[EMAIL PROTECTED] wrote: > I had a global variable holding a count. One source Google found > suggested that I wouldn't need the global if I used an object. So I > created a Singleton class that now holds the former global as an > instance attribute. Bye, bye, global. > > But later I thought about it. I cannot see a single advantage to the > object approach. Am I missing something? Or was the original global a > better, cleaner solution to the "I need a value I can read/write from > several places" problem?
A singleton is simply another form of global. The benefit of a singleton is that it can be used to encapsulate several related values and methods in a single globally accessible space. The easiest way in Python to implement a singleton is just to use a module: all modules are singletons and there is a defined mechanism (import) for accessing them. It sounds as though you simply replaced a value stored in a singleton object (global in a module) with a value stored in a singleton object (attribute of your class) which is referenced from a singleton object (your module). The real benefit from using an object comes when you stop making it a singleton. Create it at some high level in your code and pass it around as a parameter, or hold a reference to it in some other object which is passed around. When you do this you no longer have a global, and the benefits you get include: * for your counter example, you can have multiple counters counting different things without having to duplicate counter code for each thing you want to count. * it becomes much easier to write tests for your code, because each test can create its own context and be completely independant from the other tests. e.g. you can test a generic counter without having to know you are testing the foo counter or the bar counter, and you can test something which counts foos without having to worry that other tests may already have counted some foos. There is another lesser (and Python specific) benefit to storing a value as an attribute of a class rather than a global in a module: if you later want to intercept assignments to the attribute you can turn it into a property, but doing the same thing on a module is much harder. -- http://mail.python.org/mailman/listinfo/python-list
