[Tutor] singleton pattern

[Michael P. Reilly]

alan.gauld@bt.com wrote
> Alan G.
> > Guido once also suggested that replacing __class__ (which is 
> > something I believe Alan was attempting to do) 
> 
> I wasn't replacing class just returning a pointer to the 
> original instance as an attribute of the Exception.

This was in an earlier email you had sent out (something about
"self.class = ..." within the __init__ method).

It was something more like:

class FactInst:
  pass
class Factory:
  def __init__(self, klass):
    self.klass = klass
  def __call__(self, *args, **kws)
    # create a dummy instance
    inst = FactInst()
    # change its class
    inst.__class__ = self.klass
    # call the initialization routine
    apply(inst.__init__, args, kws)
    # return the re-class-fied instance
    return inst

class Spam:
  def __init__(self, msg):
    self.msg = msg
  def __str__(self):
    return msg
FactSpam = Factory(Spam)  # a new factory instance to be used like a class
brkfast = FactSpam("spam, spam, eggs & spam")  # a new "Spam" instance
print brkfast

It's been a few years since I read that thread, and I probably don't have
things exactly right, but the idea is to replace some of the "important"
bindings within an instance with other class info altogether.

  -Arcege

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+----------------------------------+-----------------------------------+
| Michael P. Reilly                | arcege@speakeasy.net              |