__del__

The method __del__ is called on the object by the garbage collector when the last reference to the object is removed:

class A:
  def __del__(self):
    print('destroying')

a = b = A()
del a
del b
# destroying

def f():
  a = A()

f()
# destroying

The method is used by Python's file object to close the descriptor when you don't need it anymore:

def f():
    a_file = open('a_file.txt')
    ...

However, you cannot safely rely on that the destructor (this is how it's called in other languages, like C) will be ever called. For instance, it can be not true in PyPy, MicroPython, or just if the garbage collector is disabled using gc.disable().

The thumb-up rule is to use the destructor only for unimportant things. For example, aiohttp.ClientSession uses __del__ to warn about an unclosed session:

def __del__(self) -> None:
  if not self.closed:
    warnings.warn(
      f"Unclosed client session {self!r}", ResourceWarning
    )