Why aren't destructors guaranteed to be called on interpreter exit?

Question

From the python docs:

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits.

Answer 1

One example where the destructor is not called is, if you exit inside a method. Have a look at this example:

class Foo(object):
    def __init__(self):
        print("Foo init running")

    def __del__(self):
        print("Destructor Foo")


class Bar(object):
    def __init__(self):
        print("Bar1 init running")
        self.bar = self
        self.foo = Foo()

    def __del__(self):
        print("Destructor Bar")

    def stop(self):
        del self.foo
        del self
        exit(1)

b = Bar()
b.stop()

The output is:

Bar1 init running
Foo init running
Destructor Foo

As we destruct foo explicitly, the destructor is called, but not the destructor of bar!

And, if we do not delete foo explicitly, it is also not destructed properly:

class Foo(object):
    def __init__(self):
        print("Foo init running")

    def __del__(self):
        print("Destructor Foo")


class Bar(object):
    def __init__(self):
        print("Bar1 init running")
        self.bar = self
        self.foo = Foo()

    def __del__(self):
        print("Destructor Bar")

    def stop(self):
        exit(1)

b = Bar()
b.stop()

Output:

Bar1 init running
Foo init running