[General lang] how to call a parent

Jeff Epler jepler at unpythonic.net
Sun Jun 15 18:53:12 CEST 2003

What is the "parent" of an object?  For instance given

class Z: pass

a = Z()
b = Z()
c = Z()

a.kids = [c]
b.kids = [c]

it's clear that c is-a kid of a, and c is-a kid of b, so what is c's
parent?  Python can have any sort of directed graph for objects, not just a
tree, so the language doesn't define the parent of an object.

However, if you work only with objects of your own devising, you can keep
track of the parent/child relationship:
	class FamilyTree:
		def __init__(self, name, parent=None):
			self.name = name
			self.kids = []
			self.parent = None
			if parent is not None: parent.add(self)

		def add(self, kid):
			if kid.parent:
			kid.parent = self

		def newkid(self, name):
			return FamilyTree(name, self)

		def __repr__(self):
			return ("<%s with %d kids>"
				% (self.name, len(self.kids)))
>>> from ft import FamilyTree
>>> a = FamilyTree("a")
>>> b = FamilyTree("b", a) # or b = a.newkid("b")
>>> c = FamilyTree("c", b)

>>> print b, b.parent      # B has a kid, and the right parent
<b with 1 kids> <a with 1 kids>
>>> print c, c.parent      # C has no kids and the right parent
<c with 0 kids> <b with 1 kids>
>>> a.add(c)               # A adopts C (B was bad parent?)
>>> print b, b.parent      # B now has 1 kid, A now has 2
<b with 0 kids> <a with 2 kids>
>>> print c.parent         # sure enough, C's parent is A
<a with 2 kids>

a = FamilyTree("a")
b = FamilyTree("b", a)
c = FamilyTree("c", b)


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