[Python-Dev] Re: redefining is (original) (raw)

Thomas Heller theller at python.net
Fri Mar 19 14:06:01 EST 2004

Casey Duncan <casey at zope.com> writes:

On Fri, 19 Mar 2004 19:31:16 +0100 Thomas Heller <theller at python.net> wrote:

Barry Warsaw <barry at python.org> writes:

> On Fri, 2004-03-19 at 12:59, Michael Chermside wrote: > >> Identity Objects can be (meaningfully) compared by: >> >> * Object Identity >> To see if these are "the same object". Also useful >> for low-level memory stuff according to Tim. >> (just use '==', although 'is' would work too) > > Using == for identity objects is the wrong thing. We should > discourage tests like "if obj == None" in favor of "if obj is None". The problem (if there is any) with 'is' is that it exposes implementation details, therefore it should not be used unless on really knows what one is doing. And 'if obj is None' gains performance by relying on one of these. So I would consider 'if obj == None' correct, but unoptimized code. The problem is that 'obj == None' is not the assertion you want to make usually. 'obj == None' means "obj claims it is equal to None", whereas 'obj is None' means 'obj is the None object'. The latter is a much more stringent assertion than the former which relies on the particular implementation of obj.

Ok, so I propose to add a isNone operator or builtin function =:)

Thomas

More information about the Python-Dev mailing list