[newbie] Looking for a good introduction to object oriented programming with Python

[Steven D'Aprano]

On Sun, 05 Aug 2012 21:14:04 -0400, Dennis Lee Bieber wrote:

> While I've probably used singletons (usually as sentinels in queues,

I don't know your code, but if I were to take a wild guess, I would say 
that apart from None, and True/False, you probably haven't.

NotImplemented and Ellipsis are two other singletons in Python. It is 
possible to program a Singleton class in pure Python, but most people 
don't bother, because 1) singleton is probably the most over-used Design 
Pattern of them all and 2) it's quite trivial to bypass the singleton-ness 
of classes written in Python and create a second instance.

Or they use a module, which is not really a singleton but behaves like 
one. Or if they want to show off their Pythonicity, they use the Borg 
pattern.

The common sentinel idiom in Python is to do this:

SENTINEL = object()
# and later...
if value is SENTINEL:
    pass


but of course that's not a singleton because it's only one instance out 
of a vast number, and anyone can create another instance. Also it misses 
the point of the Singleton pattern, that the (lone) instance should hold 
state. The sentinel does not usually hold state.



> I suspect), but can't say that I've ever used a "factory function"...

If you've ever used an ordinary function decorator, you almost certainly 
have.

If you've every created a closure, you definitely have.




-- 
Steven