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OO issues in python

Terry Reedy tjreedy at udel.edu
Wed Dec 17 22:19:01 EST 2003 
"Ali El Dada" <eldada at mdstud.chalmers.se> wrote in
message news:brpar5$c47$1 at eol.dd.chalmers.se...
> in python, when a class, say Father, has a
member that itself is an
> instance of another class, say Child, an
instance of Father actually
> only has a reference to a Child, not the Child
object itself, right?

Good question.  As you defined the difference by
quoting from Eiffel, yes.  The association
relating names and collection slots to objects is
many to one.  A given object can be associated
with (referenced by) multiple names, lists, dicts,
and instances.  This is essential to Python's
operation.  If you want to restrict the
association to be one to one (as in engine belongs
to only one car), you must exert the discipline
yourself.  You can facilitate this by making the
restricted class private to a module or another
class.  I have never heard of this being a
problem.

> eg code that doesn't work:
>
 class Father:
     def __init__(self):
        self.a = 'foo'
        self.son = Child()

class Child:
     def __init__(self):
         print f.a

f = Father()

Put on your Python interpreter hat and try to
execute this.  Father calls Father.__init__ calls
Child calls Child.__init__ tries to access object
named f, but there is no such object because the
calls have not returned and hence the binding of
global f to Father instance has not yet occurred
and hence you get a NameError.

>>> f = Father()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 4, in __init__
  File "<stdin>", line 3, in __init__
NameError: global name 'f' is not defined

(Please, when you post code that 'does not work',
cut and paste the error traceback as above.)

If you want to create a circular reference between
Father and Child, pass the Father instance to the
Child initialization.

class Father:
     def __init__(self):
        self.a = 'foo'
        self.son = Child(self)

class Child:
     def __init__(self, father):
         self.father = father
         print father.a

>>> f = Father()
foo

Terry J. Reedy
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