it is the only object in the graph. - -We've also registered a view configuration for another circumstance: -the ``goodbye_world`` view callable has a ``name`` predicate of -``goodbye``, meaning that it will match for requests that have the -:term:`view name` ``goodbye`` unlike the ``hello_world`` view -configuration registration, which will only match the default view -(view name ``''``) of a request. Because :mod:`repoze.bfg` chooses -the best view configuration for any request, the ``goodbye_world`` -view callable will be used when the URL contains path information that -ends with ``/goodbye``. - -Ending Configuration -~~~~~~~~~~~~~~~~~~~~ - -.. ignore-next-block -.. code-block:: python - - config.end() - -The :meth:`repoze.bfg.configuration.Configurator.end` method tells the -the system that application configuration has ended. It is the -inverse of :meth:`repoze.bfg.configuration.Configurator.begin`. In -particular, this causes the :term:`application registry` associated -with this configurator to no longer be the "current" application -registry, meaning that code which attempts to use the application -registry :term:`thread local` will no longer obtain the registry -associated with the configurator. - -.. index:: - single: make_wsgi_app - pair: WSGI; application - triple: WSGI; application; creation - -WSGI Application Creation -~~~~~~~~~~~~~~~~~~~~~~~~~ - -.. ignore-next-block -.. code-block:: python - - app = config.make_wsgi_app() - -After configuring views and ending configuration, the script creates a -WSGI *application* via the -:meth:`repoze.bfg.configuration.Configurator.make_wsgi_app` method. A -call to ``make_wsgi_app`` implies that all configuration is finished -(meaning all method calls to the configurator which set up views, and -various other configuration settings have been performed). The -``make_wsgi_app`` method returns a :term:`WSGI` application object -that can be used by any WSGI server to present an application to a -requestor. - -The :mod:`repoze.bfg` application object, in particular, is an -instance of the :class:`repoze.bfg.router.Router` class. It has a -reference to the :term:`application registry` which resulted from -method calls to the configurator used to configure it. The Router -consults the registry to obey the policy choices made by a single -application. These policy choices were informed by method calls to -the ``Configurator`` made earlier; in our case, the only policy -choices made were implied by two calls to the ``add_view`` method, -telling our application that it should effectively serve up the -``hello_world`` view callable to any user agent when it visits the -root URL, and the ``goodbye_world`` view callable to any user agent -when it visits the URL with the path info ``/goodbye``. - -WSGI Application Serving -~~~~~~~~~~~~~~~~~~~~~~~~ - -.. ignore-next-block -.. code-block:: python - - serve(app) - -Finally, we actually serve the application to requestors by starting -up a WSGI server. We happen to use the :func:`paste.httpserver.serve` -WSGI server runner, using the default TCP port of 8080, and we pass it -the ``app`` object (an instance of the -:class:`repoze.bfg.router.Router` class) as the application we wish to -serve. This causes the server to start listening on the TCP port. It -will serve requests forever, or at least until we stop it by killing -the process which runs it. - -Conclusion -~~~~~~~~~~ - -Our hello world application is one of the simplest possible -:mod:`repoze.bfg` applications, configured "imperatively". We can see -a good deal of what's going on "under the hood" when we configure a -:mod:`repoze.bfg` application imperatively. However, another mode of -configuration exists named *declarative* configuration. +We won't talk much about what this application does yet. Just note +that the "configuration' statements take place underneath the ``if +__name__ == '__main__':`` stanza in the form of method calls on a +:term:`Configurator` object (e.g. ``config.begin()``, +``config.add_view(...)