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?


I see two options in this case:

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

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):

    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__() = name

x = X("x")
y = X("y")
for r in X.get_instances():
del y
for r in X.get_instances():

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.

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)) = name

a1 = A('a1')
a2 = A('a2')
a3 = A('a3')
a4 = A('a4')

for instance in A.instances:

Very nice and useful code, but it has a big problem: list is always bigger and it is never cleaned-up, to test it just add print(len(cls.__refs__[cls])) at the end of the get_instances method.

Here a fix for the get_instances method:

__refs__ = defaultdict(list)

def get_instances(cls):
    refs = []
    for ref in cls.__refs__[cls]:
        instance = ref()
        if instance is not None:
            yield instance
    # print(len(refs))
    cls.__refs__[cls] = refs

or alternatively it could be done using WeakSet:

from weakref import WeakSet

__refs__ = defaultdict(WeakSet)

def get_instances(cls):
    return cls.__refs__[cls]

Same as almost all other OO languages, keep all instances of the class in a collection of some kind.

You can try this kind of thing.

class MyClassFactory( object ):
    theWholeList= []
    def __call__( self, *args, **kw ):
         x= MyClass( *args, **kw )
         self.theWholeList.append( x )
         return x

Now you can do this.

object= MyClassFactory( args, ... )
print MyClassFactory.theWholeList

Python doesn't have an equivalent to Smallktalk's #allInstances as the architecture doesn't have this type of central object table (although modern smalltalks don't really work like that either).

As the other poster says, you have to explicitly manage a collection. His suggestion of a factory method that maintains a registry is a perfectly reasonable way to do it. You may wish to do something with weak references so you don't have to explicitly keep track of object disposal.

It's not clear if you need to print all class instances at once or when they're initialized, nor if you're talking about a class you have control over vs a class in a 3rd party library.

In any case, I would solve this by writing a class factory using Python metaclass support. If you don't have control over the class, manually update the __metaclass__ for the class or module you're tracking.

See for more information.

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