What other languages use the same data model as Python?

[John Nagle]

On 5/5/2011 6:59 AM, Steven D'Aprano wrote:
> On Thu, 05 May 2011 21:48:20 +1000, Chris Angelico wrote:
>
>> On Thu, May 5, 2011 at 9:44 PM, Mel<mwilson at the-wire.com>  wrote:
>>> John Nagle wrote:
>>>> On 5/4/2011 5:46 PM, harrismh777 wrote:
>>>>> Or, as stated earlier, Python should not allow 'is' on immutable
>>>>> objects.
>>>>
>>>>      A reasonable compromise would be that "is" is treated as "==" on
>>>> immutable objects.
>>>
>>> I foresee trouble testing among float(5), int(5), Decimal(5) ...
>>
>> Define 'x is y' as 'type(x)==type(y) and
>> isinstance(x,(int,float,tuple,etc,etc,etc)) and x==y' then.

    That's close to the right answer.
>
> `is` is supposed to be a *fast* operator, not even slower than equality
> testing.

    That's an implementation problem.  Those are cheap tests at the
machine code level.  An efficient test looks like this:

     def istest(a, b) :
         if id(a) == id(b) :	# the cheap address test
             return(True)
         if type(x) != type(y) :	# cheap binary comparison
	    return(False)
         if mutable(x) : 	# the interpreter knows this
             return(False)
         return(x == y)		# equality test for mutables

Probably about 12 machine instructions, and the full "==" test
is only reached for cases in which "is" now produces wrong answers.

It's encouraging that a Google code search finds no matches of

	if .* is \"
or
	if .* is 1

in Python code.

				John Nagle