Question

I am curious about the details of __del__ in python, when and why it should be used and what it shouldn’t be used for. I’ve learned the hard way that it is not really like what one would naively expected from a destructor, in that it is not the opposite of __new__ / __init__.

class Foo(object):

    def __init__(self):
        self.bar = None

    def open(self):
        if self.bar != 'open':
            print 'opening the bar'
            self.bar = 'open'

    def close(self):
        if self.bar != 'closed':
            print 'closing the bar'
            self.bar = 'close'

    def __del__(self):
        self.close()

if __name__ == '__main__':
    foo = Foo()
    foo.open()
    del foo
    import gc
    gc.collect()

I saw in the documentation that it is not guaranteed __del__() methods are called for objects that still exist when the interpreter exits.

1. how can it be guaranteed that for any Foo instances existing when the interpreter exits, the bar is closed?
2. in the code snippet above does the bar get closed on del foo or on gc.collect()… or neither? if you want finer control of those details (e.g. the bar should be closed when the object is unreferenced) what is the usual way to implement that?
3. when __del__ is called is it guaranteed that __init__ has already been called? what about if the __init__ raised?

Answer 1

The way to close resources are context managers, aka the with statement:

class Foo(object):

  def __init__(self):
    self.bar = None

  def __enter__(self):
    if self.bar != 'open':
      print 'opening the bar'
      self.bar = 'open'
    return self # this is bound to the `as` part

  def close(self):
    if self.bar != 'closed':
      print 'closing the bar'
      self.bar = 'close'

  def __exit__(self, *err):
    self.close()

if __name__ == '__main__':
  with Foo() as foo:
    print foo, foo.bar

output:

opening the bar
<__main__.Foo object at 0x17079d0> open
closing the bar

---

2) Python’s objects get deleted when their reference count is 0. In your example the del foo removes the last reference so __del__ is called instantly. The GC has no part in this.

class Foo(object):

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    import gc
    gc.disable() # no gc
    f = Foo()
    print "before"
    del f # f gets deleted right away
    print "after"

output:

before
deling <__main__.Foo object at 0xc49690>
after

The gc has nothing to do with deleting your and most other objects. It’s there to clean up when simple reference counting does not work, because of self-references or circular references:

class Foo(object):
    def __init__(self, other=None):
        # make a circular reference
        self.link = other
        if other is not None:
            other.link = self

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    import gc
    gc.disable()   
    f = Foo(Foo())
    print "before"
    del f # nothing gets deleted here
    print "after"
    gc.collect()
    print gc.garbage # The GC knows the two Foos are garbage, but won't delete
                     # them because they have a __del__ method
    print "after gc"
    # break up the cycle and delete the reference from gc.garbage
    del gc.garbage[0].link, gc.garbage[:]
    print "done"

output:

before
after
[<__main__.Foo object at 0x22ed8d0>, <__main__.Foo object at 0x22ed950>]
after gc
deling <__main__.Foo object at 0x22ed950>
deling <__main__.Foo object at 0x22ed8d0>
done

---

3) Lets see:

class Foo(object):
    def __init__(self):

        raise Exception

    def __del__(self):
        print "deling", self

if __name__ == '__main__':
    f = Foo()

gives:

Traceback (most recent call last):
  File "asd.py", line 10, in <module>
    f = Foo()
  File "asd.py", line 4, in __init__
    raise Exception
Exception
deling <__main__.Foo object at 0xa3a910>

Objects are created with __new__ then passed to __init__ as self. After a exception in __init__, the object will typically not have a name (ie the f = part isn’t run) so their ref count is 0. This means that the object is deleted normally and __del__ is called.