how to give an object as argument for a method

[Jeff Shannon]

Marco Herrn wrote:

> [snip]
> > ...  def sayNo(self):
> > ...   caller = sys._getframe(1).f_locals['self']
> this is the only line, that looks a bit ugly. It actually does the same
> as when giving caller as parameter. So what can be a reason to implement
> it this way? It looks much less readable to me. And I have to say I not
> even understand it fully....

Actually, it does *not* do the same as giving the caller as a parameter, in
general.  What this does, is that it gets the intepreter's stack frame in which
the current object was created.  In this case, that happens to be the
Egg.__init__() frame.  It then searches this stack frame for a local name
'self';  if that name doesn't exist, this will throw an exception.  This means
that Spam can *only* be created from inside of another method which uses the self
convention;  you cannot create a Spam object globally or inside of a top level
function.  And, indeed, in the rare instances where a method uses some name other
than self to represent its bound instance, this will fail as well.  A metaphor
would be that, instead of simply being told who your parents are, you're
searching through a phone book for a name that you *think* belongs to them -- but
you might have the name wrong, they might have an unlisted number, or they might
not even *be* in that phone book.  (Note also that even if it *does* work, it's
inappropriate to call the object found the "owner" or "caller" of the current
instance; it would be the *creator*, which is not necessarily the same thing at
all, especially when you consider object factories and such.)

In short, this is black magic.  It might work, most of the time... but if you
turn your back on it, it's liable to blow up on you.  Use only with caution, and
only when you're *sure* that you know what you're asking for.  :)

Jeff Shannon
Technician/Programmer
Credit International