``, and ``config.end()``. These statements take +place one after the other, and are executed in order, so the full +power of Python, including conditionals, can be employed in this mode +of configuration. .. index:: - pair: helloworld; declarative - single: helloworld + pair: declarative; configuration -.. _helloworld_declarative: +.. _declarative_configuration: -Hello World, Configured Declaratively -------------------------------------- +Declarative Configuration +------------------------- -:mod:`repoze.bfg` can be configured for the same "hello world" -application "declaratively", if so desired. Declarative configuration -relies on *declarations* made external to the code in a configuration -file format named :term:`ZCML` (Zope Configuration Markup Language), -an XML dialect. +A :mod:`repoze.bfg` application can be alternatively be configured +"declaratively", if so desired. Declarative configuration relies on +*declarations* made external to the code in a configuration file +format named :term:`ZCML` (Zope Configuration Markup Language), an XML +dialect. Declarative configuration mode is the configuration mode in which developers cede the most amount of control to the framework itself. Because application developers cede more control to the framework, it -is also harder to understand than purely imperative configuration. -However, using declarative configuration has a number of benefits, the -primary benefit being that applications configured declaratively can -be *overridden* and *extended* by third parties without requiring the -third party to change application code. - -.. note:: +is also sometimes harder to understand than purely imperative +configuration. - See :ref:`extending_chapter` for a discussion of extending and - overriding :mod:`repoze.bfg` applications. - -Unlike the simplest :mod:`repoze.bfg` application configured -imperatively, the simplest :mod:`repoze.bfg` application, configured -declaratively requires not one, but two files: a Python file and a -:term:`ZCML` file. +A :mod:`repoze.bfg` application configured declaratively requires not +one, but two files: a Python file and a :term:`ZCML` file. In a file named ``helloworld.py``: @@ -585,9 +111,6 @@ In a file named ``helloworld.py``: def hello_world(request): return Response('Hello world!') - def goodbye_world(request): - return Response('Goodbye world!') - if __name__ == '__main__': config = Configurator() config.begin() @@ -610,19 +133,16 @@ previously created ``helloworld.py``: view="helloworld.hello_world" /> - <view - name="goodbye" - view="helloworld.goodbye_world" - /> - </configure> This pair of files forms an application functionally equivalent to the -application we created earlier. Let's examine the differences between -the code described in :ref:`helloworld_imperative` and the code above. +application we created earlier in :ref:`imperative_configuration`. + +Let's examine the differences between the code listing in +:ref:`imperative_configuration` and the code above. -In :ref:`helloworld_imperative_appconfig`, we had the following lines -within the ``if __name__ == '__main__'`` section of ``helloworld.py``: +In :ref:`imperative_configuration`, we had the following lines within +the ``if __name__ == '__main__'`` section of ``helloworld.py``: .. code-block:: python :linenos: @@ -631,16 +151,14 @@ within the ``if __name__ == '__main__'`` section of ``helloworld.py``: config = Configurator() config.begin() config.add_view(hello_world) - config.add_view(goodbye_world, name='goodbye') config.end() app = config.make_wsgi_app() - simple_server.make_server('', 8080, app).serve_forever() + serve(app) -In our "declarative" code, we've added a call to the -:meth:`repoze.bfg.configuration.Configurator.load_zcml` method with -the value ``configure.zcml``, and we've removed the lines which read -``config.add_view(hello_world)`` and ``config.add_view(goodbye_world, -name='goodbye')``, so that it now reads as: +In our "declarative" code, we've removed the call to ``add_view`` and +replaced it with a call to the +:meth:`repoze.bfg.configuration.Configurator.load_zcml` method so that +it now reads as: .. code-block:: python :linenos: @@ -651,14 +169,14 @@ name='goodbye')``, so that it now reads as: config.load_zcml('configure.zcml') config.end() app = config.make_wsgi_app() - simple_server.make_server('', 8080, app).serve_forever() + serve(app) Everything else is much the same. The ``config.load_zcml('configure.zcml')`` line tells the configurator -to load configuration declarations from the ``configure.zcml`` file -which sits next to ``helloworld.py``. Let's take a look at the -``configure.zcml`` file now: +to load configuration declarations from the file named +``configure.zcml`` which sits next to ``helloworld.py`` on the +filesystem. Let's take a look at that ``configure.zcml`` file again: .. code-block:: xml :linenos: @@ -671,166 +189,45 @@ which sits next to ``helloworld.py``. Let's take a look at the view="helloworld.hello_world" /> - <view - name="goodbye" - view="helloworld.goodbye_world" - /> - </configure> -The ``<configure>`` Tag -~~~~~~~~~~~~~~~~~~~~~~~ - -The ``configure.zcml`` ZCML file contains this bit of XML: - -.. code-block:: xml - :linenos: - - <configure xmlns="http://namespaces.repoze.org/bfg"> - - <!