singletons

Craig Allen callen314 at gmail.com
Thu Jul 17 00:20:04 CEST 2008


Hey, forgive me for just diving in, but I have a question I was
thinking of asking on another list but it really is a general question
so let me ask it here.  It's about how to approach making singletons.
Background: I've been programming in python seriously for about a year
now, maybe a little longer depending how you count, and the system I
am making is sophisticated enough that I've had to enter into a few
idioms which were beyond my knowledge of python, and I had to do quick
research and test my ideas with test code (which of course can miss
subtle problems).  Otoh, I have decades of programming experience now
and wasn't totally without footing.  I think I have a solution I like
for creating something to act as a singleton but I'm curious what
other's think.

I have several classes in our system which need to act like
singletons, they are libraries of data classifications, and other such
libraries of configurations for the system which need to be global.

The first thing I found searching for singleton, early in this
project, trying to be a good citizen and find a decent idiom from the
python community itself, knowing someone had mentioned "singleton" and
"python" together at some point, was a recommendation to do this:

(option 1)

class TehLibrary(object):
   __single = None
   def __init__(self):
      if (TehLibrary.__single):
         raise AlreadyExistsException # or whatever

This sucks because that means creation of the object has to be in a
try block as a matter of course, something I promptly hid in a factory
function, but still.

But the way this worked made me realize that the Class itself is a
full fledged object, quite instance-like from my C++ addled (I love
you C++) perspective and it's single.  If I can store that instance, I
can make a class that shares member at the class level. The class
doesn't even have to be a singleton exactly.

(option 2)
Therefore option two is a family of options where class level members
can be used to share whatever needs to be shared, though strictly the
class is not a singleton since multiple instances are created which
merely share the data that should be single (say a big dictionary of
configuration information the class manages).

(option 3)
I still wanted actual singletons and realized that since I had to
create a factory function even in option 1, that I could use module
level variables to control the behavior of those factories, which led
me to realize I'm basically just using the module itself as a
singleton. And this is sort of where I have arrived... when I import
the modules it runs code to build up it's basic services, much like an
object construction.  It only runs once no matter how many times it's
imported. When client code asks for the library that should be a
singleton, it gets a singleton which has been stored in a module level
variable.

Anyone have any comments?  Is there anything wrong, evil, or ugly
about using a module this way, or am I correct to think that actually,
this is a common approach in python.

Is it pythonic?



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