.. _pyxbgen:

Generating Binding Classes
==========================

The following sections reference example schema and programs that are
available in the ``examples/manual`` subdirectory of the PyXB
distribution.

.. note::
   PyXB is developed and tested assuming a POSIX file system, as is used
   on Linux.  While PyXB works perfectly well on Windows, some
   accommodation is required.  In particular, when providing file URIs
   as command-line arguments to ``pyxbgen`` it may be necessary to
   explicitly note that the parameter is a URI by using the `Windows
   file URI form
   <http://blogs.msdn.com/b/ie/archive/2006/12/06/file-uris-in-windows.aspx>`_
   of the file path.

   For example, something like this will generally not work::

     pyxbgen -m x -u "c:\\Windows\My Documents\x.xsd" # DO NOT USE

   This should be expressed as::

     pyxbgen -m x -u file://c:/Windows/My%20Documents/x.xsd


Self-contained schema
---------------------

The following schema ``po1.xsd`` is a condensed version of the `purchase
order schema <http://www.w3.org/TR/xmlschema-0/#POSchema>`_ in the XMLSchema
Primer:

.. literalinclude:: ../examples/manual/po1.xsd

Translate this into Python with the following command:

.. literalinclude:: ../examples/manual/demo1.sh

The :ref:`-u<pyxbgen--schema-location>` parameter identifies a schema
document describing contents of a namespace.  The parameter may be a path to
a file on the local system, or a URL to a network-accessible location like
http://www.weather.gov/forecasts/xml/DWMLgen/schema/DWML.xsd.  The
:ref:`-m<pyxbgen--module>` parameter specifies the name to be used by the
Python module holding the bindings generated for the namespace in the
preceding schema.  After running this, the Python bindings will be in a
file named ``po1.py``.

With the bindings available, this program (``demo1.py``):

.. literalinclude:: ../examples/manual/demo1.py

processing this document:

.. literalinclude:: ../examples/manual/po1.xml

produces the following output:

.. literalinclude:: ../examples/manual/demo1.out

Multi-document schema
---------------------

Complex schema are more easy to manage when they are separated into multiple
documents, each of which contains a cohesive set of types.  In the example
above, the ``USAddress`` type can be abstracted to handle a variety of
addresses, and maintained as its own document ``address.xsd``:

.. _address_xsd:

.. literalinclude:: ../examples/manual/address.xsd

The XMLSchema `include directive
<http://www.w3.org/TR/xmlschema-1/#compound-schema>`_ can be used to
incorporate this document into ``po2.xsd``:

.. literalinclude:: ../examples/manual/po2.xsd

Translation of this document and execution of the test program is just as it
was in the previous section:

.. literalinclude:: ../examples/manual/demo2.sh

Note that you do not need to explicitly list the ``address.xsd`` file.  PyXB
detects the ``include`` directive and reads the second schema by resolving
its ``schemaLocation`` relative to the base URI of the containing document.
Because the contents of the two schema files belong to the same namespace,
their combined bindings are placed into the ``po2.py`` module.

Working with Namespaces
-----------------------

Documents of significant complexity are likely to require references to
multiple namespaces.  Notice that the schemas we've looked at so far have
:ref:`no namespace <absentNamespaces>` for both their target and default
namespaces.  The following schema ``nsaddress.xsd`` places the types that
are in :ref:`address.xsd <address_xsd>` into the namespace ``URN:address``
by defining a `target namespace
<http://www/Documentation/W3C/www.w3.org/TR/xmlschema-0/index.html#QualLocals>`_
then including the namespace-less schema:

.. _nsaddress_xsd:

.. literalinclude:: ../examples/manual/nsaddress.xsd

Note that this technique takes advantage of the `chameleon schema
<http://www.xfront.com/ZeroOneOrManyNamespaces.html#mixed>`_ pattern.

There are several ways you can prepare to process documents with multiple
namespaces.  If you have no expectation of using the imported namespace
directly, you can process the importing schema just as before:

.. literalinclude:: ../examples/manual/demo3a.sh

PyXB will detect the ``import`` statement, read the corresponding schema,
and create bindings for its types.  However, since the ``pyxbgen``
invocation did not mention the ``URN:address`` namespace, the bindings are
written into a :ref:`private <pyxbgen--private-namespace>` binding file.
The generated module file ``_address.py`` is created with a prefixed
underscore indicating that it is not expected to be referenced directly.
The public module ``po3.py`` will locally import module ``_address`` so that
the required classes are available, but will not expose them to code that
imports only module ``po3``.  The demonstration program ``demo3.py`` shows
that things work as expected without the new namespace being made explicit.

