Prevent Create/Delete/Update subclasses from overriding _execute and __init__ methods

[mscott]

The best practice for customizing Create, Delete, and Update subclasses is to define _setup, _before_execute, and _after_execute methods, and to do nothing with __init__ and _execute.

Because of the complexity of __init__ and _execute themselves for these types of transactions, we need to prevent them from even being defined for subclasses of those transaction types, thus enforcing the practice described above.

Proposed implementation:

• Add _restrict_subclasses = False to base Transaction class.

• In Create, Delete, and Update subclasses, set _restrict_subclasses = True

• Create a TransactionMeta(schema_metaclass('T')) class in schevo.transaction, and extend it to check for _restrict_subclasses; if that is True, then raise an exception if __init__ or _execute are in the class_dict passed to the metaclass.

[mscott]

Milestone 2006 October software release deleted

[mscott]

Seems worthwhile for inclusion in both 3.0.1 and 3.1; it enforces behavior that is desirable no matter which version you use.

[mscott]

Milestone Schevo 3.0.1 deleted

[mscott]

implementation in r3174. tests and docs to follow.

[mscott]

docs and doctest in r3176.

ready for review and merge.

[mscott]

Reviewed by pobrien, merged at r3189