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# 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.

## Creating Instances from XML Documents

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

    from __future__ import print_function
    import po3

    order = po3.CreateFromDocument(open('po3.xml').read())

    print('%s is sending %s %d thing(s):' % (order.billTo.name, order.shipTo.name, len(order.items.item)))
    for item in order.items.item:
        print('  Quantity %d of %s at $%s' % (item.quantity, item.productName, item.USPrice))

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.

### Locating Invalid Content

If a document does not validate, PyXB will generally through an 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:

    from __future__ import print_function
    import pyxb
    import po1

    xml = open('badcontent.xml').read()
    try:
        order = po1.CreateFromDocument(xml, location_base='badcontent.xml')
    except pyxb.UnrecognizedContentError as e:
        print('Unrecognized element "%s" at %s' % (e.content.expanded_name, e.content.location))

which produces:

    Unrecognized element "streeet" at badcontent.xml[5:4]

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

Some web services and binding tools mis-use xsi:type, providing attribute values that either are not types, or do not specify a type that is derived from an abstract type. The pyxb.namespace.builtin.XMLSchema_instance.ProcessTypeAttribute method can be used to relax how PyXB processes those attributes.

## 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 Binding Model, 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, 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 namespace-aware address schema, we can begin to construct the example document in Python:

    from __future__ import print_function
    import address

    addr = address.USAddress()
    addr.name = 'Robert Smith'
    addr.street = '8 Oak Avenue'
    addr.city = 'Anytown'
    addr.state = 'AK'
    addr.zip = 12341

    print(addr.toxml("utf-8"))

This produces:

    <?xml version="1.0" encoding="utf-8"?><ns1:USAddress xmlns:ns1="URN:address"><name>Robert Smith</name><street>8 Oak Avenue</street><city>Anytown</city><state>AK</state><zip>12341</zip></ns1:USAddress>

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 BadTypeValueError exception to be raised:

    Traceback (most recent call last):
      File "demo4a1.py", line 7, in <module>
        addr.state = 'NY'
      File "/opt/pyxb/pyxb/binding/basis.py", line 49, in __setattr__
        return super(_TypeBinding_mixin, self).__setattr__(name, value)
      File "/opt/pyxb/pyxb/binding/content.py", line 557, in set
        value = self.__elementBinding.compatibleValue(value, is_plural=self.isPlural())
      File "/opt/pyxb/pyxb/binding/basis.py", line 1415, in compatibleValue
        return self.typeDefinition()._CompatibleValue(value, **kw)
      File "/opt/pyxb/pyxb/binding/basis.py", line 280, in _CompatibleValue
        return cls(value)
      File "/opt/pyxb/pyxb/binding/basis.py", line 791, in __init__
        self.xsdConstraintsOK()
      File "/opt/pyxb/pyxb/binding/basis.py", line 924, in xsdConstraintsOK
        return self.XsdConstraintsOK(self)
      File "/opt/pyxb/pyxb/binding/basis.py", line 918, in XsdConstraintsOK
        raise pyxb.SimpleTypeValueError('%s violation for %s in %s' % (f.Name(), value, cls.__name__))
    pyxb.exceptions_.SimpleTypeValueError: enumeration violation for NY in USState

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.

    from __future__ import print_function
    import address

    addr = address.USAddress()
    addr.street = '8 Oak Avenue'
    addr.state = 'AK'
    addr.city = 'Anytown'
    addr.zip = 12341
    addr.name = 'Robert Smith'

    print(addr.toxml("utf-8"))

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

    # examples/manual/demo4b.py
    from __future__ import print_function

    import address

    addr = address.USAddress('Robert Smith', '8 Oak Avenue', 'Anytown', 'AK', 12341)

    print(addr.toxml("utf-8"))

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:

    # examples/manual/demo4c.py
    from __future__ import print_function

    import pyxb
    import po4
    import address
    import pyxb.binding.datatypes as xs

    po = po4.purchaseOrder(orderDate=xs.date(1999, 10, 20))
    po.shipTo = address.USAddress('Alice Smith', '123 Maple Street', 'Anytown', 'AK', 12341)
    po.billTo = address.USAddress('Robert Smith', '8 Oak Avenue', 'Anytown', 'AK', 12341)

    pyxb.RequireValidWhenGenerating(False)
    print(po.toxml("utf-8"))

This example produces (after reformatting):

    <?xml version="1.0" encoding="utf-8"?>
    <ns1:purchaseOrder xmlns:ns1="URN:purchase-order" orderDate="1999-10-20">
      <ns1:billTo>
        <city>Anytown</city>
        <state>AK</state>
        <street>8 Oak Avenue</street>
        <name>Robert Smith</name>
        <zip>12341</zip>
      </ns1:billTo>
      <ns1:shipTo>
        <city>Anytown</city>
        <state>AK</state>
        <street>123 Maple Street</street>
        <name>Alice Smith</name>
        <zip>12341</zip>
      </ns1:shipTo>
    </ns1:purchaseOrder>

