Printing all instances of a class

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

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.