Really, Really Confused...

[Ben Mitchell]

Hi Folks,

I've encountered a very strange problem that'll take a little space to
explain.  I hope some of you will brave your way to the end though because
I'm well and truly stumped by this one.

First some history:  I got started on the road to confusion because I was
trying to turn my Python class into a COM server, though the issue I
ultimately ran into has nothing to do with COM.  It looks like the VB app I
was using to test the COM server i holding a long running reference to the
COM object, and then calling __init__ to initialize it instead of recreating
it from scratch when I  "create" it.  I don't know enough VB/COM to know why
this would be the case, but whatever - the init function was being invoked
repeatedly and it was causing problems.  I decided to remove some complexity
and just reinvoke the initializer a second time from the basic script to
debug.  I have since been able to reduce the issue down to the following
snippet of code:

class cApp:
    object = None
    def __init__(self):
        self.object = None
        self.object = cObject()

class cObject:
    myDict = {}
    def __init__(self):
        if self.myDict.has_key("foo"):
            raise "Already set"
        self.myDict["foo"] = "some value"

testApp = cApp()
testApp.__init__()
print "Finished"


When the init function runs the second time, the "Already set" exception is
raised.  Now I really don't understand why this would be.  When
cApp.__init__() is invoked, it releases its reference to self.object before
creating a new one (self.object = None).  As best I'm aware, it then creates
a completely new cObject with no common references to the old one.  In what
conceivable way could the myDict of the new cObject have a value already set
for "foo"?

And now it gets more interesting.  If I substitute a string in place of a
dict as the value in cObject, everything works fine:

class cApp:
    object = None
    def __init__(self):
        self.object = None
        self.object = cObject()

class cObject:
    myString = None
    def __init__(self):
        if self.myString != None:
            raise "Already set"
        self.myString = "some value"

testApp = cApp()
testApp.__init__()
print "Finished"

This runs to completion without griping.  HOW IS THIS DIFFERENT?

I think the real root of my confusion is that I don't understand what the
constructor is doing here.  Why is it that when I create a seemingly new
cObject, it has any value at all stored within it?


Thanks for reading all this, and thanks in advance for your help.


Best,

-Ben