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<div class="section" id="binding-model">
<span id="bindingmodel"></span><h1>Binding Model<a class="headerlink" href="#binding-model" title="Permalink to this headline">¶</a></h1>
<p>The binding model represents the relations between Python classes that
correspond to schema components. The class hierarchy for the binding model
is depicted in the following diagram.</p>
<img alt="_images/BindingModel.jpg" src="_images/BindingModel.jpg" />
<p>There are three primary groups of classes, which in turn depend on some
supporting capabilities, all of which are described in the following
sections.</p>
<div class="section" id="supporting-capabilities">
<h2>Supporting Capabilities<a class="headerlink" href="#supporting-capabilities" title="Permalink to this headline">¶</a></h2>
<div class="section" id="common-binding-instance-features">
<h3>Common Binding Instance Features<a class="headerlink" href="#common-binding-instance-features" title="Permalink to this headline">¶</a></h3>
<p><a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html">_TypeBinding_mixin</a> is a marker class to indicate
that the incorporating class represents a binding associated with a type
definition (whether simple or complex). The key features of this mixin are:</p>
<blockquote>
<ul class="simple">
<li>The <a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html#_ExpandedName">_ExpandedName</a> class variable is
overridden in each class to identify the type definition corresponding to
the class.</li>
<li>The <a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html#_namespaceContext">namespace context</a> of the type
definition is recorded to allow users to perform QName resolution of values
within instance documents (see, for example, the customized bindings in
<tt class="docutils literal"><span class="pre">pyxb.bundles.wssplat.wsdl11</span></tt>).</li>
<li>Each instance records the <a class="reference external" href="api/pyxb.binding.basis.element-class.html">element</a> instance that
determines where the type came from. The element is required in order to
provide the correct name when converting the binding instance to a DOM
instance on its way to expression as a text XML document.</li>
<li>The mixin is also where <a class="reference external" href="http://www.w3.org/TR/xmlschema-1/#xsi_nil">xsi:nil</a> information for the instance
is stored.</li>
<li>A <a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html">Factory</a> infrastructure is
provided to allow creation of new instances of the binding while permitting
developers to customize the generated binding classes; see
<a class="reference internal" href="#binding-customization"><em>Support for Customization</em></a>.</li>
</ul>
</blockquote>
</div>
<div class="section" id="deconflicting-names">
<span id="binding-deconflictingnames"></span><h3>Deconflicting Names<a class="headerlink" href="#deconflicting-names" title="Permalink to this headline">¶</a></h3>
<p>In XML schema, the namespaces for element declarations, type definitions, and
attribute definitions are all distinct. Python uses the same namespace for
everything. So, if you have a schema that defines a simple type <tt class="docutils literal"><span class="pre">color</span></tt>,
the schema can also have an element named <tt class="docutils literal"><span class="pre">color</span></tt> with a complex type that
itself has both a child element named <tt class="docutils literal"><span class="pre">color</span></tt> and a distinct attribute (of
type <tt class="docutils literal"><span class="pre">color</span></tt>) that is also named <tt class="docutils literal"><span class="pre">color</span></tt>. Since the natural
representation of elements and attributes inside complex types is also by
their XML name, the chances of conflict are high.</p>
<p>PyXB resolves this by ensuring every identifiable object has a unique
identifier within its context. The steps involved are:</p>
<ol class="arabic simple">
<li>Make object name into an <a class="reference external" href="api/pyxb.utils.utility-module.html#MakeIdentifier">identifier</a> by stripping out non-printable
characters, replacing characters that cannot appear in identifiers with
underscores, stripping leading underscores, and prefixing an initial
digit with the character <tt class="docutils literal"><span class="pre">n</span></tt>.</li>
<li><a class="reference external" href="api/pyxb.utils.utility-module.html#DeconflictKeyword">Deconflict</a> the resulting
identifier from Python <a class="reference external" href="api/pyxb.utils.utility-module.html#_Keywords">reserved identifiers</a> and other context-specific keywords.</li>
<li>Prepend the standard prefix that denotes the identifier’s <a class="reference external" href="maintref.html#identifier-naming"><em>visibility</em></a> (public, protected, private)</li>
<li>Make the resulting identifier <a class="reference external" href="api/pyxb.utils.utility-module.html#MakeUnique">unique</a>
within its context (containing class or module).</li>
</ol>
<p>These steps are encapsulated into a single function
<a class="reference external" href="api/pyxb.utils.utility-module.html#PrepareIdentifier">PrepareIdentifier</a> which takes parameters that
customize the context for the identifier.</p>
<p>In addition to name conflicts with namespace-global identifiers appearing
directly in the module, conflicts may also appear within a binding class as a
result of collision with names from Python keywords, public class names, and
public field or method names in the class. The
<a class="reference external" href="api/pyxb.utils.utility._DeconflictSymbols_mixin-class.html">_DeconflictSymbols_mixin</a> is used to refine the set
of type-specific public names. If you customize a generated binding class by
extending from it, you must specify your own class variable