-- other directives --> - - </configure> - -Because :term:`ZCML` is XML, and because XML requires a single root -tag for each document, every ZCML file used by :mod:`repoze.bfg` must -contain a ``configure`` container directive, which acts as the root -XML tag. It is a "container" directive because its only job is to -contain other directives. - -See also :ref:`configure_directive` and :ref:`word_on_xml_namespaces`. - -The ``<include>`` Tag -~~~~~~~~~~~~~~~~~~~~~ - -The ``configure.zcml`` ZCML file contains this bit of XML within the -``<configure>`` root tag: - -.. code-block:: xml - - <include package="repoze.bfg.includes" /> - -This singleton (self-closing) tag instructs ZCML to load a ZCML file -from the Python package with the :term:`dotted Python name` -:mod:`repoze.bfg.includes`, as specified by its ``package`` attribute. -This particular ``<include>`` declaration is required because it -actually allows subsequent declaration tags (such as ``<view>``, which -we'll see shortly) to be recognized. The ``<include>`` tag -effectively just includes another ZCML file; this causes its -declarations to be executed. In this case, we want to load the -declarations from the file named ``configure.zcml`` within the -:mod:`repoze.bfg.includes` Python package. We know we want to load -the ``configure.zcml`` from this package because ``configure.zcml`` is -the default value for another attribute of the ``<include>`` tag named -``file``. We could have spelled the include tag more verbosely, but -equivalently as: - -.. code-block:: xml - :linenos: - - <include package="repoze.bfg.includes" - file="configure.zcml"/> +Note that this file contains some XML, and that the XML contains a +``<view>`` :term:`configuration declaration` tag that references a +:term:`dotted Python name`. This dotted name refers to the +``hello_world`` function that lives in our ``helloword`` Python +module. -The ``<include>`` tag that includes the ZCML statements implied by the -``configure.zcml`` file from the Python package named -:mod:`repoze.bfg.includes` is basically required to come before any -other named declaration in an application's ``configure.zcml``. If it -is not included, subsequent declaration tags will fail to be -recognized, and the configuration system will generate a traceback. -However, the ``<include package="repoze.bfg.includes"/>`` tag needs to -exist only in a "top-level" ZCML file, it needn't also exist in ZCML -files *included by* a top-level ZCML file. +This ``<view>`` declaration tag performs the same function as the +``add_view`` method that was employed within +:ref:`imperative_configuration`. In fact, the ``<view>`` tag is +effectively a "macro" which calls the +:meth:`repoze.bfg.configuration.Configurator.add_view` method on your +behalf. -See also :ref:`include_directive`. - -The ``<view>`` Tag -~~~~~~~~~~~~~~~~~~ - -The ``configure.zcml`` ZCML file contains these bits of XML *after* the -``<include>`` tag, but *within* the ``<configure>`` root tag: - -.. code-block:: xml - :linenos: - - <view - view="helloworld.hello_world" - /> - - <view - name="goodbye" - view="helloworld.goodbye_world" - /> - -These ``<view>`` declaration tags direct :mod:`repoze.bfg` to create -two :term:`view configuration` registrations. The first ``<view>`` -tag has an attribute (the attribute is also named ``view``), which -points at a :term:`dotted Python name`, referencing the -``hello_world`` function defined within the ``helloworld`` package. -The second ``<view>`` tag has a ``view`` attribute which points at a -:term:`dotted Python name`, referencing the ``goodbye_world`` function -defined within the ``helloworld`` package. The second ``<view>`` tag -also has an attribute called ``name`` with a value of ``goodbye``. - -These effect of the ``<view>`` tag declarations we've put into our -``configure.zcml`` is functionally equivalent to the effect of lines -we've already seen in an imperatively-configured application. We're -just spelling things differently, using XML instead of Python. - -In our previously defined application, in which we added view -configurations imperatively, we saw this code: - -.. ignore-next-block -.. code-block:: python - :linenos: - - config.add_view(hello_world) - config.add_view(goodbye_world, name='goodbye') - -Each ``<view>`` declaration tag encountered in a ZCML file effectively -invokes the :meth:`repoze.bfg.configuration.Configurator.add_view` -method on