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 explicitly disables the requirement for validation when generating XML from a binding instance. A consequence of this is that the generated XML is not valid, and validation must be disabled for parsing 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:

    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
       targetNamespace="URN:purchase-order"
       xmlns:tns="URN:purchase-order"
       xmlns:address="URN:address"
       elementFormDefault="qualified">
      <xsd:import namespace="URN:address" schemaLocation="nsaddress.xsd"/>
      <xsd:element name="purchaseOrder" type="tns:PurchaseOrderType"/>
      <xsd:element name="comment" type="xsd:string"/>
      <xsd:complexType name="PurchaseOrderType">
        <xsd:sequence>
          <xsd:element name="shipTo" type="address:USAddress"/>
          <xsd:element name="billTo" type="address:USAddress"/>
          <xsd:element ref="tns:comment" minOccurs="0"/>
          <xsd:element name="items"  type="tns:Items"/>
        </xsd:sequence>
        <xsd:attribute name="orderDate" type="xsd:date"/>
      </xsd:complexType>
      <xsd:complexType name="Items">
        <xsd:sequence>
          <xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="productName" type="xsd:string"/>
                <xsd:element name="quantity">
                  <xsd:simpleType>
                    <xsd:restriction base="xsd:positiveInteger">
                      <xsd:maxExclusive value="100"/>
                    </xsd:restriction>
                  </xsd:simpleType>
                </xsd:element>
                <xsd:element name="USPrice"  type="xsd:decimal"/>
                <xsd:element ref="tns:comment"   minOccurs="0"/>
                <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
              </xsd:sequence>
              <xsd:attribute name="partNum" type="tns:SKU" use="required"/>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
      </xsd:complexType>
      <!-- Stock Keeping Unit, a code for identifying products -->
      <xsd:simpleType name="SKU">
        <xsd:restriction base="xsd:string">
          <xsd:pattern value="\d{3}-[A-Z]{2}"/>
        </xsd:restriction>
      </xsd:simpleType>

    </xsd:schema>

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

    from __future__ import print_function
    import pyxb
    import po4
    import address
    import pyxb.binding.datatypes as xs

    po = po4.purchaseOrder(orderDate=xs.date(1999, 10, 20))
    po.shipTo = address.USAddress('Alice Smith', '123 Maple Street', 'Anytown', 'AK', 12341)
    po.billTo = address.USAddress('Robert Smith', '8 Oak Avenue', 'Anytown', 'AK', 12341)
    po.items = pyxb.BIND(pyxb.BIND('Lapis necklace', 1, 99.95, partNum='833-AA'),
                         pyxb.BIND('Plastic necklace', 4, 3.95, partNum='833-AB'))

    print(po.toxml("utf-8"))

The 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:

    <?xml version="1.0" encoding="utf-8"?>
    <ns1:purchaseOrder xmlns:ns1="URN:purchase-order" orderDate="1999-10-20">
      <ns1:shipTo>
        <name>Alice Smith</name>
        <street>123 Maple Street</street>
        <city>Anytown</city>
        <state>AK</state>
        <zip>12341</zip>
      </ns1:shipTo>
      <ns1:billTo>
        <name>Robert Smith</name>
        <street>8 Oak Avenue</street>
        <city>Anytown</city>
        <state>AK</state>
        <zip>12341</zip>
      </ns1:billTo>
      <ns1:items>
        <ns1:item partNum="833-AA">
          <ns1:productName>Lapis necklace</ns1:productName>
          <ns1:quantity>1</ns1:quantity>
          <ns1:USPrice>99.95</ns1:USPrice>
        </ns1:item>
        <ns1:item partNum="833-AB">
          <ns1:productName>Plastic necklace</ns1:productName>
          <ns1:quantity>4</ns1:quantity>
          <ns1:USPrice>3.95</ns1:USPrice>
        </ns1:item>
      </ns1:items>
    </ns1:purchaseOrder>

The complete document is generated by the following program:

    from __future__ import print_function
    import pyxb
    import po4
    import address
    import pyxb.binding.datatypes as xs
    import datetime

    po = po4.purchaseOrder(orderDate=xs.date(1999, 10, 20))
    po.shipTo = address.USAddress('Alice Smith', '123 Maple Street', 'Anytown', 'AK', 12341)
    po.billTo = address.USAddress('Robert Smith', '8 Oak Avenue', 'Anytown', 'AK', 12341)
    po.items = pyxb.BIND(pyxb.BIND('Lapis necklace', 1, 99.95, partNum='833-AA'),
                         pyxb.BIND('Plastic necklace', 4, 3.95, partNum='833-AB'))

    po.shipTo.country = po.billTo.country = po.shipTo.country

    lapis = po.items.item[0]
    lapis.shipDate = po.orderDate + datetime.timedelta(days=46)
    lapis.comment = 'Want this for the holidays!'
    po.items.item[1].shipDate = po.items.item[0].shipDate + datetime.timedelta(days=19)

    print(po.toxml("utf-8"))