<tt class="docutils literal"><span class="pre">_ReservedSymbols</span></tt> with a value that is the union of your symbols and those
of the superclass(es) (see <a class="reference external" href="api/pyxb.utils.utility._DeconflictSymbols_mixin-class.html">_DeconflictSymbols_mixin</a>
for details).</p>
<p>Deconfliction of module-level names occurs prior to <a class="reference external" href="api/pyxb.binding.generate-module.html#GeneratePython">code generation</a>. Identifiers are deconflicted in
favor of higher items on this list:</p>
<ul class="simple">
<li>Python keywords</li>
<li>Public class identifiers</li>
<li>Element tags</li>
<li>Complex or simple type definition tags</li>
<li>Enumeration tags</li>
<li>Attribute tags</li>
</ul>
</div>
<div class="section" id="support-for-customization">
<span id="binding-customization"></span><h3>Support for Customization<a class="headerlink" href="#support-for-customization" title="Permalink to this headline">¶</a></h3>
<p>One of the primary goals of PyXB is to support Python modules which customize
the generated bindings by adding both functionality and derived content.
Maintenance issues require that these extensions exist separately from the
automatically-generated binding modules; usability requires that they inherit
from the automatically-generated modules. This is supported by the
<a class="reference external" href="api/pyxb.binding.basis._DynamicCreate_mixin-class.html">_DynamicCreate_mixin</a> class.</p>
<p>This class provides a <a class="reference external" href="api/pyxb.binding.basis._DynamicCreate_mixin-class.html#_DynamicCreate">method</a> which is used by
the generated bindings to create new instances of themselves. The raw
bindings are generated into a sub-module with the prefix <tt class="docutils literal"><span class="pre">raw</span></tt>, and the
extensions modify the generated class to record the real class that should
be used when new instances are created as a result of converting an XML
document into a binding object.</p>
<p>For example, if a binding is to be created in a module <tt class="docutils literal"><span class="pre">dinner</span></tt>, the
<tt class="docutils literal"><span class="pre">--generate-raw-binding</span></tt> flag would be used on <a class="reference external" href="userref_pyxbgen.html#pyxbgen"><em>pyxbgen</em></a> to
generate the binding in a file named <tt class="docutils literal"><span class="pre">raw/dinner.py</span></tt>. The wrapper module
<tt class="docutils literal"><span class="pre">dinner.py</span></tt> would contain the following code (assuming that the class
<tt class="docutils literal"><span class="pre">parsnip</span></tt> was to be extended):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Bring all public symbols up from the generated one</span>
<span class="kn">from</span> <span class="nn">raw.dinner</span> <span class="kn">import</span> <span class="o">*</span>
<span class="c"># Bring them in again, but left with their original module path</span>
<span class="kn">import</span> <span class="nn">raw.dinner</span>
<span class="c"># Replace the generated parsnip with a customizing extension</span>
<span class="k">class</span> <span class="nc">parsnip</span> <span class="p">(</span><span class="n">raw</span><span class="o">.</span><span class="n">dinner</span><span class="o">.</span><span class="n">parsnip</span><span class="p">):</span>
<span class="c"># Customization here</span>
<span class="k">pass</span>
<span class="c"># Register the customization for use by the binding infrastructure</span>
<span class="n">raw</span><span class="o">.</span><span class="n">dinner</span><span class="o">.</span><span class="n">parsnip</span><span class="o">.</span><span class="n">_SetSupersedingClass</span><span class="p">(</span><span class="n">parsnip</span><span class="p">)</span>
</pre></div>
</div>
<p>With this pattern, objects created by the user through <tt class="docutils literal"><span class="pre">dinner.parsnip()</span></tt>
and from XML documents by the <tt class="docutils literal"><span class="pre">CreateFromDOM</span></tt> infrastructure will both be
instances of the extending wrapper class.</p>
</div>
</div>
<div class="section" id="simple-type-definitions">
<h2>Simple Type Definitions<a class="headerlink" href="#simple-type-definitions" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="http://www.w3.org/TR/xmlschema-1/#Simple_Type_Definitions">Simple type definitions</a> derive from
<a class="reference external" href="api/pyxb.binding.basis.simpleTypeDefinition-class.html">simpleTypeDefinition</a> and a standard Python type.</p>
<p>For simple types that are not derived by list or union, you can construct
instances using the <a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html">Factory</a>
method or directly, providing the value as an argument. New instance creation
is validated against the <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#rf-facets">facets</a> recorded in the binding class.</p>
<div class="section" id="constraining-facets">
<h3>Constraining Facets<a class="headerlink" href="#constraining-facets" title="Permalink to this headline">¶</a></h3>
<p>Each class corresponding to a simple type definition has class variables for
the <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#rf-facets">constraining facets</a>
that are valid for that class. These variables are named by prefixing the
facet name with <tt class="docutils literal"><span class="pre">_CF_</span></tt>, and have a value that is an instance of the
corresponding <a class="reference external" href="api/pyxb.binding.facets-module.html">facet class</a>. Where possible, the
variables are inherited from the parent class; when a simple type is derived
by restriction, the restricted class overrides its parent with a new value
for the corresponding facet.</p>
<p>Facets incorporate schema-specific constraining values with some code that
validates potential instances of the type against the constraints.
