.. _namespaces: Namespaces ========== The most important concept to understand when working with complex XML documents is the `namespace `_. A namespace is nothing more than a map from names to objects, partitioned into groups within which the names must uniquely identify an object. A namespace is identified by its name, which is a `URI `_. Although it is common to use URIs like ``http://www.w3.org/2001/XMLSchema`` as namespace names, the name is simply an opaque identifier: it does not have to resolve to a Web site or anything helpful. ``dinner:ParsnipsOnTuesday`` is a perfectly valid namespace name. Equally, namespaces and XML schemas are not the same thing. A schema is simply a mechanism for specifying the contents of a namespace. It is common to use the ``include`` directive in XMLSchema to combine multiple schema into a single namespace. It is less common, though equally valid, to use ``xmlns`` or ``xs:schemaLocation`` to select alternative schemas to use for the same namespace in different instance documents, as in the `dangling type `_ pattern. This diagram shows the class structure of the PyXB namespace infrastructure. The central object is the :api:`pyxb.namespace.Namespace`. Four mix-in classes provide implementations of separate namespace functions. :api:`pyxb.namespace.ExpandedName` is used ubiquitously to pair local names with their namespaces. :api:`pyxb.namespace.NamedObjectMap` maps names to the objects (generally, schema components) for a particular category of object. The :api:`pyxb.namespace.NamespaceContext` class provides information related to the use of namespaces in XML documents, including mappings from prefixes to namespaces. .. image:: Images/NamespaceCore.jpg Namespace Category Maps ----------------------- The :api:`pyxb.namespace._NamespaceCategory_mixin` provides support for discrete categories of named objects. It allows arbitrary, runtime-identified, groups of objects to be registered in individual dictionaries within the namespace. For example, XML Schema require that type definitions, element declarations, and attribute declarations be distinct categories of named objects in a namespace. PyXB also maintains separate categories for attribute groups, model groups, identity constraint definitions, and notation declarations, which also must be unique within their category. Other groups of objects can be stored in a namespace. For example, the WSDL definition of a service may choose to use the same namespace name for its types as for its definitions, adding services, ports, messages, bindings, and portTypes as named objects that can be identified. Namespace Component Associations -------------------------------- The :api:`pyxb.namespace._NamespaceComponentAssociation_mixin` provides support for associating schema components with a namespace. Of particular interest is that a namespace can be comprised of components defined from multiple sources (generally, distinct schema documents). In addition, there are anonymous components (e.g., complex types defined within element members of complex types) which are implicitly associated with the namespace although they cannot be named within it. These must all be stored within the namespace so that a complete set of bindings can be generated in a single Python module. .. _resolution: Namespace Resolution -------------------- Named objects are often associated with namespaces through XML elements in a document. For example:: specifies an attribute declaration. In turn, references to names appear within XML elements, usually as values of specific attributes. The ``type`` portion of the attribute declaration above also identifies an object by name, and it must be possible to resolve the named object. The work involved in associating names with schema components is encapsulated in the :api:`pyxb.namespace.resolution._NamespaceResolution_mixin` class. The following `concepts `_ are important to understand: - An `NCName `_ ("no-colon name") is an identifier, specifically one without any colon (":") characters, serving as a local name. - A `QName `_ ("qualified name") is an local name with an optional prefix, separated from it by a colon, which identifies a context for the local name. - The prefix is mapped using `xmlns `_ attributes to a namespace name, which is a URI. - The combination of a namespace URI and the local name comprise an `expanded namespace name `_, which is represented by :api:`pyxb.namespace.ExpandedName`. - The category within which the local name must be resolved in the namespace is determined through external information, in the above case the fact of the QName's appearance in a ``type`` attribute in an ``attribute`` declaration of an XML schema. .. index: pair: resolution; name pair: resolution; object (component) :api:`pyxb.namespace._NamespaceCategory_mixin` is used to define the set of categories supported by a namespace and to add named objects to those categories. A name is **resolved** when the object with which it is associated has been identified. Objects are **resolved** when any names on which they depend have been resolved. :api:`pyxb.namespace.resolution._NamespaceResolution_mixin` provides a mechanism to hold on to names that have been encountered but whose associated objects have not yet been resolved (perhaps because the named object on which they depend has not been defined). Because one named object (e.g., a model group definition) might require resolution of another (e.g., an element reference), resolution is an iterative process, implemented by :api:`pyxb.namespace.resolution._NamespaceResolution_mixin.resolveDefinitions`, and executed when all named objects have been added to the namespace. It depends on :api:`pyxb.namespace.resolution.NamespaceContext` to identify named objects using the :api:`pyxb.namespace.resolution.NamespaceContext.interpretQName` method. Expanded Names -------------- An :api:`pyxb.namespace.ExpandedName` instance couples a local name with (optionally) a namespace, resulting in a QName. This class also integrates with namespace categories, permitting lookup of the object with its name in a specific category by using the category name as a method. For example, the following two expressions are equivalent:: # Short-hand method en.typeDefinition() # Detailed equivalent en.namespace().categoryMap('typeDefinition').get(en.localName()) Both produce the type definition with the given name, or ``None`` if there is no such definition. The short-hand method interface works for any category defined within the expanded name's namespace; it is not limited to the standard set of component categories. Methods are also present to test whether the name matches a DOM node, and to retrieve the named attribute (if present) from a DOM node. In this version of PyXB, the hash codes and comparison methods for :api:`ExpandedName ` have been overridden so that an expanded name with no namespace is treated equivalently to the string value of the local name. This simplified management of default namespace lookups in earlier versions of PyXB, but may no longer be necessary; reliance on this feature is discouraged. Namespace Context ----------------- `Namespaces