[PYTHON] Use weak references

When you create something like a collection of classes in Python, you may want to have a reference to the parent object.

In such a case, in other languages, it is sometimes said that a variable called ʻowner` is created to have a reference to the parent object, but if it is implemented as it is, it becomes a circular reference, and the acquired memory was intended. It may not be released at the timing.

Code where circular references can occur

class ParentClass:
  def __init__(self):
    self.child = []

class ChildClass:
  def __init__(self, owner):
    self._owner = owner

p = ParentClass()
p.child.append(ChildClass(p))
# ... 

For this case, there is a ** weak reference (weakref module) ** that does not increment the reference counter when referencing an object.

• weakref ---weak reference — Python 3 \ .8 \ .3 documentation

Code with weak references

import weakref

class ParentClass:
  def __init__(self):
    self.child = []

class ChildClass:
  def __init__(self, owner):
    self._owner = weakref.ref(owner)

p = ParentClass()
p.child.append(ChildClass(p))
# ... 

If you want to follow the reference created by this weakref object, call it with () like a method.

Code with weak references

import weakref

class ParentClass:
  def __init__(self):
    self.child = []

  def message(self):
    print("called")

class ChildClass:
  def __init__(self, owner):
    self._owner = weakref.ref(owner)

  def message(self): 
    self._owner().message()

p = ParentClass()
p.child.append(ChildClass(p))
p.child[0].message()
# ...