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

for i in spisok: #список экземпляров любых классов
    if i.__class__==Class1:
        [y for y in i.__dict__]

[y for y in i.__dict__]

>>> class A(object):
...     pass
>>> a, b = A(), A()
>>> filter(lambda obj: isinstance(obj, A), map(eval, dir()))
[<__main__.A object at 0xa2699ec>, <__main__.A object at 0xa26994c>]

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