Creating a variable of a specific type

[Terry Reedy]

"Jp Calderone" <exarkun at intarweb.us> wrote in message
news:20040205143140.GA22941 at intarweb.us...
> On Thu, Feb 05, 2004 at 08:54:29AM -0500, Aahz wrote:
> > In article <bvtei9$10enm4$1 at ID-111250.news.uni-berlin.de>,
> > Diez B. Roggisch <nospam-deets at web.de> wrote:
> > >
> > >Now back to your problem: I don't know right from my head what socket
> > >requires as inet-type, but I guess its a tuple of some sort. You don't
need
> > >to declare a variable for that - just use it. If you need to check for
a
> > >symbol not beeing initialized, simply use None as value, like here:
> > >
> > >address = None
> > >if address:
> > >  do_something(address)
> > >
> > >None is considered false. You might be more explicit, by using
> > >
> > >if address == None:
> > >
> > >but thats a matter of taste (to me, at least...)
> >
> > No, that's not a matter of taste, it's a matter of incorrect coding.
> > Using ``==`` calls a method on address, which could return true even if
> > address isn't None.  Much better to use ``is``, which is guaranteed to
> > return true only if address really *is* None.
> >
> > Note that in the absence of special methods for comparison, all Python
> > objects are true, so your original formulation is especially
appropriate.
>
>   Just as using "==" calls a method on address, which could return true
even
> if address isn't None, calling bool() with address may return false, even
if
> address isn't None!  "if address:" may work in some cases, but it will
> return incorrect results when address has been initialized to another
false
> value ([] is especially common, I find), when it is initalized to a class
> defining __nonzero__/__len__ in certainly ways, and in some unfortunate
> cases it may even raise an exception (eg, cgi.FieldStorage).

Identity to None should be tested for with the 'is' operator (if address is
None:...), which quickly tests for identity of the two objects.  Equality
of value and boolean value are both slower and a bit slippery (type
specific).

Terry J. Reedy