.. _usebind:

Using Binding Classes
=====================

Python instances corresponding to XML structures can be created in two
primary ways: from XML documents, and directly within Python code.
Generating XML documents from bindings can also be controlled.

.. _from-xml:

Creating Instances from XML Documents
-------------------------------------

XML documents are converted into Python bindings by invoking the
``CreateFromDocument`` function in a binding module.  For example:

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

The ``CreateFromDocument`` function in a given binding module is configured
so that documents with no default namespace are assumed to be in the
namespace from which the binding was generated.

.. _invalid-content:

Locating Invalid Content
^^^^^^^^^^^^^^^^^^^^^^^^

If a document does not validate, PyXB will generally through an
:api:`pyxb.UnrecognizedContentError` exception.  You can determine where the
problem lies, and what was not recognized, by examining attributes present
on the exception as shown in this example:

.. literalinclude:: ../examples/manual/badcontent.py

which produces:

.. literalinclude:: ../examples/manual/badcontent.out

Coping With Wrong ``xsi:type`` Attributes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Some web services and binding tools mis-use `xsi:type
<http://www.w3.org/TR/xmlschema-1/#xsi_type>`_, providing attribute values
that either are not types, or do not specify a type that is derived from an
abstract type.  The
:api:`pyxb.namespace.builtin.XMLSchema_instance.ProcessTypeAttribute
<pyxb.namespace.builtin._XMLSchema_instance.ProcessTypeAttribute>` method
can be used to relax how PyXB processes those attributes.

.. _from-python:

Creating Instances in Python Code
---------------------------------

Creating bindings from XML documents is straightforward, because the
documents contain enough information to identify each element and attribute,
locate the corresponding use in the binding class, and store a value that is
converted to the appropriate type.  Creating values in Python is inherently
more complex, because native Python objects like strings and integers do not
contain this information.

As described in :ref:`bindingModel`, binding classes corresponding to simple
types extend the underlying Python type (such as ``str`` or ``int``), and
add XML-specific information like the canonical representation of the value
in `Unicode <http://www.unicode.org/>`_, which is the natural representation
as XML text.  These classes also maintain a set of facets that constrain the
values that can be stored as instances when validation is active.  Binding
classes for complex types have constructors that parse positional and
keyword parameters to determine the appropriate element or attribute to
which the value belongs.  Attributes are assigned using keyword parameters.
Content is assigned using positional parameters.  The order of the
positional parameters must be consistent with the order expected by the
content model.

Using the schema in the :ref:`namespace-aware address schema
<nsaddress_xsd>`, we can begin to construct the example document in Python:

.. literalinclude:: ../examples/manual/demo4a.py

.. note::

   It is necessary to provide an ``element_name`` parameter to ``to_xml``
   because in this case ``USAddress`` is the name of a complex type, not an
   top-level schema element.  PyXB cannot generate XML for an instance unless
   it knows the name to use for the root element.  In most situations PyXB can
   figure out what element the instance belongs to, as when the instance is
   created through an element binding instead of a type binding and when it is
   assigned into another instance, both of which are seen in `demo4c`_.

This produces:

.. literalinclude:: ../examples/manual/demo4a.out

Assigning to individual fields like this bypasses the complex type content
model, although each field itself is validated.  For example, the address
schema does not include New York as a state, so the following assignment::

  addr.state = 'NY'

will cause a :api:`pyxb.exceptions_.BadTypeValueError` exception to be raised:

.. literalinclude:: ../examples/manual/demo4a1.out

However, the order of the field assignments does not matter, as long as all
required fields are present by the time the XML document is generated.

.. literalinclude:: ../examples/manual/demo4a2.py

Alternatively, you can provide the content as positional parameters in the
object creation call:

.. literalinclude:: ../examples/manual/demo4b.py

This has the same effect, and is much more compact, but it does require that
the order match the content model.

.. _demo4c:

Attributes are set using keyword parameters:

.. literalinclude:: ../examples/manual/demo4c.py

This example produces (after reformatting):

.. literalinclude:: ../examples/manual/demo4c.out

.. index::
   pair: disabling; validation

Note that, because we're in the middle of the example and have not provided
the ``items`` element that the content model requires, the code
:api:`explicitly disables the requirement for
validation <pyxb.RequireValidWhenGenerating>` when generating XML from a
binding instance.  A consequence of this is that the generated XML is not
valid, and validation must be :api:`disabled for parsing
<pyxb.RequireValidWhenParsing>` as well if the resulting document is to be
re-converted into a binding with ``CreateFromDocument``.

