Singletons

[Erik Max Francis]

Roy Smith wrote:

> It seems like a pretty reasonable implementation, but I don't
> understand
> one thing about it.  What is the point of:
> 
> # Store instance reference as the only member in the handle
> self.__dict__[ '_Singleton__instance' ] = Singleton.__instance
> 
> I don't see that it does anything useful.  Is there something subtle
> that I'm missing?

I presume you're confused about the '_Singleton__instance' key?  It's
because a double underscore for a class or instance attribute is
mangled.  For instance:

>>> class C:
...  def __init__(self):
...   self.__x = 123
... 
>>> c = C()
>>> dir(c)
['_C__x', '__doc__', '__init__', '__module__']
>>> c.x
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: C instance has no attribute 'x'
>>> c._C__x
123

So this pattern is really just diddling with the new object and
assigning something to a "private" attribute (e.g., one prefixed with a
double underscore).

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