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

## 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 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:

    import pyxb.utils.domutils
    pyxb.utils.domutils.BindingDOMSupport.DeclareNamespace(address.Namespace, 'addr')
    pyxb.utils.domutils.BindingDOMSupport.DeclareNamespace(po4.Namespace, 'po')

    print(po.toxml("utf-8"))

With this, the final document produced is:

    <?xml version="1.0" encoding="utf-8"?>
    <po:purchaseOrder xmlns:po="URN:purchase-order" orderDate="1999-10-20">
      <po:shipTo country="US">
        <name>Alice Smith</name>
        <street>123 Maple Street</street>
        <city>Anytown</city>
        <state>AK</state>
        <zip>12341</zip>
      </po:shipTo>
      <po:billTo country="US">
        <name>Robert Smith</name>
        <street>8 Oak Avenue</street>
        <city>Anytown</city>
        <state>AK</state>
        <zip>12341</zip>
      </po:billTo>
      <po:items>
        <po:item partNum="833-AA">
          <po:productName>Lapis necklace</po:productName>
          <po:quantity>1</po:quantity>
          <po:USPrice>99.95</po:USPrice>
          <po:comment>Want this for the holidays!</po:comment>
          <po:shipDate>1999-12-05</po:shipDate>
        </po:item>
        <po:item partNum="833-AB">
          <po:productName>Plastic necklace</po:productName>
          <po:quantity>4</po:quantity>
          <po:USPrice>3.95</po:USPrice>
          <po:shipDate>1999-12-24</po:shipDate>
        </po:item>
      </po:items>
    </po:purchaseOrder>

(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.)

### 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:

    # -*- coding: utf-8 -*-
    from __future__ import print_function
    import pyxb.bundles.common.xhtml1 as xhtml
    import pyxb.utils.domutils

    pyxb.utils.domutils.BindingDOMSupport.SetDefaultNamespace(xhtml.Namespace)

    head = xhtml.head(title='A Test Document')
    body = xhtml.body()
    body.append(xhtml.h1('Contents'))
    body.append(xhtml.p('''Here is some text.

    It doesn't do anything special.'''))

    p2 = xhtml.p('Here is more text.  It has ',
                 xhtml.b('bold'),
                 ' and ',
                 xhtml.em('emphasized'),
                 ' content with ',
                 xhtml.b('more bold'),
                 ' just to complicate things.')
    body.append(p2)

    # Verify we have two b's and an em
    assert 2 == len(p2.b)
    assert 1 == len(p2.em)

    # Generate the document and externally verify that the em is between the two bs.
    doc = xhtml.html(head, body)
    try:
        xmls = doc.toDOM().toprettyxml()
    except pyxb.ValidationError as e:
        print(e.details())
        raise
    open('genout.xhtml', 'w').write(xmls)

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

    <?xml version="1.0" ?>
    <html xmlns="http://www.w3.org/1999/xhtml">
    	<head>
    		<title/>
    	</head>
    	<body>
    		<h1/>
    		<p/>
    		<p>
    			<b/>
    			<b/>
    			<em/>
    		</p>
    	</body>
    </html>

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 [1], PyXB appends both element and non-element content to a list in each complex binding instance. The list may be obtained using the orderedContent method. The list comprises instances of ElementContent and 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 append or extend methods to add content consistent with the content model.

The contentInfluencesGeneration flag of ValidationConfig controls how the `orderedContent` list affects generation of documents (both DOM directly and XML indirectly). With the default value of 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:

- ALWAYS for contentInfluencesGeneration
- RAISE_EXCEPTION for orphanElementInContent
- RAISE_EXCEPTION for 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:

    <?xml version="1.0" ?>
    <html xmlns="http://www.w3.org/1999/xhtml">
    	<head>
    		<title>A Test Document</title>
    	</head>
    	<body>
    		<h1>Contents</h1>
    		<p>Here is some text.

    It doesn't do anything special.</p>
    		<p>
    			Here is more text.  It has
    			<b>bold</b>
    			 and
    			<em>emphasized</em>
    			 content with
    			<b>more bold</b>
    			 just to complicate things.
    		</p>
    	</body>
    </html>

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:

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

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

The value returned by 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 orphanElementInContent and invalidElementInContent flags.

[1] | 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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