Re: Help with Borg design Pattern

[Maric Michaud]

Le Saturday 28 June 2008 03:47:43 Casey McGinty, vous avez écrit*:

On Fri, Jun 27, 2008 at 3:21 PM, Casey McGinty <ca***********@gmail.com>

wrote:

Hi,

I'm trying to implement a simple Borg or Singleton pattern for a class
that inherits from 'dict'. Can someone point out why this code does not
work?

class MyDict( dict ):
__state = {}
def __init__(self):
self.__dict__ = self.__state

a = MyDict()
a['one'] = 1
a['two'] = 2

print a
print MyDict()

Well, it works !

>>>[156]: class MyDict( dict ):

__state = {}
def __init__(self):
self.__dict__ = self.__state
.....:
.....:

>>>[160]: MyDict().toto = 5

>>>[161]: MyDict().toto

...[161]: 5

but the __dict__ attribute is the container of attributes of an instance,
which are not used in the underlying implementation of dictinnaries.

This looks like a good solution:

class MyDict( dict ):
def __new__(cls,*p,**k):
if not '_instance' in cls.__dict__:
cls._instance = dict.__new__(cls)
return cls._instance

Yes it is, but it's rather unneeded in Python, we prefer simply create a
module level dictionnary, these tricks are used in language like C++ or Java.

In python :

mymodule.py :

ModuleOptions = {}

othermodule.py :

import mymodule

mymodule.ModuleOptions['Verbose'] = True

or if you think encapsulation is important :

mymodule.py :

_ModuleOptions = {}

def get_option(opt) :
return _ModuleOptions[opt]
....

And you're done.

--
_____________

Maric Michaud