Constraining values may:</p>
<ul class="simple">
<li>be of a fixed type, as with <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#rf-length">length</a>;</li>
<li>take on a value in the value space of the simple type in which the facet
appears, as with <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#rf-minInclusive">minInclusive</a>; or</li>
<li>take on a value in the value space of the superclass of the simple type in
which the facet appears, as with <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#rf-minExclusive">minExclusive</a>;</li>
</ul>
<p>Enumeration and pattern constraints maintain a list of the respective
acceptable enumeration and pattern values.</p>
<p>Facets implement the
<a class="reference external" href="api/pyxb.binding.facets.ConstrainingFacet-class.html#validateConstraint">ConstrainingFacet.validateConstraint</a> method, which
in turn is invoked by the
<a class="reference external" href="api/pyxb.binding.basis.simpleTypeDefinition-class.html#XsdConstraintsOK">simpleTypeDefinition.XsdConstraintsOK</a> class method
when given a value that may or may not satisfy the constraints. The
<a class="reference external" href="api/pyxb.binding.basis._TypeBinding_mixin-class.html#Factory">Factory</a> will normally
validate the constraints before allowing a new instance to be returned.</p>
</div>
<div class="section" id="list-types">
<h3>List Types<a class="headerlink" href="#list-types" title="Permalink to this headline">¶</a></h3>
<p>Simple types that derive by <a class="reference external" href="http://www.w3.org/TR/xmlschema-2/#atomic-vs-list">list</a> extend from
<a class="reference external" href="api/pyxb.binding.basis.STD_list-class.html">STD_list</a> which in turn descends from the Python
<tt class="docutils literal"><span class="pre">list</span></tt> type. These derived classes must override the base class
<a class="reference external" href="api/pyxb.binding.basis.STD_list-class.html#_ItemType">STD_list._ItemType</a> value with the appropriate
class to use when creating or validating list members.</p>
<p>When constructing an instance of a simple list type, you can provide a list as
the initializer. The members of the list must be valid initializers to the
underlying item type.</p>
</div>
<div class="section" id="union-types">
<h3>Union Types<a class="headerlink" href="#union-types" title="Permalink to this headline">¶</a></h3>
<p>Union types are classes that are never instantiated. Instead, the binding
classes define a <a class="reference external" href="api/pyxb.binding.basis.STD_union-class.html#_MemberTypes">STD_union._MemberTypes</a> variable
which contains a list of binding classes that are permitted as members of the
union. The <a class="reference external" href="api/pyxb.binding.basis.STD_union-class.html#Factory">STD_union.Factory</a> method attempts, in
turn, to create an instance of each potential member type using the arguments
passed into it. The returned value is the first instance that was
successfully created.</p>
<p>Note that this means the fact that a particular attribute in an element is a
member of a union is not recorded in the attribute value. See
<a class="reference external" href="arch_content.html#attributeuse"><em>Attribute Uses</em></a>.</p>
<p>It is not possible to construct an instance of a union type directly. You
must use the <a class="reference external" href="api/pyxb.binding.basis.STD_union-class.html#Factory">Factory</a> method,
with an argument that is acceptable as an initializer for one of the member
types.</p>
</div>
</div>
<div class="section" id="complex-type-definitions">
<h2>Complex Type Definitions<a class="headerlink" href="#complex-type-definitions" title="Permalink to this headline">¶</a></h2>
<p><a class="reference external" href="http://www.w3.org/TR/xmlschema-1/#Complex_Type_Definitions">Complex type definitions</a> derive from
<a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html">complexTypeDefinition</a>. Classes representing complex
type definitions record maps that specify <a class="reference external" href="api/pyxb.binding.content.AttributeUse-class.html">attribute</a> and <a class="reference external" href="api/pyxb.binding.content.ElementUse-class.html">element</a> use structures to record the attributes and
elements that comprise the type. Each such structure is stored as a private
class field and is used by Python properties to provide access to element and
attribute values in a binding instance.</p>
<p>The base <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html">complexTypeDefinition</a> class provides the
infrastructure to identify the appropriate attribute or element given an XML
tag name. For classes corresponding to types that enable <a class="reference external" href="http://www.w3.org/TR/xmlschema-1/#Wildcards">wildcards</a>, it also provides a mechanism
to access unrecognized elements and attributes. <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#wildcardElements">Wildcard elements</a> are converted to
