.. _declarative_chapter:

Declarative Configuration
=========================

The mode of configuration most comprehensively detailed by examples in
narrative chapters in this book is "imperative" configuration. 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 issue configuration statements.  However, another mode of
configuration exists within :mod:`pyramid`, which often provides better
extensibility and configuration conflict detection.

A complete listing of ZCML directives is available within
:ref:`zcml_directives`.  This chapter provides an overview of how you might
get started with ZCML and highlights some common tasks performed when you use
ZCML.  You can get a better understanding of when it's appropriate to use
ZCML from :ref:`extending_chapter`.

.. index::
   single: declarative configuration

.. _declarative_configuration:

Declarative Configuration
-------------------------

A :mod:`pyramid` application can 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.

A :mod:`pyramid` application configured declaratively requires not
one, but two files: a Python file and a :term:`ZCML` file.

In a file named ``helloworld.py``:

.. code-block:: python
   :linenos:

   from paste.httpserver import serve
   from pyramid.response import Response
   from pyramid.configuration import Configurator

   def hello_world(request):
       return Response('Hello 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')

In a file named ``configure.zcml`` in the same directory as the
previously created ``helloworld.py``:

.. code-block:: xml
   :linenos:

   <configure xmlns="http://pylonshq.com/pyramid">

     <include package="pyramid.includes" />

     <view
        view="helloworld.hello_world"
        />

   </configure>

This pair of files forms an application functionally equivalent to the
application we created earlier in :ref:`imperative_configuration`.
Let's examine the differences between that code listing and the code
above.

In :ref:`imperative_configuration`, 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.end()
       app = config.make_wsgi_app()
       serve(app, host='0.0.0.0')

In our "declarative" code, we've removed the call to ``add_view`` and
replaced it with a call to the
:meth:`pyramid.configuration.Configurator.load_zcml` method 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 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:

   <configure xmlns="http://pylonshq.com/pyramid">

      <include package="pyramid.includes" />

      <view
         view="helloworld.hello_world"
         />

   </configure>

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 ``helloworld`` Python
module.

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:`pyramid.configuration.Configurator.add_view` method on your
behalf.

The ``<view>`` tag is an example of a :mod:`pyramid` declaration
tag.  Other such tags include ``<route>`` and ``<scan>``.  Each of
these tags is effectively a "macro" which calls methods of a
:class:`pyramid.configuration.Configurator` object on your behalf.

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.

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.  If you want to build a
framework or an extensible application, using declarative
configuration is a good idea.

Declarative configuration has an obvious downside: you can't use
plain-old-Python syntax you probably already know and understand to
configure your application; instead you need to use :term:`ZCML`.

.. index::
   single: ZCML conflict detection

ZCML Conflict Detection
~~~~~~~~~~~~~~~~~~~~~~~

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:

.. code-block:: xml
   :linenos:

    <configure xmlns="http://pylonshq.com/pyramid">

      <include package="pyramid.includes" />

      <view
        view="helloworld.hello_world"
        />

      <view
        view="helloworld.hello_world"
        />

    </configure>

If you try to use this ZCML file as the source of ZCML for an
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: helloworld (declarative)

.. _helloworld_declarative:

Hello World, Goodbye World (Declarative)
----------------------------------------

Another almost entirely equivalent mode of application configuration
exists named *declarative* configuration.  :mod:`pyramid` 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 pyramid.configuration import Configurator
   from pyramid.response import Response
   from paste.httpserver import serve

   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:

   <configure xmlns="http://pylonshq.com/pyramid">

     <include package="pyramid.includes" />

     <view
        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 in :ref:`helloworld_imperative`.  We can run
it the same way.

.. code-block:: bash

   $ python helloworld.py
   serving on 0.0.0.0:8080 view at http://127.0.0.1:8080

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:`pyramid.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:

   <configure xmlns="http://pylonshq.com/pyramid">

      <include package="pyramid.includes" />

      <view
         view="helloworld.hello_world"
         />

      <view
         name="goodbye"
         view="helloworld.goodbye_world"
         />

   </configure>

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 ``<configure>`` Tag
~~~~~~~~~~~~~~~~~~~~~~~

