Newbie: is a variable defined?

[Erik Max Francis]

Chad Netzer wrote:

>    The preferred way to test against None is with the "is" keyword,
> presumably because something can equal None without being None
> (from memory, if I'm wrong, someone correct this).

You're correct.  Someone could override __eq__ or __cmp__, for instance,
to make a custom instance test equal (==) to None.  But (unless None is
rebound), it could never test identical (is) to it.

It's a fairly minor quibble, I'd say, since only someone up to some
serious trickery (who's going to get his come-uppance) would make stoop
to making something equal to None, but nevertheless I myself always test
Noneness with is, not ==, and suggest that others do the same.

-- 
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