Checking for an "undefined" variable - newbie question

[Damien Wyart]

* Alex Martelli <aleax at aleax.it> in comp.lang.python:
> If you can't rule out (e.g.) None as a valid value for your variable,
> just make a unique placeholder value such as [be sure to use a MUTABLE
> value, else uniqueness is not guaranteed]:

Could you expand on this "uniqueness" point ? I don't see very well what
you mean.


Thanks in advance,
-- 
Damien Wyart