Permanent objects?

[Dave Brueck]

On Tue, 24 Dec 2002, Tim Peters wrote:

> [Tim]
> >> For example, never use "is" to compare immutable objects, unless you
> >> don't care what the result is <wink>.  The optimizations can (and do)
> >> change across Python releases.
>
> [Dave Brueck]
> > Does this also apply to None?
>
> Yes, provided you supply the missing paragraph <wink> about
> guaranteed-singleton objects, where "is" is fine.  None and type objects are
> guaranteed unique, so "x is None" and "type(d) is dict" are fine (and
> idiomatic).  No guarantees about uniqueness are made wrt ints, longs,
> floats, complexes, strings, Unicode strings, or tuples.  Uniqueness is
> guaranteed for True and False in Python 2.3.

Good, thanks for that explanation. I just remembered having written "if x
is None" a lot and suddenly worried I was just lucky that it always
worked.

-Dave