.. _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 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. 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``. 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.)
