Comparing revisions for page pyBorg

  ==Headline==
The Borg Pattern in [[Language:Python]]

==Motivation==

This implementation illustrates the Borg Pattern in Python, it is similar to the
singleton pattern but the singleton pattern ensures instance equality which 
the Borg pattern does not, instead it ensures state equality. Using this pattern,
one has several instances but they all have the exact same state. An advantage of
this pattern is the possibility to write normal python classes and deriving from
the Borg class is sufficient to make it a "Borg".

==Illustration==

The data model is mostly adapted from [[Contribution:py3k]]. 



class Borg(object):

    _state = {}
    def __init__(self):
        self.__dict__ = Borg._state
        


This class makes sure that all instances of a class derived from the Borg class
have the exact same state by writing to the "  dict  " attribute which holds all
attributes and methods of an object in Python. The Company class is very similar
to the normal version.



class Company(Borg):
    
    def __init__(self, name, subunits):
        Borg.__init__(self)
        self.name = name
        self.subunits = subunits


 
By deriving from the Borg class, you can do this:


company = Company("Meganalysis", ["..."])
first_total = company.total()
company2 = Company("Meganalysis",["..."])
company2.cut()
assert company.total() * 2.0 == first_total



== Usage ==

Just run python program.py

== Metadata ==

* [[uses::Language:Python]]
* [[implements::Feature:Hierarchical company]]
* [[implements::Feature:Total]]
* [[implements::Feature:Cut]]
* [[developedBy::Contributor:kevin-klein]]