The ``configure.zcml`` ZCML file contains this bit of XML:

.. code-block:: xml
   :linenos:

    <configure xmlns="http://pylonshq.com/pyramid">

       <!-- 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:`pyramid` 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="pyramid.includes" />

This self-closing tag instructs :mod:`pyramid` to load a ZCML file
from the Python package with the :term:`dotted Python name`
``pyramid.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, causing its declarations
to be executed.  In this case, we want to load the declarations from
the file named ``configure.zcml`` within the
:mod:`pyramid.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="pyramid.includes" 
            file="configure.zcml"/>

The ``<include>`` tag that includes the ZCML statements implied by the
``configure.zcml`` file from the Python package named
:mod:`pyramid.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 ``<include package="pyramid.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.

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:`pyramid` 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:`pyramid.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:`pyramid.configuration.Configurator.add_view` doesn't matter
(see the sidebar entitled *View Dispatch and Ordering* within
:ref:`adding_configuration`), 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"
       />

.. topic:: Goodbye Before Hello

  .. code-block:: xml
     :linenos:

     <view
       name="goodbye"
       view="helloworld.goodbye_world"
       />

     <view
       view="helloworld.hello_world"
       />

We've now configured a :mod:`pyramid` helloworld application
declaratively.  More information about this mode of configuration is
available in :ref:`declarative_configuration` and within
:ref:`zcml_reference`.

Scanning
--------

: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 pyramid.response import Response
      from pyramid.view import view_config
     
      @view_config()
      def hello(request):
          return Response('Hello')

      if __name__ == '__main__':
          from pyramid.configuration import Configurator
          config = Configurator()
          config.begin()
          config.load_zcml('configure.zcml')
          config.end()
          app = config.make_wsgi_app()
          serve(app, host='0.0.0.0')

   .. code-block:: xml
      :linenos:

      <configure xmlns="http://namespaces.repoze.org">

        <!-- configure.zcml -->

        <include package="pyramid.includes"/>
        <scan package="."/>

      </configure>

Which Mode Should I Use?
------------------------

A combination of imperative configuration, declarative configuration
via ZCML and scanning can be used to configure any application.  They
are not mutually exclusive.

The :mod:`pyramid` authors often recommend using mostly declarative
configuration, because it's the more traditional form of configuration
used in :mod:`pyramid` 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.

.. index::
   single: ZCML directive; route

.. _zcml_route_configuration:

Configuring a Route via ZCML
----------------------------

Instead of using the imperative
:meth:`pyramid.configuration.Configurator.add_route` method to add a new
route, you can alternately use :term:`ZCML`.  :ref:`route_directive`
statements in a :term:`ZCML` file used by your application is a sign that
you're using :term:`URL dispatch`.  For example, the following :term:`ZCML
declaration` causes a route to be added to the application.

.. code-block:: xml
   :linenos:

   <route
       name="myroute"
       pattern="/prefix/:one/:two"
       view=".views.myview"
    />

.. note::

   Values prefixed with a period (``.``) within the values of ZCML
   attributes such as the ``view`` attribute of a ``route`` mean
   "relative to the Python package directory in which this
   :term:`ZCML` file is stored".  So if the above ``route``
   declaration was made inside a ``configure.zcml`` file that lived in
   the ``hello`` package, you could replace the relative
   ``.views.myview`` with the absolute ``hello.views.myview`` Either
   the relative or absolute form is functionally equivalent.  It's
   often useful to use the relative form, in case your package's name
   changes.  It's also shorter to type.

The order that routes are evaluated when declarative configuration is used
is the order that they appear relative to each other in the ZCML file.

See :ref:`route_directive` for full ``route`` ZCML directive
documentation.

