Python: thinking of a module and its variables as a singleton — Clean approach?

Question

I\'d like to implement some sort of singleton pattern in my Python program. I was thinking of doing it without using classes; that is, I\'d like to put all the singleton-rel

Answer 1

One approach to implementing a singleton pattern with Python can also be:

have singleton __init()__ method raise an exception if an instance of the class already exists. More precisely, class has a member _single. If this member is different from None, exception is raised.

class Singleton:
    __single = None
    def __init__( self ):
        if Singleton.__single:
            raise Singleton.__single
        Singleton.__single = self  

It could be argued that handling the singleton instance creation with exceptions is not very clean also. We may hide implementation details with a method handle() as in

def Handle( x = Singleton ):
    try:
        single = x()
    except Singleton, s:
        single = s
    return single 

this Handle() method is very similar to what would be a C++ implementation of the Singleton pattern. We could have in Singleton class the handle()

Singleton& Singleton::Handle() {

  if( !psingle ) {
    psingle = new Singleton;
  }
  return *psingle;
}

returning either a new Singleton instance or a reference to the existing unique instance of class Singleton.

Handling the whole hierarchy

If Single1 and Single2 classes derive from Singleton, a single instance of Singleton through one of the derived class exists. This can be verify with this:

>>> child = S2( 'singlething' )
>>> junior = Handle( S1)
>>> junior.name()
'singlething'