Printing all instances of a class

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.

Answer 2

You'll want to create a static list on your class, and add a weakref to each instance so the garbage collector can clean up your instances when they're no longer needed.

import weakref  class A:     instances = []     def __init__(self, name=None):         self.__class__.instances.append(weakref.proxy(self))         self.name = name  a1 = A('a1') a2 = A('a2') a3 = A('a3') a4 = A('a4')  for instance in A.instances:     print(instance.name)