gc.is_finalized

By using __del__ and global variables, it is possible to leave a reference to the object after it was "destroyed":

runner = None
class Lazarus:
  def __del__(self):
    print('destroying')
    global runner
    runner = self

lazarus = Lazarus()
print(lazarus)
# <__main__.Lazarus object at 0x7f853df0a790>
del lazarus
# destroying
print(runner)
# <__main__.Lazarus object at 0x7f853df0a790>

In the example above, runner points to the same object as lazarus did and it's not destroyed. If you remove this reference, the object will stay in the memory forever because it's not tracked by the garbage collector anymore:

del runner  # it will NOT produce "destroying" message

This can lead to a strange situation when you have an object that escapes the tracking and will be never collected.

In Python 3.9, the function gc.is_finalized was introduced that tells you if the given object is a such runner:

import gc
lazarus = Lazarus()
gc.is_finalized(lazarus) # False
del lazarus
gc.is_finalized(runner)  # True

It's hard to imagine a situation when you'll need it, though. The main conclusion you can make out of it is that you can break things with a destructor, so don't overuse it.