.. literalinclude:: ../examples/manual/demo3.py

More often, you will want to be able to import the module defining bindings
from the additional namespaces.  To do this, explicitly reference the
additional schema and provide it with a module name:

.. literalinclude:: ../examples/manual/demo3b.sh

Here each namespace is represented in its own module (``address`` for
``URN:address`` and ``po3`` for module with an absent namespace).  In this
case, the demonstration program is unchanged; see :ref:`from-python` for
additional examples.

Sharing Namespace Bindings
--------------------------

Most often, if you have a common utility namespace like ``URN:address``, you
will want to generate its bindings once, and reference them in other schema
without regenerating them.  To do this, PyXB must be provided with an
archive containing the schema components that were defined in that
namespace, so they can be referenced in independent generation activities.

To generate the archive, you add the :ref:`pyxbgen--archive-to-file`
flag to the binding generation command:

.. literalinclude:: ../examples/manual/demo3c1.sh

In addition to generating the ``address`` Python module, this causes a
:ref:`archive <namespace-archive>` of the schema contents to be saved in the
corresponding file, which by convention ends with the extension ``.wxs``.
Any anonymous names that were generated with the bindings are also recorded
in this archive, so that cross-namespace extension works correctly.

You can then generate bindings for importing namespaces by providing PyXB
with the information necessary to locate this archive:

.. literalinclude:: ../examples/manual/demo3c2.sh

The :ref:`pyxbgen--archive-path` directive indicates that the current
directory (``.``) should be searched for files that end in ``.wxs``, and any
namespaces found in such files implicitly made available for reference when
they are encountered in an ``import`` instruction.  (The second path
component ``+`` causes the standard search path to be used after searching
the current directory.)

In this case, when the ``import`` instruction is encountered, PyXB detects
that it has an archive ``address.wxs`` that defines the contents of the
imported namespace.  Instead of reading and processing the schema, it
generates references to the existing binding modules.  Again, the
demonstration program is unchanged.

Advanced Topics
---------------

Schemas Defined in WSDL Documents
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It is a common, if regrettable, practice that web services define the
structure of their documents using XML schema elements encoded directly into
a ``types`` element of a WSDL specification rather than having that elements
import complete standalone schema.  To accommodate this, pyxbgen supports
the :ref:`pyxbgen--wsdl-location` argument as an alternative to
:ref:`pyxbgen--schema-location`.  For example, the following will generate a
module ``ndfd`` containing bindings required to communicate with the
`National Digital Forecast Database <http://www.nws.noaa.gov/xml>`_::

  pyxbgen \
   --wsdl-location=http://www.weather.gov/forecasts/xml/DWMLgen/wsdl/ndfdXML.wsdl --module=ndfd \
   --archive-path=${PYXB_ROOT}/pyxb/bundles/wssplat//:+

Note that it will be necessary to have the :ref:`WS-* <bundle_wssplat>`
bindings available, as provided by the :ref:`pyxbgen--archive-path` option
above.

.. _customized_bindings:

Customizing Binding Classes
^^^^^^^^^^^^^^^^^^^^^^^^^^^

PyXB permits you to customize the bindings that it generates by creating a
module that imports the generated classes and instances, then extends them
with subclasses with additional behavior.  As long as you do not make major
changes to the structure and names used in your namespaces, you can
fine-tune the schema without changing the custom code.

The :ref:`pyxbgen--write-for-customization` option causes PyXB to generate
all the Python modules in a subdirectory ``raw``.  Then you write a module
that imports the generated bindings and extends them.

Until this documentation is enhanced significantly, users interested in
generating custom bindings are referred to the extensions for WSDL 1.1 that
are provided in the WS-* support bundle as
``pyxb.bundles.wssplat.wsdl11.py``.  An excerpt of the sort of thing done
there is::

  from pyxb.bundles.wssplat.raw.wsdl11 import *
  import pyxb.bundles.wssplat.raw.wsdl11 as raw_wsdl11

  class tParam (raw_wsdl11.tParam):
    def __getMessageReference (self):
        return self.__messageReference
    def _setMessageReference (self, message_reference):
        self.__messageReference = message_reference
    __messageReference = None
    messageReference = property(__getMessageReference)
  raw_wsdl11.tParam._SetSupersedingClass(tParam)

The first line brings in all the public identifiers from the generated
binding.  The second makes them available in a qualified form that ensures
we use the generated value rather than the customized value.

The class definition shows how to extend the generated bindings for the
``tParam`` complex type so that it has a field that can hold the instance
of ``tMessage`` that was identified by the ``message`` attribute in an
``operation`` element.  Following the class is a directive that tells PyXB to
create instances of the customized class when automatically generating
``tParam`` instances from XML documents.