.. index::
   triple: view; zcml; static resource

.. _zcml_static_resources_section:

Serving Static Resources Using ZCML
------------------------------------

Use of the ``static`` ZCML directive makes static files available at a name
relative to the application root URL, e.g. ``/static``.  

Note that the ``path`` provided to the ``static`` ZCML directive may be a
fully qualified :term:`resource specification`, a package-relative path, or
an *absolute path*.  The ``path`` with the value ``a/b/c/static`` of a
``static`` directive in a ZCML file that resides in the "mypackage" package
will resolve to a package-qualified resource such as
``some_package:a/b/c/static``.

Here's an example of a ``static`` ZCML directive that will serve files
up under the ``/static`` URL from the ``/var/www/static`` directory of
the computer which runs the :mod:`pyramid` application using an
absolute path.

.. code-block:: xml
   :linenos:

   <static
      name="static"
      path="/var/www/static"
      />

Here's an example of a ``static`` directive that will serve files up
under the ``/static`` URL from the ``a/b/c/static`` directory of the
Python package named ``some_package`` using a fully qualified
:term:`resource specification`.

.. code-block:: xml
   :linenos:

   <static
      name="static"
      path="some_package:a/b/c/static"
      />

Here's an example of a ``static`` directive that will serve files up
under the ``/static`` URL from the ``static`` directory of the Python
package in which the ``configure.zcml`` file lives using a
package-relative path.

.. code-block:: xml
   :linenos:

   <static
      name="static"
      path="static"
      />

Whether you use for ``path`` a fully qualified resource specification,
an absolute path, or a package-relative path, When you place your
static files on the filesystem in the directory represented as the
``path`` of the directive, you will then be able to view the static
files in this directory via a browser at URLs prefixed with the
directive's ``name``.  For instance if the ``static`` directive's
``name`` is ``static`` and the static directive's ``path`` is
``/path/to/static``, ``http://localhost:6543/static/foo.js`` will
return the file ``/path/to/static/dir/foo.js``.  The static directory
may contain subdirectories recursively, and any subdirectories may
hold files; these will be resolved by the static view as you would
expect.

While the ``path`` argument can be a number of different things, the
``name`` argument of the ``static`` ZCML directive can also be one of
a number of things: a *view name* or a *URL*.  The above examples have
shown usage of the ``name`` argument as a view name.  When ``name`` is
a *URL* (or any string with a slash (``/``) in it), static resources
can be served from an external webserver.  In this mode, the ``name``
is used as the URL prefix when generating a URL using
:func:`pyramid.url.static_url`.

For example, the ``static`` ZCML directive may be fed a ``name``
argument which is ``http://example.com/images``:

.. code-block:: xml
   :linenos:

   <static
      name="http://example.com/images"
      path="mypackage:images"
      />

Because the ``static`` ZCML directive is provided with a ``name`` argument
that is the URL prefix ``http://example.com/images``, subsequent calls to
:func:`pyramid.url.static_url` with paths that start with the ``path``
argument passed to :meth:`pyramid.url.static_url` will generate a URL
something like ``http://example.com/logo.png``.  The external webserver
listening on ``example.com`` must be itself configured to respond properly to
such a request.  The :func:`pyramid.url.static_url` API is discussed in more
detail later in this chapter.

The :meth:`pyramid.configuration.Configurator.add_static_view` method offers
an imperative equivalent to the ``static`` ZCML directive.  Use of the
``add_static_view`` imperative configuration method is completely equivalent
to using ZCML for the same purpose.  See :ref:`static_resources_section` for
more information.

.. _zcml_authorization_policy:

Enabling an Authorization Policy Via ZCML
-----------------------------------------

If you'd rather use :term:`ZCML` to specify an authorization policy
than imperative configuration, modify the ZCML file loaded by your
application (usually named ``configure.zcml``) to enable an
authorization policy.

