Generic functions and business rules support systems
PEAK-Rules is a highly-extensible framework for creating and using generic functions, from the very simple to the very complex. Out of the box, it supports multiple-dispatch on positional arguments using tuples of types, full predicate dispatch using strings containing Python expressions, and CLOS-like method combining. (But the framework allows you to mix and match dispatch engines and custom method combinations, if you need or want to.)
>>> from peak.rules import abstract, when, around, before, after >>> @abstract() ... def pprint(ob): ... """A pretty-printing generic function""" >>> @when(pprint, (list,)) ... def pprint_list(ob): ... print "pretty-printing a list" >>> @when(pprint, "isinstance(ob,list) and len(ob)>50") ... def pprint_long_list(ob): ... print "pretty-printing a long list" >>> pprint([1,2,3]) pretty-printing a list >>> pprint(*1000) pretty-printing a long list >>> pprint(42) Traceback (most recent call last): ... NoApplicableMethods: ...
PEAK-Rules works with Python 2.3 and up – just omit the @ signs if your code needs to run under 2.3. Also, note that with PEAK-Rules, any function can be generic: you don’t have to predeclare a function as generic. (The abstract decorator is used to declare a function with no default method; i.e., one that will give a NoApplicableMethods if no rules match the arguments it’s invoked with, as opposed to executing a default implementation.)
PEAK-Rules is still under development; it lacks much in the way of error checking, so if you mess up your rules, it may not be obvious where or how you did. User documentation is also lacking, although there are extensive doctests describing and testing most of its internals, including:
Introduction (Method combination, porting from RuleDispatch)
Core Design Overview (Terminology, method precedence, etc.)
(Please note that these documents are still in a state of flux and some may still be incomplete or disorganized, prior to the first official release.)
Source distribution snapshots are generated daily, but you can also update directly from the development version in SVN.