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XMLToObjects

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Contact

Chris McDonough

(Credit to Shane Hathaway for this idea)

Problem

XML is an object-friendly data format. Unlike flat data formats such as CSV, it can have an arbitrarily complex data containment hierarchy. In all cases, XML data can be expressed in the form of a Document Object Model (DOM) tree, where the DOM represents the hierarchical structure imposed by the schema of a chunk of well-formed XML, and where methods can be called on the DOM to return various pieces of information about the structure. Having a DOM tree that can be poked and prodded for information makes it slightly easier to transform XML encoded information into other things, like instances of Python objects. But it would be even more convenient to have a generically useful facility for transforming XML data into Python objects less circuitously. The Zope Product XMLDocument allows you to import an XML document into Zope and have it converted into nodes which are instances of a single generic Python class. But a facility to transform a well-understood XML document into instances of arbitrary Python classes does not exist.

Proposed Solutions

It sure would be keen if we could transform a well-understood XML document into another structured format which directly represented a hierarchy of Python/Zope objects.

A format already exists for representing Zope objects in XML in the "xml pickle" ("ppml") format used by the XML import/export machinery. This format encodes Python "pickles" (serialized Python object representations) into XML and vice versa.

It may be conceivable that a transform operation on a given XML file could turn it in to the "xml pickle" format, after which it could be imported into Zope as a set of Zope objects.

The transform could be performed any number of ways, including via XSLT or via Python. The transforming operation would need to know enough about the detail of the input XML to associate elements with "known" Zope classes and other object types (strings, integers, etc.) It would obviously also need to know the output format.

A Zope Product could be made to help perform this type of transform and to handle the details of actually importing the generated ppml into Zope.

Risk Factors

Overgeneralization

creating a given XML-to-ppml transform method might end up in many cases being more complicated than just parsing the input XML and generating objects manually. If possible, we should shoot for a way to let people transform very simple XML into very simple Python structures easily.

Undergeneralization

such a beast would be useful outside Zope, within raw Python. "ppml" may not be the best format to use for this purpose, as it contains Zope-specific data. We may want to create another XML format to represent generic Python objects, and give the option to choose between this format and ppml at transform time. Someone may have already worked out a more generic format.

Scope

The project will provide a Python package which facilitates the transform of arbitrary XML into an XML format which directly represents Python objects.

Additionally, the project will provide a Zope product which wraps the Python package and provides a mechanism for importing arbitrary XML as Python objects. It will also provide management functionality for the code involved in the transform. It will also provide a (possibly) "wizard-like" mechanism for creating and performing relatively simple transforms.

Deliverables

A set of use cases for XMLToObjects.

The DTD for a suitable format to represent Python objects (if not ppml).

A Zope product wrapping XMLToObjects, which includes common-case wizardry for transforming "simple" XML into "simple" Python instances.

A sample transformer and input XML document.

Documentation.

k_vertigo some interesting links. first one is using xml schema to generate java objects. second one a python xml_pickle format thats currently in progress.

• http://www-4.ibm.com/software/developer/library/data-binding2/?dwzone=xml

• http://www-4.ibm.com/software/developer/library/xml-matters1/?dwzone=xml