To customize bindings, you will need to be familiar with the
:py:obj:`pyxb.binding.basis._DynamicCreate_mixin` class.

Be aware that :py:obj:`_SetSupersedingClass
<pyxb.binding.basis._DynamicCreate_mixin>` only affects the behavior of
:py:obj:`Factory <pyxb.binding.basis._TypeBinding_mixin>`, and does not change
the Python inheritance tree.  This means that the superseding class is only
invoked when the content model requires an instance of the original type.
When an instance of a subclass of a superseded class (that is not itself
superseded) is needed by the content model, this infrastructure is bypassed,
the normal Python inheritance mechanism takes control, and the instance will
not be an instance of the superseding class.  This will happen both when
instances are created in Python directly and when they are created due to
presence in the binding model.

This is probably not what you will want, and to avoid it you must customize
all subclasses of a customized class.  A detailed example customization is
in the :file:`examples/customization` subdirectory of the distribution.  In
particular, it shows how to introspect the binding model extracted from the
generated Python module and programmatically create custom binding classes
without manually reproducing the content hierarchy, making the customizing
module more compact and stable.

Generating Related Namespaces
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The :ref:`pyxbgen--module-prefix` option permits you to add a fixed prefix
to the generated modules.  For example, when generating bindings for the
OpenGIS schemas it is desirable to aggregate them into a Python module
hierarchy so the imported name incorporates the namespace collection::

 pyxbgen \
   --schema-location=${SCHEMA_DIR}/gml/3.2.1/gml.xsd --module=gml_3_2 \
   --schema-location=${SCHEMA_DIR}/iso/19139/20070417/gmd/gmd.xsd --module=iso19139.gmd \
   --schema-location=${SCHEMA_DIR}/iso/19139/20070417/gts/gts.xsd --module=iso19139.gts \
   --schema-location=${SCHEMA_DIR}/iso/19139/20070417/gsr/gsr.xsd --module=iso19139.gsr \
   --schema-location=${SCHEMA_DIR}/iso/19139/20070417/gss/gss.xsd --module=iso19139.gss \
   --schema-location=${SCHEMA_DIR}/iso/19139/20070417/gco/gco.xsd --module=iso19139.gco \
   --module-prefix=opengis \
   --archive-to-file=opengis/iso19139.core.wxs

When generated this way, your Python code imports these modules with
directives like::

  import opengis.gml_3_2 as gml
  import opengis.iso19139.gmd

PyXB comes with :ref:`pre-defined bundles <bundles>` for related namespaces
in the ``pyxb.bundles`` module hierarchy.  The command above is an excerpt
from an early version of the script that generates the :ref:`OpenGIS
<bundle_opengis>` bundles.  See :ref:`Layout of a Bundle Directory
<bundle_layout>` for more information and the location of the current
version of the script.

Fine-Grained Namespace Control
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In certain cases, schema developers will presume that it is within their
purview to re-declare or extend the contents of namespaces that belong to
others.  Supporting this while preserving or re-using the original namespace
contents requires finesse.

For example, when generating the bindings for the OpenGIS `Sensor
Observation Service <http://www.opengeospatial.org/standards/sos>`_, you
would find that this service extends the ``http://www.opengis.net/ogc``
namespace, normally defined in the OpenGIS `Filter Encoding
<http://www.opengeospatial.org/standards/filter>`_, with temporal operators
that are defined in a local schema ``ogc4sos.xsd``.

Because ``http://www.opengis.net/ogc`` is defined in a namespace archive,
PyXB would normally assume that any ``import`` commands related to that
namespace are redundant with the contents of that archive, and would ignore
the import directive.  In this case, that assumption is mistaken, and the
``ogc4sos.xsd`` schema must be read to define the additional elements and
types.  The required build command is::

  pyxbgen \
    --schema-location=${SCHEMA_DIR}/sos/1.0.0/sosAll.xsd --module sos_1_0 \
    --archive-path=${ARCHIVE_DIR} \
    --import-augmentable-namespace=http://www.opengis.net/ogc

The :ref:`pyxbgen--import-augmentable-namespace` directive causes PyXB to
allow import directives within the schema to add material to the content
already loaded from an archive.  Consequently, when reference to the
``ogc4sos.xsd`` schema is encountered, PyXB detects that, although it
already has definitions for components in that namespace, this particular
schema has not yet been read.  PyXB reads the additional components, and
generates bindings for the additional material into a private module
``_ogc`` which is then imported into the ``sos_1_0`` module.
