Question

I’m building my own lightweight orm. I’d like to keep instantiated objects in a class variable (perhaps a dictionary). When I request an object (through a class method) like get(id), I’d like to return the object from the instantiated list, or create one if it does not exist.

Is there a way to prevent the instantiation of an object (if its id already exists in the cls list)?

Answer 1

There are two straightforward ways of doing it – and many other ways,as well. One of them, as you suggest, is to write the __new__ method for your objects, which could return an already existing object or create a new instance.

Another way is to use a factory function for your objects – and call this factory function instead of the class – more or less like this:

class _MyClass(object):
    pass

def MyClass(*args, **kw):
    if not "my_class_object" in all_objects:
         all_objects["my_class_object"] = _MyClass(*args, **kw)
    return all_objects["my_class_object"]