.. _firstapp_chapter: Creating Your First :mod:`repoze.bfg` Application ================================================= We will walk through the creation of a tiny :mod:`repoze.bfg` application in this chapter. After we're finished creating it, we'll explain in more detail how the application works. .. note:: If you're a "theory-first" kind of person, you might choose to read :ref:`contextfinding_chapter` and :ref:`views_chapter` to augment your understanding before diving into the code that follows, but it's not necessary if -- like many programmers -- you're willing to "go with the flow". .. _helloworld_imperative: Hello World, Goodbye World (Imperative) --------------------------------------- Here's one of the very simplest :mod:`repoze.bfg` applications, configured imperatively: .. code-block:: python :linenos: from webob import Response from paste.httpserver import serve from repoze.bfg.configuration import Configurator 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, host='0.0.0.0') 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 TCP port 8080. When port 8080 is visited by a browser on the root URL (``/``), the server will simply serve up the text "Hello world!" When visited by a browser on the URL ``/goodbye``, the server will serve up the text "Goodbye world!" Now that we have a rudimentary understanding of what the application does, let's examine it 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 the ``paste`` package is a dependency of :mod:`repoze.bfg` itself. 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 Callable Declarations ~~~~~~~~~~~~~~~~~~~~~~~~~~ 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!') These functions don't do anything very taxing. Both functions accept a single argument (``request``). The ``hello_world`` function does nothing but return a response instance with the body ``Hello world!``. The ``goodbye_world`` function returns a response instance with the body ``Goodbye world!``. Each of these functions is known as a :term:`view callable`. View callables in a :mod:`repoze.bfg` application accept a single argument, ``request`` and are expected to return a :term:`response` object. A view callable doesn't need to be a function; it 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 always called with a :term:`request` object. A request object is a representation of an HTTP request sent to :mod:`repoze.bfg` via the active :term:`WSGI` server. 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 by the upstream :term:`WSGI` server and sent back to the requesting browser. To return a response, each view callable creates 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. .. index:: single: imperative configuration single: Configurator single: helloworld (imperative) .. _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() serve(app, host='0.0.0.0') 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 in the code sample above represents a Python idiom: the code inside this if clause is not invoked unless the script containing this code is run directly from the command line. For example, if the file named ``helloworld.py`` contains the entire script body, the code within the ``if`` statement will only be invoked when ``python helloworld.py`` is executed from the operating system command line. ``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. Methods called on the Configurator will cause registrations to be made in a :term:`application registry` associated with the application. Beginning Configuration ~~~~~~~~~~~~~~~~~~~~~~~ .. ignore-next-block .. code-block:: python config.begin() The :meth:`repoze.bfg.configuration.Configurator.begin` method tells 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". .. note:: See :ref:`threadlocals_chapter` for a discussion about what it means for an application registry to be "current". .. _adding_configuration: 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 set of circumstances related to the :term:`request` that will cause a specific :term:`view callable` to be invoked. This "set of circumstances" is provided as one or more keyword arguments to the ``add_view`` method. Each of these keyword arguments is known as a view configuration :term:`predicate`. 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 the ``hello_world`` function. This line calls ``add_view`` with a *default* value for the :term:`predicate` argument, named ``name``. The ``name`` predicate defaults to a value equalling the empty string (``''``). This means that we're instructing :mod:`repoze.bfg` to invoke the ``hello_world`` view callable when the :term:`view name` is the empty string. We'll learn in later chapters what a :term:`view name` is, and under which circumstances a request will have a view name that is the empty string; in this particular application, it means that the ``hello_world`` view callable will be invoked when the root URL ``/`` is visted by a browser. 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'``. The ``name`` argument supplied in this :term:`view configuration` implies that only a request that has a :term:`view name` of ``goodbye`` should cause the ``goodbye_world`` view callable to be invoked. In this particular application, this means that the ``goodbye_world`` view callable will be invoked when the URL ``/goodbye`` is visted by a browser. 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 most specific set of predicates) is always invoked. In this application, :mod:`repoze.bfg` chooses the most specific view callable based only on view :term:`predicate` applicability. 