in XML `_ specifies how the ``xmlns`` attributes are used to associate prefix strings with namespaces. The :api:`pyxb.namespace.NamespaceContext` class supports this by associating with each node in a DOM document the contextual information extracted from ``xmlns`` and other namespace-relevant attributes. The namespace context consists of three main parts: - The `default namespace `_ specifies the namespace in which unqualified names are resolved. - The `target namespace `_ is the namespace into which new name-to-component associations will be recorded. - The `in-scope namespaces `_ of a DOM node are those which can be identified by a prefix applied to names that appear in the node. Methods are provided to define context on a per-node basis within a DOM structure, or to dynamically generate contexts based on parent contexts and local namespace declarations as needed when using the SAX parser. Other Concepts -------------- .. index:: pair: namespace; absent single: no namespace .. _absentNamespaces: Absent Namespaces ^^^^^^^^^^^^^^^^^ Some schemas fail to specify a default namespace, a target namespace, or both. These cases are described by the term "absent namespace"; sometimes it is said that an object for which the target namespace is absent is in "no namespace". If the target namespace for a schema is absent, we still need to be able to store things somewhere, so we represent the target namespace as a normal :api:`pyxb.namespace.Namespace` instance, except that the associated URI is ``None``. If in the same schema there is no default namespace, the default namespace is assigned to be this absent (but valid) target namespace, so that QName resolution works. Absence of a target namespace is the only situation in which resolution can succeed without some sort of namespace declaration. The main effect of this is that some external handle on the Namespace instance must be retained, because the namespace cannot be identified in other contexts. PyXB supports this by defining a ``Namespace`` variable within each binding module, thus allowing access to the namespace instance via syntax like ``pyxb.bundles.wssplat.wsdl11.Namespace``. .. _namespaceStorage: Storage of Namespaces --------------------- In PyXB, when the :ref:`componentModel` is used to define various elements, attributes, and types by representing them in Python instances, those instance objects are stored in a :api:`pyxb.namespace.Namespace` instance. In addition to generating code corresponding to those objects, it is possible to save the pre-computed objects into a file so that they can be referenced in other namespaces. PyXB uses the Python pickling infrastructure to store the namespace component model into a file in the same directory as the generated binding, but with a suffix ``.wxs``. When a schema is processed that refers to a namespace, the serialized component model for the namespace is read in so that the referring namespace can resolve types in it. .. _namespace-archive: The Namespace Archive Model ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Recall that the contents of a namespace can be defined from multiple sources. While in the simplest cases the namespace is defined by combining components from one or more schemas, the set of schemas that define a namespace may be different for different documents; see http://www.xfront.com/VariableContentContainers.html. Another not uncommon situation is to use a namespace `profile`, which is a subset of the full namespace intended for use in a particular application. For example, the Geography Markup Language defines three profiles denoted "GML-SF" for "simple features"; these profiles do not include the more complex structures that are needed for unusual situations. To support namespace profiles, PyXB must do two things: - Use a different Python binding module for the profile as opposed to the full namespace definition - Use a different archive for the profile Naive management of these multiple information sources will cause havoc, since namespaces do not allow multiple objects to share the same name. The relations of the various classes involved in storing namespace data are depicted in the following diagram: .. image:: Images/NamespaceArchive.jpg The namespace archive facility must support the following situations: - The archive stores the complete set of components for a single namespace (most common) - The archive stores components from multiple namespaces which are interdependent, but together completely define the expected contents of the namespaces - The archive stores a complete subset of the standard components of a namespace (the `profile` situation) - The archive extends a namespace with additional components, often required for a particular application. It is usually necessary to read another archive to determine the full namespace content. Because of interdependencies between namespaces stored in a single archive, archives are read as complete entities: i.e., from a single archive you cannot read the components corresponding to one namespace while ignoring those from another. The component model for a namespace is read from a namespace archive only when it is necessary to generate new bindings for a namespace that refers to it, through import or namespace declarations. The component model is defined by invoking the :api:`pyxb.namespace.Namespace.validateComponentModel` method. Within an archive, each namespace can be marked as `private` or `public`. When the component model for a namespace is validated, all archives in which that namespace is present and marked `public` are read and integrated into the available component models. When an archive is read, namespaces in it that are marked `private` are also integrated into the component model. Prior to this integration, the namespace component model is validated, potentially requiring the load of other archives in which the namespace is marked `public`. The contents of the namespace archive are: - A set of :api:`pyxb.namespace.archive._ModuleRecord` instances which identify namespaces and mark whether they are public or private in the archive. Each instance in turn contains (for namespace ``A``): - the set of :api:`pyxb.namespace.archive._ObjectOrigin` instances which identify the origins for components that are defined in the archive. In turn, each of these origins identifies by category and name the objects that were defined by this origin and consequently are stored in the containing archive. Due to use of the `include `_ directive, multiple origins may be associated with a single module record - the set of :api:`pyxb.namespace.Namespace` instances that were referenced by ``A``. This includes namespaces that were imported into one of the origin objects, as well as those that were incorporated simply through reference to an object in a declared namespace - The objects within the namespace that were defined by the various origins In addition to the raw component model, the namespace archive includes the names of the Python modules into which bindings for each namespace were generated. .. ignored ## Local Variables: ## fill-column:78 ## indent-tabs-mode:nil ## End: