Cyril Bazin wrote:
> Ok, ok, there was a mistake in the code.
> (Of course, it was done on prupose in order to verify if everybody is
> aware ;-)
> I don't know why it is preferable to compare an object to the object
> "None" using "is not".
> "==" is, IMHO, more simple. Simple is better than complex.. So I use "==".
The archives could tell you more, but basically on is usually interested
in *identity* with a singleton object (None), not in whether the object
on is examining happens to compare equal. A custom object could be
designed to compare equal to None in certain cases, even though it *is
not* None, leading to the "== None" approach being defective code.
In the specific code in question, it won't make any differences, but I
pointed it out to help folks who don't know this to start developing the
safer habit, which is always to use "is" and "is not" with None (or,
generally, with other singletons).
> The correct program is:
>
> import urllib2
> for line in open("fileName.txt"):
> addr, port = urllib2.splitport (line)
> print (port == None) and '' or port
Nope, sorry... the point is that the "and/or" pseudo-ternary operator is
inherently flawed if the operand after "and" evaluates False. Check
this out:
>>> port = None
>>> print (port == None) and '' or port
None
>>> print (port != None) and '' or port
None
and even:
>>> print (port is None) and '' or port
None
So the bug isn't in using "!=" instead of "==", or using equality
instead of identity comparison, it is in trying to use the and/or
expression for a purpose it wasn't intended for.
> or
>
> import urllib2
> for line in open("fileName.txt"):
> addr, port = urllib2.splitport(line)
> if port == None:
> print ''
> else:
> print port
That should work nicely... and it's more readable too!
Note that in Python 2.5 you should be able to write this as
print '' if port is None else port
or print '' if (port is None) else port
but it's quite arguable whether that is better than the simple if/else
statement in this case.
-Peter
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