weakref_callback.py

#!/usr/bin/env python3.4

import weakref


class Node:
    def __init__(self, name):
        self.name = name
        self._closed = False
        print('Node.__init__: {}'.format(self))

    def __str__(self):
        return 'Node(name="{}", id={}, closed={})'.format(
            self.name, id(self), self._closed)

    def close(self):
        self._closed = True
        print('Node.close: {}'.format(self))

    def __del__(self):
        print('Node.__del__: {}'.format(self))
        self.close()


def main():
    '''
    $ python3.4 -u "weakref_callback.py"
    Node.__init__: Node(name="Node01", id=139814978159200, closed=False)
    deleting n ...
    Node.__del__: Node(name="Node01", id=139814978159200, closed=False)
    Node.close: Node(name="Node01", id=139814978159200, closed=True)
    weakref finalizer: 139814978159200, <weakref at 0x7f293610e858; dead>
    weakref callback:
      ref: <weakref at 0x7f293610e858; dead>
      ref(): None
    <weakref at 0x7f293610e858; dead>
    <finalize object at 0x7f29372eb100; dead>
    '''

    import sys

    n = Node('Node01')

    def callback(ref):
        print('weakref callback:')
        print('  ref: {}'.format(ref))
        print('  ref(): {}'.format(ref()))

    w = weakref.ref(n, callback)

    if sys.version_info >= (3, 4):
        def finalizer(msg, w):
            print('weakref finalizer: {}, {}'.format(msg, w))

        f = weakref.finalize(n, finalizer, id(n), w)

    print('deleting n ...')
    del n
    print(w)

    if sys.version_info >= (3, 4):
        print(f)


if __name__ == '__main__':
    main()

Shouldn't if sys.version_info > (3, 4): be if sys.version_info >= (3, 4):?