is None or == None ?

[Steven D'Aprano]

On Sat, 07 Nov 2009 14:22:28 -0800, sturlamolden wrote:

> On 6 Nov, 14:35, "Alf P. Steinbach" <al... at start.no> wrote:
> 
>> As I understand it, 'is' will always work and will always be efficient
>> (it just checks the variable's type), while '==' can depend on the
>> implementation of equality checking for the other operand's class.
> 
> '==' checks for logical equality. 'is' checks for object identity.

So far so good, although technically == merely calls __eq__, which can be 
over-ridden to do (nearly) anything you like:

>>> class Funny(object):
...     def __eq__(self, other):
...             return self.payload + other
...
>>> f = Funny()
>>> f.payload = 5
>>> f == 10
15


> None is a singleton of type NoneType. Since None evaluates to True only
> when compared against itself, 

That's wrong. None never evaluates to True, it always evaluates as None, 
in the same way that 42 evaluates as 42 and [1,2,3] evaluates as [1,2,3]. 
Python literals evaluate as themselves, always.

Perhaps you mean that *comparisons* of None evaluate to True only if both 
operands are None. That's incorrect too:

>>> None > None
False

You have to specify the comparison. It would be a pretty strange language 
if both None==None and None!=None returned True.



> it is safe to use both operators.

Only if you want unexpected results if somebody passes the wrong sort of 
object to your code.


>>> class NoneProxy:
...     def __eq__(self, other):
...             if other is None: return True
...             return False
...
>>> o = NoneProxy()
>>> o is None
False
>>> o == None
True

You should use == *only* if you want to test for objects which are equal 
to None, *whatever that object may be*, and is if you want to test for 
None itself.



-- 
Steven