Question

With a class in Python, how do I define a function to print every single instance of the class in a format defined in the function?

Answer 1

I see two options in this case:

Garbage collector

import gc for obj in gc.get_objects():     if isinstance(obj, some_class):         dome_something(obj) 

This has the disadvantage of being very slow when you have a lot of objects, but works with types over which you have no control.

Use a mixin and weakrefs

from collections import defaultdict import weakref  class KeepRefs(object):     __refs__ = defaultdict(list)     def __init__(self):         self.__refs__[self.__class__].append(weakref.ref(self))      @classmethod     def get_instances(cls):         for inst_ref in cls.__refs__[cls]:             inst = inst_ref()             if inst is not None:                 yield inst  class X(KeepRefs):     def __init__(self, name):         super(X, self).__init__()         self.name = name  x = X('x') y = X('y') for r in X.get_instances():     print r.name del y for r in X.get_instances():     print r.name 

In this case, all the references get stored as a weak reference in a list. If you create and delete a lot of instances frequently, you should clean up the list of weakrefs after iteration, otherwise there’s going to be a lot of cruft.

Another problem in this case is that you have to make sure to call the base class constructor. You could also override __new__, but only the __new__ method of the first base class is used on instantiation. This also works only on types that are under your control.

Edit: The method for printing all instances according to a specific format is left as an exercise, but it’s basically just a variation on the for-loops.