For example, to enable a policy which compares the value of an "auth
ticket" cookie passed in the request's environment which contains a
reference to a single :term:`principal` against the principals present
in any :term:`ACL` found in model data when attempting to call some
:term:`view`, modify your ``configure.zcml`` to look something like
this:

.. code-block:: xml
   :linenos:

   <configure xmlns="http://pylonshq.com/pyramid">

     <!-- views and other directives before this... -->

     <authtktauthenticationpolicy
          secret="iamsosecret"/>

     <aclauthorizationpolicy/>

    </configure>

"Under the hood", these statements cause an instance of the class
:class:`pyramid.authentication.AuthTktAuthenticationPolicy` to be
injected as the :term:`authentication policy` used by this application
and an instance of the class
:class:`pyramid.authorization.ACLAuthorizationPolicy` to be
injected as the :term:`authorization policy` used by this application.

:mod:`pyramid` ships with a number of authorization and
authentication policy ZCML directives that should prove useful.  See
:ref:`authentication_policies_directives_section` and
:ref:`authorization_policies_directives_section` for more information.

.. index::
   pair: ZCML directive; authentication policy

.. _authentication_policies_directives_section:

Built-In Authentication Policy ZCML Directives
----------------------------------------------

Instead of configuring an authentication policy and authorization
policy imperatively, :mod:`pyramid` ships with a few "pre-chewed"
authentication policy ZCML directives that you can make use of within
your application.

``authtktauthenticationpolicy``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When this directive is used, authentication information is obtained
from an "auth ticket" cookie value, assumed to be set by a custom
login form.

An example of its usage, with all attributes fully expanded:

.. code-block:: xml
   :linenos:

   <authtktauthenticationpolicy
    secret="goshiamsosecret"
    callback=".somemodule.somefunc"
    cookie_name="mycookiename"
    secure="false"
    include_ip="false"
    timeout="86400"
    reissue_time="600"
    max_age="31536000"
    path="/"
    http_only="false"
    />

See :ref:`authtktauthenticationpolicy_directive` for details about
this directive.

``remoteuserauthenticationpolicy``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When this directive is used, authentication information is obtained
from a ``REMOTE_USER`` key in the WSGI environment, assumed to
be set by a WSGI server or an upstream middleware component.

An example of its usage, with all attributes fully expanded:

.. code-block:: xml
   :linenos:

   <remoteuserauthenticationpolicy
    environ_key="REMOTE_USER"
    callback=".somemodule.somefunc"
    />

See :ref:`remoteuserauthenticationpolicy_directive` for detailed
information.

``repozewho1authenticationpolicy``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When this directive is used, authentication information is obtained
from a ``repoze.who.identity`` key in the WSGI environment, assumed to
be set by :term:`repoze.who` middleware.

An example of its usage, with all attributes fully expanded:

.. code-block:: xml
   :linenos:

   <repozewho1authenticationpolicy
    identifier_name="auth_tkt"
    callback=".somemodule.somefunc"
    />

See :ref:`repozewho1authenticationpolicy_directive` for detailed
information.

.. index::
   pair: ZCML directive; authorization policy

.. _authorization_policies_directives_section:

Built-In Authorization Policy ZCML Directives
---------------------------------------------

``aclauthorizationpolicy``

When this directive is used, authorization information is obtained
from :term:`ACL` objects attached to model instances.

An example of its usage, with all attributes fully expanded:

.. code-block:: xml
   :linenos:

   <aclauthorizationpolicy/>

In other words, it has no configuration attributes; its existence in a
``configure.zcml`` file enables it.

See :ref:`aclauthorizationpolicy_directive` for detailed information.

.. Todo
.. ----

.. - ``narr/project.rst`` chapter describes execution of a paster template that
..   is based on XML.

.. - Skipped views chapter.

.. - i18n chapter still has topics for ZCML

.. - events chapter still has topics for ZCML

.. - hooks chapter still has topics for ZCML

.. - resources chapter still has topics for ZCML