confused by bindings

[Sam Falkner]

Mitchell Morris <mitchell.morris at cingular.com> writes:

> Sam Falkner <samf+usenet at frii.com> wrote in 
> news:ii7k7yvqi87.fsf at central.sun.com:
> 
> > I'm confused.  I'm a relative newbie, but am still surprised that I
> > can't make this work.
> > 
> > What I want to do is to have my test module set "stuff" in one of the
> > other modules, do its tests, and then check the results.  Code might
> > be more clear:
> > 
> > --- file data.py ---
> > 
> > class Cell:
> >     count = 0
> >     def __init__(self, stuff):
> >         self.count += 1
> >         self.stuff = stuff
> >         print 'count is', self.count

> Well, I learn something new every day. I would have expected this to
> raise a NameError on the "self.count += 1" line. It doesn't though
> ... I'll have to revisit my understanding of scoping now.

I might have thought this too, but look at the wonderful Borg recipe:

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66531

Aren't they doing exactly what I was doing?
    self.__dict__ = self.__shared_state

As I mentioned before, I tried using the "borg pattern" for my test
suite, and it didn't work either.  If anyone is curious, I can
re-create the failure.

> The short answer is that "self.count" isn't bound to the same thing as 
> "Cell.count". "self.count" is an attribute of the instance: each new Cell 
> instance gets its own copy. If you want to manipulate the class attribute, 
> then you have to say so via "Cell.count += 1" instead.

I've tried it this way before.  I just (re)tried this, and the test
passes, if I run only that test case.  But, if I run all my test
cases, it fails again with 5677 != 0; in other words, data.Cell.count
is 0 when looked at from my test module.

re-inserting my test case:

--- file test.py ---

class CellTestCase(unittest.TestCase):
    def testforleaks(self):
        data.Cell.count = 0
        # do a whole bunch of stuff
        # x = a gnarly collection of Cell objects
        # count number of Cell objects in x
        self.failUnlessEqual(count-of-Cells-from-x, data.Cell.count)

You can see that I reset the counter, via data.Cell.count = 0, to
clean up from the other test cases.  As I mentioned above, my test now 
succeeds when I try it your way, but not if I run the whole test suite.

> > Okay, I feel stupid here.  What am I doing wrong?

> My guess is you're still wearing your Java hat, but I'm not entirely sure.

I'm offended, but only slightly.  ;-)

Help!  I'm running Python 2.1.1; I don't yet suspect a Python bug, but
I'm getting more confused by the minute.

- sam