Why do circular references prevent the destructors of objects that aren't on the loop?

Question

In this example:

class Foo(object):
    def __del__(self):
        print "Foo died"

class Bar(object):
    def __init__(self):
        self.foo = Foo()
        self.baz = Baz(self)

class Baz(object):
    def __init__(self, bar):
        self.bar = bar

b = Bar()

I'd expect the Foo destructor to be called in spite of the loop between Bar and Baz, as since Foo holds no references to any bar or baz collecting them and decrementing reference counts should be completely safe to do before collecting Foo. Why doesn't python behave this way? How can destructors possibly be useful if they can be prevented from being called by completely unrelated actions of other classes?

@LevLevitsky It is no longer referenced once b goes out of scope (i.e. when the program ends). Try this example with the `self.baz` line commented out, you'll see the destructor called. — Shea Levy, Jan 31 '13 at 14:30
@DanielRoseman But they're the only way to safely handle certain cleanup situations... context managers don't work across threads. And as far as I can tell this use case _could_ be safe! — Shea Levy, Jan 31 '13 at 14:31

score 1 · Accepted Answer · answered Jan 31 '13 at 14:32

1

Note that the destructor does not need to be called when the interpreter exits.

A quick modification to your script and all works as you expected:

class Foo(object):
    def __del__(self):
        print "Foo died"

class Bar(object):
    def __init__(self):
        self.foo = Foo()
        self.baz = Baz(self)

class Baz(object):
    def __init__(self, bar):
        self.bar = bar

b = Bar()

del b

answered Jan 31 '13 at 14:32

mgilson

300,191
65
633
696

Oh! Well now I feel dumb. I assumed it would work the same as when I didn't have the `self.baz` line. Thanks, will mark as accepted when the time limit ends. – Shea Levy Jan 31 '13 at 14:34

Why do circular references prevent the destructors of objects that aren't on the loop?

1 Answers1