binding instances if their namespace and name are recognized, and otherwise
are left as DOM nodes. <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#wildcardAttributeMap">Wildcard attributes</a> are stored in
a map from their <a class="reference external" href="api/pyxb.namespace.ExpandedName-class.html">expanded name</a> to the
unicode value of the attribute.</p>
<p>For information on validating the content of a complex type, see
<a class="reference external" href="arch_content.html#arch-content-automata-parsing"><em>Parsing With Automata</em></a>.</p>
<p>When creating a complex type definition, you can provide the values for its
attributes and fields through arguments and keywords. Keywords whose name
matches an attribute or element identifier are used to set that element,
bypassing the content model. Arguments are processed in order using the
content model for identification and validation. See the <a class="reference internal" href="#ex-showcontent"><em>example below</em></a>.</p>
<div class="section" id="elements">
<h3>Elements<a class="headerlink" href="#elements" title="Permalink to this headline">¶</a></h3>
<p>Each element corresponds to a field in the binding instance; the field is
managed through a <a class="reference external" href="arch_content.html#elementuse"><em>element use</em></a> structure. Element names
are disambiguated, and a Python property is defined to retrieve and set the
element value.</p>
<p>When the content model permits multiple occurrences of the element, its value
is a Python list.</p>
</div>
<div class="section" id="attributes">
<h3>Attributes<a class="headerlink" href="#attributes" title="Permalink to this headline">¶</a></h3>
<p>Each attribute corresponds to a field in the binding instance; the field is
managed through a <a class="reference external" href="arch_content.html#attributeuse"><em>attribute use</em></a> structure. Attribute
names are disambiguated, and a Python property is defined to retrieve and set
each attribute value.</p>
<p>Note that if the same name is used for both an attribute and an element, the
element use takes priority. See <a class="reference internal" href="#binding-deconflictingnames"><em>Deconflicting Names</em></a>.</p>
</div>
<div class="section" id="simple-content">
<h3>Simple Content<a class="headerlink" href="#simple-content" title="Permalink to this headline">¶</a></h3>
<p>Complex types with simple content (i.e., those in which the body of the
element is an octet sequence in the lexical space of a specified simple type)
are distinguished by providing a value for the class-level
<a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#_TypeDefinition">complexTypeDefinition._TypeDefinition</a> variable.
For these types, the <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#content">complexTypeDefinition.content</a>
method returns the instance of that type that corresponds to the content of
the object.</p>
<p>Users of bindings must be aware of whether a particular value is a true simple
type, or a complex type with simple content. In the former case, the value
descends from the corresponding Python type and can be used directly in Python
expressions. In the latter, the value must be retrieved using the <tt class="docutils literal"><span class="pre">value</span></tt>
method before it can be used.</p>
<p>As an example, consider this schema (available in <tt class="docutils literal"><span class="pre">examples/content</span></tt>):</p>
<div class="highlight-python"><pre><xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="numbers">
<xs:complexType>
<xs:sequence>
<xs:element name="simple" type="xs:integer"/>
<xs:element name="complex">
<xs:complexType>
<xs:simpleContent>
<xs:extension base="xs:integer">
<xs:attribute name="style" type="xs:string"/>
</xs:extension>
</xs:simpleContent>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="attribute" type="xs:integer"/>
</xs:complexType>
</xs:element>
</xs:schema>
</pre>
</div>
<p>With the generated bindings, the following program:</p>
<div class="highlight-python" id="ex-showcontent"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">pyxb</span> <span class="kn">import</span> <span class="n">BIND</span>
<span class="kn">import</span> <span class="nn">content</span>
<span class="n">v</span> <span class="o">=</span> <span class="n">content</span><span class="o">.</span><span class="n">numbers</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="n">BIND</span><span class="p">(</span><span class="mi">2</span><span class="p">),</span> <span class="n">attribute</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>
<span class="n">v</span><span class="o">.</span><span class="n">complex</span><span class="o">.</span><span class="n">style</span> <span class="o">=</span> <span class="s">"decimal"</span>
<span class="k">print</span> <span class="n">v</span><span class="o">.</span><span class="n">toxml</span><span class="p">()</span>
