dm.zopepatches.zclasses 0.1.1

Through the Web application development without deep programming skills (former ZClasses functionality in the Zope core).

This functionality used to be in the Zope core as ZClasses. It allows the menu driven construction of Web applications. Due to the menu support, only limited programming skills are necessary.

More precisely: the ZClasses functionality allowed to define classes (in the object oriented sense of the word) via the Zope Management Interface (ZMI), so called ZClasses. A ZClass could inherit from base classes and other ZClasses (multiple inheritance), manage data in a set of associated property sheets, define methods to operate on its instances (present, modify) via arbitrary other Zope objects (e.g. templates, scripts), define permissions appropriate for the class and map class relevant permissions to the standard permissions used by the objects implementing the class methods. Classes could then be instantiated and the instances used in the Web application, to hold data, to provide fundamental services, to glue things together. Quite impressive….

It has been a valuable selling point in the past. In fact, back in 2000, I used ZClasses to build a large multilingual Web application comprising about half a dozen ZClasses and allowing to perform queries against a European tender database and subscribe to email notifications for changed query results.

Unfortunately, some influential Zope developers (especially the Zope 2 release manager) did not like ZClasses and started to fight them. Finally (with Zope 2.12), they removed the functionality from the Zope core – originally claiming the high maintenance burden (although since 2.8 nothing was done with ZClasses) but finally conceeding that the removal happened because they did not want ZClasses.

True, ZClasses had some problems:

• ZClasses are Zope objects, stored and maintained in the ZODB (the Zope Object Database). There is no version control system documenting who changed what for what reason. Such documentation can be vital for large projects with a long lifetime.

• ZClasses provided a vast functionality but there have been no tests to verify all this functionality. As a consequence, ZClassess broke once partially and this was only detected by Zope users and not during the development by the tests.

• For Zope 2.8, ZClasses needed an overhaul and only Jim Fulton (the initial author of Zope and ZClasses) was able to do this work. The developers feared that a future Zope version might need a similar effort with the resources missing.

• ZClasses do not play well with storage mounting.

It is right to caution users against such deficiencies but in my view it was wrong to break backward compatibility by removing the functionality. Fortunately, Zope is open source which somewhat limits the power of developers and release managers. This package uses so called monkey patching (runtime code modifications) to reinsert into Zope again the ZClasses functionality the Zope developers had ripped off.

Some warning notes: monkey patching is potentially dangerous. Partially, it replaces core functionality by a slightly modified one. I have tried hard that these replacements should be okay for Zope 2.12 but future Zope versions might need other replacements. The breakage might occur not in ZClasses itself but in the core functionality. Moreover, the Zope developers hostility towards ZClasses may be large enough that they remove further parts required by ZClasses. While in principle, these parts can also be put back, the costs increase and may finally get too large (such that I abandon the project). Therefore, it is probably wise not to rely on this package for long living new projects but restrict its use to get some additional time to reimplement ZClasses based projects or for short living prototypes.

ZClasses used to be documented at “http://www.plope.com/Books/2_7Edition/CustomZopeObjects.stx”. The Zope developers did not forget to rip this documentation off as well. But, I have anticipated the step and have saved a copy. You find it in the package’s doc subdirectory. It still presents the “plope” look&feel and links will not to work. Screenshots are lost but you can use your Zope to get them.

Activation

This package does nothing until it is imported. On import, it performs its monkey patching.

ZClasses require a special class factory for the root ZODB. To this end, the root ZODB configuration must use the configuration option (in the Zope configuration file):

class-factory dm.zopepatches.zclasses.ClassFactory

Usually, the import (mentioned in the first paragraph) is performed when this option is processed.

Limitations

• This version does not yet reinstate the ZClasses help pages.

• This version does not yet reinstate the distibution tab of products. Likewise, distribution import is likely not to work.