.. _pyxb_BIND:

Creating Instances of Anonymous Types
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The style of XML schema used for purchase orders uses anonymous types for
the deeper elements of the purchase order:

.. literalinclude:: ../examples/manual/po4.xsd

.. index::
   single: BIND

In particular, there is no global ``item`` element that can be used to
create the individual items.  For situations like this, we use 
:api:`pyxb.BIND`:

.. literalinclude:: ../examples/manual/demo4c1.py

The :api:`pyxb.BIND` reference wraps the content of the inner elements, and
is a cue to PyXB to attempt to build an instance of whatever type of object
would satisfy the content model at that point.  The resulting document
(after reformatting) is:

.. literalinclude:: ../examples/manual/demo4c1.out

The complete document is generated by the following program:

.. literalinclude:: ../examples/manual/demo4c2.py

The additional code demonstrates a couple additional features:

 - Fixed attribute values (such as ``country``) are present in the bindings,
   even though they are only printed if they are set explicitly

 - The PyXB types for representing dates and times are extensions of those
   used by Python for the same purpose, including the ability to use them in
   expressions

.. _to-xml:

Creating XML Documents from Binding Instances
---------------------------------------------

All along we've been seeing how to generate XML from a binding instance.
The ``toxml`` method is short-hand for a sequence that converts the binding
to a DOM instance using ``xml.dom.minidom``, then uses the DOM interface to
generate the XML document.

The :api:`pyxb.utils.domutils.BindingDOMSupport` class provides ways to
control this generation.  In particular, you may want to use something more
informative than ``ns#`` to denote namespaces in the generated documents.
This can be done using the following code:

.. literalinclude:: ../examples/manual/demo4c3.py
   :lines: 20-

With this, the final document produced is:

.. literalinclude:: ../examples/manual/demo4c3.out

(Surprise: ``addr`` does not appear, because the ``nsaddress.xsd`` schema
uses the default element form ``unqualified``, so none of the address
components in the document have a namespace.)

.. _mixed_content:

Influencing Element and Mixed Content Order
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

PyXB generally expects that any information reflected in the order of elements
is controlled by the content model in the schema.  Where content includes
multiple instances of the same element, they are maintained in order within
the binding attribute corresponding to the name.  Historically relative order
with other elements or with mixed content historically was not rigorously
maintained, and generated documents applied only the order enforced by the
content model.

The following example from ``examples/xhtml/generate.py`` hints at the difficulty:

.. literalinclude:: ../examples/xhtml/generate.py

If the relative order of elements and mixed content were not maintained, this
might produce something like:

.. literalinclude:: eltonly.xhtml

Here mixed content is lost, and element content is emitted in the order that
elements appear in the original schema.

As of release 1.2.1 [#content]_, PyXB appends both element and non-element content to a
list in each complex binding instance.  The list may be obtained using the
:api:`orderedContent
<pyxb.binding.basis.complexTypeDefinition.orderedContent>` method.  The list
comprises instances of :api:`pyxb.binding.basis.ElementContent` and
:api:`pyxb.binding.basis.NonElementContent` added in the order in which they were
added to the binding instance: when creating the instance from a document or
through a constructor, or by invoking the
:api:`append <pyxb.binding.basis.complexTypeDefinition.append>` or
:api:`extend <pyxb.binding.basis.complexTypeDefinition.extend>` methods to add
content consistent with the content model.

The :api:`contentInfluencesGeneration
<pyxb.ValidationConfig.contentInfluencesGeneration>` flag of
:api:`pyxb.ValidationConfig` controls how the ``orderedContent`` list affects
generation of documents (both DOM directly and XML indirectly).  With the
default value of :api:`MIXED_ONLY <pyxb.ValidationConfig.MIXED_ONLY>` the
``orderedContent`` list is only consulted when a complex type allows both
element and non-element content.

The bundle for XHTML has been modified to use:

- :api:`ALWAYS <pyxb.ValidationConfig.ALWAYS>` for :api:`contentInfluencesGeneration
  <pyxb.ValidationConfig.contentInfluencesGeneration>`
- :api:`RAISE_EXCEPTION <pyxb.ValidationConfig.RAISE_EXCEPTION>` for
  :api:`orphanElementInContent <pyxb.ValidationConfig.orphanElementInContent>`
- :api:`RAISE_EXCEPTION <pyxb.ValidationConfig.RAISE_EXCEPTION>` for
  :api:`invalidElementInContent <pyxb.ValidationConfig.invalidElementInContent>`

for all binding classes in that module.  (See
``pyxb/bundles/common/xhtml1.py`` for the technique used.)  This ensures
preservation of element order in cases where no non-element content may appear
(such as the top-level ``body`` element).

With this capability the following document is generated:

.. literalinclude:: ../examples/xhtml/expout.xhtml

Be aware that the automatically-maintained ``orderedContent`` list will be
incorrect in at least two cases:

- When the elements of an instance are mutated through Python code, the list
  no longer reflects the correct order;

- When elements are appended directly to sub-elements as with:

  .. code-block:: python

     p2.b.append('another bit of bold')

  the newly added elements do not appear in the ``orderedContent`` list.

The value returned by :api:`orderedContent
<pyxb.binding.basis.complexTypeDefinition.orderedContent>` is a mutable list
so that you can manipulate it to reflect the content you wish to have
generated.

Where the ``orderedContent`` list is not consistent with the content model
(e.g., references elements that are no longer part of the binding instance, or
proposes an order that is not valid) various exceptions may arise.  To some
extent this can be controlled through the :api:`orphanElementInContent
<pyxb.ValidationConfig.orphanElementInContent>` and
:api:`invalidElementInContent <pyxb.ValidationConfig.invalidElementInContent>`
flags.

.. [#content] Though previous versions also provided this information through
   a ``content`` list, the list did not associate content with the element to
   which it belonged making it difficult to reconstruct a valid document.

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