<span class="k">print</span> <span class="mi">3</span> <span class="o">*</span> <span class="n">v</span><span class="o">.</span><span class="n">simple</span>
<span class="k">print</span> <span class="mi">4</span> <span class="o">*</span> <span class="n">v</span><span class="o">.</span><span class="n">complex</span><span class="o">.</span><span class="n">value</span><span class="p">()</span>
<span class="k">print</span> <span class="mi">5</span> <span class="o">*</span> <span class="n">v</span><span class="o">.</span><span class="n">attribute</span>
</pre></div>
</div>
<p>produces the following output:</p>
<div class="highlight-python"><pre><?xml version="1.0" ?><numbers attribute="3"><simple>1</simple><complex style="decimal">2</complex></numbers>
3
8
15
</pre>
</div>
<p>Note that it was necessary to indicate that the second member (<tt class="docutils literal"><span class="pre">complex</span></tt>) of
the <tt class="docutils literal"><span class="pre">numbers</span></tt> element needs to be wrapped in an instance of the appropriate
complex type. Similarly, it was necessary to add the call to <tt class="docutils literal"><span class="pre">value()</span></tt> on
the value of <tt class="docutils literal"><span class="pre">v.complex</span></tt> in order to get a valid Python numeric value. This
was not necessary for <tt class="docutils literal"><span class="pre">v.simple</span></tt> or <tt class="docutils literal"><span class="pre">v.attribute</span></tt>.</p>
</div>
<div class="section" id="mixed-and-element-only-content">
<h3>Mixed and Element-Only Content<a class="headerlink" href="#mixed-and-element-only-content" title="Permalink to this headline">¶</a></h3>
<p><a class="reference external" href="http://www.w3.org/TR/xmlschema-1/#content_type">Mixed and element-only content</a> nodes assign values to
their <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#_ElementUses">element maps</a> variable to manage
the mapping from XML schema element names to class members. The element and
attribute names are distinct. These classes also associate a <a class="reference external" href="api/pyxb.binding.content-module.html#ContentModel">content
model</a> to ensure the constraints of the
schema are satisfied. These structures are described in
<a class="reference external" href="arch_content.html#contentmodel"><em>Content Model</em></a>.</p>
<p>For these types, the <a class="reference external" href="api/pyxb.binding.basis.complexTypeDefinition-class.html#content">complexTypeDefinition.content</a>
method returns a list, in parsed order, of the Python objects and (if mixed)
non-element content that belong to the instance.</p>
</div>
</div>
<div class="section" id="id5">
<h2>Elements<a class="headerlink" href="#id5" title="Permalink to this headline">¶</a></h2>
<p>Unlike the bindings for schema type definitions, which are represented as
Python classes, bindings corresponding to XML Schema element declarations are
represented as instances of the <a class="reference external" href="api/pyxb.binding.basis.element-class.html">element</a> class. The
instances can be used to create new binding instances that are associated with
the element. Elements are used in the content model to identify transitions
through a finite automaton.</p>
<p>You can use elements just like types in that they are invokable, with
arguments corresponding to the arguments of the constructor of the
corresponding type. See <a class="reference internal" href="#ex-showcontent"><em>the example above</em></a>.</p>
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<li><a class="reference external" href="#">Binding Model</a><ul>
<li><a class="reference external" href="#supporting-capabilities">Supporting Capabilities</a><ul>
<li><a class="reference external" href="#common-binding-instance-features">Common Binding Instance Features</a></li>
<li><a class="reference external" href="#deconflicting-names">Deconflicting Names</a></li>
<li><a class="reference external" href="#support-for-customization">Support for Customization</a></li>
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<li><a class="reference external" href="#constraining-facets">Constraining Facets</a></li>
<li><a class="reference external" href="#list-types">List Types</a></li>
<li><a class="reference external" href="#union-types">Union Types</a></li>
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<li><a class="reference external" href="#complex-type-definitions">Complex Type Definitions</a><ul>
<li><a class="reference external" href="#elements">Elements</a></li>
<li><a class="reference external" href="#attributes">Attributes</a></li>
<li><a class="reference external" href="#simple-content">Simple Content</a></li>
<li><a class="reference external" href="#mixed-and-element-only-content">Mixed and Element-Only Content</a></li>
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<li><a class="reference external" href="#id5">Elements</a></li>
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