the behalf of the developer. Various attributes can be -specified on the ``<view>`` tag which influence the :term:`view -configuration` it creates. - -Since the relative ordering of calls to -:meth:`repoze.bfg.configuration.Configurator.add_view` doesn't matter -(see the sidebar entitled *View Dispatch and Ordering*), the relative -order of ``<view>`` tags in ZCML doesn't matter either. The following -ZCML orderings are completely equivalent: - -.. topic:: Hello Before Goodbye - - .. code-block:: xml - :linenos: - - <view - view="helloworld.hello_world" - /> - - <view - name="goodbye" - view="helloworld.goodbye_world" - /> +The ``<view>`` tag is an example of a :mod:`repoze.bfg` declaration +tag. Other such tags include ``<route>``, ``<scan>``, ``<notfound>``, +``<forbidden>``, and others. Each of these tags is effectively a +"macro" which calls methods of a +:class:`repoze.bfg.configuration.Configurator` object on your behalf. -.. topic:: Goodbye Before Hello +Essentially, using a :term:`ZCML` file and loading it from the +filesystem allows us to put our configuration statements within this +XML file rather as declarations, rather than representing them as +method calls to a :term:`Configurator` object. Otherwise, declarative +and imperative configuration are functionally equivalent. - .. code-block:: xml - :linenos: +Using declarative configuration has a number of benefits, the primary +benefit being that applications configured declaratively can be +*overridden* and *extended* by third parties without requiring the +third party to change application code. - <view - name="goodbye" - view="helloworld.goodbye_world" - /> +.. note:: - <view - view="helloworld.hello_world" - /> + See :ref:`extending_chapter` for a discussion of extending and + overriding :mod:`repoze.bfg` applications. -The ``<view>`` tag is an example of a :mod:`repoze.bfg` declaration -tag. Other such tags include ``<route>``, ``<scan>``, ``<notfound>``, -``<forbidden>``, and others. Each of these tags is effectively a -"macro" which calls methods on the -:class:`repoze.bfg.configuration.Configurator` object on your behalf. +If you want to build a framework or an extensible application, using +ZCML is a good idea. .. index:: pair: ZCML; conflict detection @@ -838,8 +235,8 @@ tag. Other such tags include ``<route>``, ``<scan>``, ``<notfound>``, ZCML Conflict Detection ~~~~~~~~~~~~~~~~~~~~~~~ -An additional feature of ZCML is *conflict detection*. If you define -two declaration tags within the same ZCML file which logically +A minor additional feature of ZCML is *conflict detection*. If you +define two declaration tags within the same ZCML file which logically "collide", an exception will be raised, and the application will not start. For example, the following ZCML file has two conflicting ``<view>`` tags: @@ -862,33 +259,155 @@ start. For example, the following ZCML file has two conflicting </configure> If you try to use this ZCML file as the source of ZCML for an -application, a :data:`repoze.bfg.exceptions.ConfigurationError` will -be raised when you attempt to start the application with information -about which tags might have conflicted. +application, an error will be raised when you attempt to start the +application. This error will contain information about which tags +might have conflicted. + +.. index:: + single: bfg_view + pair: ZCML directive; view + single: configuration decorations + pair: code; scanning + +.. _decorations_and_code_scanning: + +Configuration Decorations and Code Scanning +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +An alternate mode of declarative configuration lends more *locality of +reference* to a :term:`configuration declaration`. It's sometimes +painful to have all configuration done in ZCML, or even in imperative +code, because you may need to have two files open at once; the file +that represents the configuration, and the file that contains the +implementation objects (such as :term:`view callable` functions) that +the configuration references. To avoid this, :mod:`repoze.bfg` allows +you to insert :term:`configuration decoration` statements very close +to code that is referred to by the declaration itself. For example: + +.. code-block:: python + :linenos: -Conclusions ------------ + from repoze.bfg.view import bfg_view + from webob import Response -.. sidebar:: Which Configuration Mode Should I Use? + @bfg_view(name='hello', request_method='GET') + def hello(request): + return Response('Hello') + +The :class:`repoze.bfg.view.bfg_view` decorator above adds an +attribute to the ``hello`` function, making it available for a +:term:`scan` to find it later. + +The mere existence of configuration decoration doesn't cause any +configuration registration to be made. Before they have any effect on +the configuration of a :mod:`repoze.bfg` application, a configuration +decoration within application code must be found through a process +known as *scanning*. + +:mod:`repoze.bfg` is willing to :term:`scan` a module or a package and +its subpackages for decorations when the +:meth:`repoze.bfg.configuration.Configurator.scan` method is invoked: +scanning implies searching for configuration declarations in a package +and its subpackages. For example: + +.. topic:: Imperatively Starting A Scan + + .. code-block:: python + :linenos: + + from paste.httpserver import serve + from repoze.bfg.view import bfg_view + from webob import Response + + @bfg_view() + def hello(request): + return Response('Hello') + + if __name__ == '__main__': + from repoze.bfg.configuration import Configurator + config = Configurator() + config.begin() + config.scan() + config.end() + app = config.make_wsgi_app() + serve(app) + +:term:`ZCML` can also invoke a :term:`scan` via its ``<scan>`` +directive. If a ZCML file is processed that contains a scan +directive, the package the ZCML file points to is scanned. + +.. topic:: Declaratively Starting a Scan + + .. code-block:: python + :linenos: + + # helloworld.py + + from paste.httpserver import serve + from repoze.bfg.view import bfg_view + from webob import Response + + @bfg_view() + def hello(request): + return Response('Hello') + + if __name__ == '__main__': + from repoze.bfg.configuration import Configurator + config = Configurator() + config.begin() + config.load_zcml('configure.zcml') + config.end() + app = config.make_wsgi_app() + serve(app) + + .. code-block:: xml + :linenos: + + <configure xmlns="http://namespaces.repoze.org"> + + <!-- configure.zcml --> + + <include package="repoze.bfg.includes"/> + <scan package="."/> + + </configure> + +The scanning machinery imports each module and subpackage in a package +or module recursively, looking for special attributes attached to +objects defined within a module. These special attributes are +typically attached to code via the use of a :term:`decorator`. For +example, the :class:`repoze.bfg.view.bfg_view` decorator can be +attached to a function or instance method: + +Once scanning is invoked, and :term:`configuration decoration` is +found by the scanner, a set of calls are made to a +:term:`Configurator` on behalf of the developer: these calls represent +the intent of the configuration decoration. + +In the example above, this is best represented as the scanner +translating the arguments to :class:`repoze.bfg.view.bfg_view` into a +call to the :meth:`repoze.bfg.configuration.Configurator.add_view` +method, effectively: - We recommend declarative configuration (:term:`ZCML`), because it's - the more traditional form of configuration used by Zope-based - systems, it can be overridden and extended by third party deployers, - and there are more examples for it "in the wild". However, - imperative mode configuration can be simpler to understand. +.. ignore-next-block +.. code-block:: python -:mod:`repoze.bfg` allows an application to perform configuration tasks -either imperatively or declaratively. You can choose the mode that -best fits your brain as necessary. + config.add_view(hello) -For more information about the API of a ``Configurator`` object, see -:class:`repoze.bfg.configuration.Configurator` . The equivalent ZCML -declaration tags are introduced in narrative documentation chapters as -necessary. +Which Mode Should I Use? +------------------------ -For more information about :term:`traversal`, see -:ref:`traversal_chapter`. +A combination of imperative configuration, declarative configuration +via ZCML and scanning can be used to configure any application. They +are not mutually exclusive. -For more information about :term:`view configuration`, see -:ref:`views_chapter`. +The :mod:`repoze.bfg` authors often recommend using mostly declarative +configuration, because it's the more traditional form of configuration +used in :mod:`repoze.bfg` applications, it can be overridden and +extended by third party deployers, and there are more examples for it +"in the wild". +However, imperative mode configuration can be simpler to understand, +and the framework is not "opinionated" about the choice. This book +presents examples in both styles, mostly interchangeably. You can +choose the mode that best fits your brain as necessary. |