Circular references and python

Martin von Loewis loewis at informatik.hu-berlin.de
Wed Feb 2 23:06:58 CET 2000


"Brian Dam Pedersen" <brian.pedersen at mail.danbbs.dk> writes:

> Now the lists are circular referenced, and dereferencing them will
> not garbage-collect them. To break up the chain just do

The essential piece here is that you have to break the circularity
with explicit code; Python will then clean-up the rest when you drop
the last reference to the structure.

Since cyclic structures are usually regular in some way, it is often
easy to break the cycles with a recursive algorithm. Consider

class ListItem:
  def __init__(self,next,prev):
    self.prev,self.next = prev,next

This could be used in a double-linked list (not that a double-linked
list is particular useful in Python :-). To allow breaking the cycles,
you can do

  def clear(self):
    if self.next:
      self.next.clear()
      self.next=None  # or self.prev=None

If you know you don't need such a list (stored in a variable head)
anymore, you do

head.clear()

and it will remove everything.

So it is very easy to deal with the problem when you know what it is.

Regards,
Martin




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