Checking for an "undefined" variable - newbie question

[Alex Martelli]

Damien Wyart wrote:

> * 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.

if you used, e.g.:

variable = 'astringthatwontactuallyoccur'

then you couldn't later test with

if variable is 'astringthatwontactuallyoccur':

because the two literals might be or not be unique so the 'is' test is
not guaranteed to work -- you'd have to use == which is slower -- AND
again because of non-uniqueness you might be in trouble is some code did
want to assign to variable such a string, e.g. because it can get read
from a file, or interactively from the user, or otherwise constructed.

The 'is' test checks for object identity, not value, so it's faster --
and using a unique mutable object (e.g. a class) made just for the
purpose also guarantees no clashes with any actual value you might 
want to assign to your variable.  I started that classname with an
underscore to signify it's meant for internal use in the module, not
for export to other modules, by the way.


Alex