confused by bindings

[Don O'Donnell]

Sam Falkner wrote:
> 
> 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
> 
> --- 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)
> 
> In other words, I'm trying to test for a leak of Cell objects.  But,
> no matter what I've tried, this doesn't work.  I've tried
> global-to-the-data-module variables for the count, I've tried the Borg
> pattern from the cookbook, but nothing works.
> 
> When I run it this way, I get
> count is 1
> count is 1
> count is 1
> count is 1
> ...
> 
> and the test fails with 5499 != 0.
> 
> Okay, I feel stupid here.  What am I doing wrong?
> 
> Thanks!
> 
> - Sam

Hi Sam,

Not stupid, just confusing instance variables with class variables.  The
argument 'self' in your __init__ method refers to the class *instance*
that is being initialized, so that the statement  self.count += 1  in
Cell.__init__ is really incrementing the new instance variable named
count. Here's how that statement works:

1.  Looks for a variable named count in the dictionary of the new
instance.
2.  Doesn't find it, so searches the Cell class dictionary.
3.  Finds it, with a value of 0.
4.  Adds 1 to it.
5.  Stores it in the local instance dict.

So every instance you create will have a count value of 1, as you
noticed.

What you need to do to maintain an instance counter by incrementing the
class variable directly.  Your class definition should look like this:

>>> class Cell:
	count = 0
	def __init__(self, stuff):
		Cell.count += 1
		self.stuff = stuff
		print 'count is', Cell.count

		
>>> x = []
>>> x.append(Cell('a'))
count is 1
>>> x.append(Cell('b'))
count is 2
>>> x.append(Cell('c'))
count is 3
>>> Cell.count
3

To be complete, you should also define a __del__ method in your Cell
class which will decrement Cell.count when an instance is deleted:

	def __del__(self):
		Cell.count -= 1

Hope this helps.

Cheers,
Don