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. Ending Configuration ~~~~~~~~~~~~~~~~~~~~ .. ignore-next-block .. code-block:: python config.end() The :meth:`repoze.bfg.configuration.Configurator.end` method tells 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. .. note:: See :ref:`threadlocals_chapter` for a discussion about what it means for an application registry to be "current". .. index:: single: make_wsgi_app single: WSGI application 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. :term:`WSGI` is a protocol that allows servers to talk to Python applications. We don't discuss :term:`WSGI` in any depth within this book, however, you can learn more about it by visiting `wsgi.org `_. The :mod:`repoze.bfg` application object, in particular, is an instance of a class representing a :mod:`repoze.bfg` :term:`router`. It has a reference to the :term:`application registry` which resulted from method calls to the configurator used to configure it. The :term:`router` consults the registry to obey the policy choices made by a single application. These policy choices were informed by method calls to the :term:`Configurator` made earlier; in our case, the only policy choices made were implied by two calls to its ``add_view`` method. WSGI Application Serving ~~~~~~~~~~~~~~~~~~~~~~~~ .. ignore-next-block .. code-block:: python serve(app, host='0.0.0.0') 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, passing it the ``app`` object (a :term:`router`) as the application we wish to serve. We also pass in an argument ``host=='0.0.0.0'``, meaning "listen on all TCP interfaces." By default, the Paste HTTP server listens only on the ``127.0.0.1`` interface, which is problematic if you're running the server on a remote system and you wish to access it with a web browser from a local system. We don't specify a TCP port number to listen on; this means we want to use the default TCP port, which is 8080. When this line is invoked, it causes the server to start listening on TCP port 8080. 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 that it's configured imperatively because the full power of Python is available to us as we perform configuration tasks. .. index:: single: helloworld (declarative) .. _helloworld_declarative: Hello World, Goodbye World (Declarative) ---------------------------------------- Another almost entirely equivalent mode of application configuration exists named *declarative* configuration. :mod:`repoze.bfg` can be configured for the same "hello world" application "declaratively", if so desired. To do so, first, create a file named ``helloworld.py``: .. code-block:: python :linenos: from webob import Response from paste.httpserver import serve from repoze.bfg.configuration import Configurator def hello_world(request): return Response('Hello world!') def goodbye_world(request): return Response('Goodbye world!') if __name__ == '__main__': config = Configurator() config.begin() config.load_zcml('configure.zcml') config.end() app = config.make_wsgi_app() serve(app, host='0.0.0.0') Then create a file named ``configure.zcml`` in the same directory as the previously created ``helloworld.py``: .. code-block:: xml :linenos: This pair of files forms an application functionally equivalent to the application we created earlier in :ref:`helloworld_imperative`. Let's examine the differences between the code in that section and the code above. In :ref:`helloworld_imperative_appconfig`, we had the following lines within the ``if __name__ == '__main__'`` section of ``helloworld.py``: .. 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() serve(app, host='0.0.0.0') 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: .. code-block:: python :linenos: if __name__ == '__main__': config = Configurator() config.begin() config.load_zcml('configure.zcml') config.end() app = config.make_wsgi_app() serve(app, host='0.0.0.0') 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: .. code-block:: xml :linenos: We already understand what the view code does, because the application is functionally equivalent to the application described in :ref:`helloworld_imperative`, but use of :term:`ZCML` is new. Let's break that down tag-by-tag. The ```` Tag ~~~~~~~~~~~~~~~~~~~~~~~ The ``configure.zcml`` ZCML file contains this bit of XML: .. code-block:: xml :linenos: 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 ```` Tag ~~~~~~~~~~~~~~~~~~~~~ The ``configure.zcml`` ZCML file contains this bit of XML within the ```` root tag: .. code-block:: xml This self-closing tag instructs :mod:`repoze.bfg` to load a ZCML file from the Python package with the :term:`dotted Python name` ``repoze.bfg.includes``, as specified by its ``package`` attribute. This particular ```` declaration is required because it actually allows subsequent declaration tags (such as ````, which we'll see shortly) to be recognized. The ```` tag effectively just includes another ZCML file, causing 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 ```` tag named ``file``. We could have spelled the include tag more verbosely, but equivalently as: .. code-block:: xml :linenos: The ```` 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 an error at startup. However, the ```` 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. See also :ref:`include_directive`. The ```` Tag ~~~~~~~~~~~~~~~~~~ The ``configure.zcml`` ZCML file contains these bits of XML *after* the ```` tag, but *within* the ```` root tag: .. code-block:: xml :linenos: These ```` declaration tags direct :mod:`repoze.bfg` to create two :term:`view configuration` registrations. The first ```` 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 ```` 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 ```` tag also has an attribute called ``name`` with a value of ``goodbye``. These effect of the ```` 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 ```` 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 ```` 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* within :ref:`adding_configuration`), the relative order of ```` tags in ZCML doesn't matter either. The following ZCML orderings are completely equivalent: .. topic:: Hello Before Goodbye .. code-block:: xml :linenos: .. topic:: Goodbye Before Hello .. code-block:: xml :linenos: We've now configured a :mod:`repoze.bfg` helloworld application declaratively. More information about this mode of configuration is available in :ref:`declarative_configuration` and within :ref:`zcml_reference`. References ---------- For more information about the API of a :term:`Configurator` object, see :class:`repoze.bfg.configuration.Configurator` . The equivalent ZCML declaration tags are introduced in :ref:`zcml_reference`. For more information about :term:`view configuration`, see :ref:`views_chapter`.