Python & XML

Physical Structures

XML text is stored in entities. Entities are identified in various ways, but most commonly by filename or URI. There is no constraint on this, however, and many systems do use alternate means for entity storage — for example, many live happily in large databases. Many XML documents involve more than one entity; perhaps the most common arrangement is that the document is in one entity and its type definition is in another. As documents get larger, increasing numbers of entities are often involved with each document. This may be more common with document-centric applications than with data-communication applications of XML.

Entities are typically given names in one or more global namespaces. XML requires that entities be given system identifiers, which are always URIs. The term has roots in the SGML community, where system identifiers were used to refer to storage locations using whatever syntax the tools in use happened to understand. An additional global namespace is shared with the SGML world; the identifiers in that space are called formal public identifiers (FPIs). Use of this namespace is very limited in the XML world, as it is not always easily mapped to URLs that can be used to retrieve arbitrary resources, although there are ways to do it. They do see some use, and extensible support for FPIs is available in the PyXML toolkit.

Entities are used for several things in XML:

Document entities

Regardless of the application, all documents start somewhere. ...