is implemented with id ?

[Ramchandra Apte]

On Wednesday, 5 September 2012 19:43:30 UTC+5:30, Steven D'Aprano  wrote:
> On Wed, 05 Sep 2012 05:48:26 -0700, Ramchandra Apte wrote:
> 
> 
> 
> > Seeing this thread, I think the is statment should be removed. It has a
> 
> > replacement syntax of id(x) == id(y)
> 
> 
> 
> A terrible idea.
> 
> 
> 
> Because "is" is a keyword, it is implemented as a fast object comparison 
> 
> directly in C (for CPython) or Java (for Jython). In the C implementation 
> 
> "x is y" is *extremely* fast because it is just a pointer comparison 
> 
> performed directly by the interpreter.
> 
> 
> 
> Because id() is a function, it is much slower. And because it is not a 
> 
> keyword, Python needs to do a name look-up for it, then push the argument 
> 
> on the stack, call the function (which may not even be the built-in id() 
> 
> any more!) and then pop back to the caller.
> 
> 
> 
> And worst, *it doesn't even do what you think it does*. In some Python 
> 
> implementations, IDs can be reused. That leads to code like this, from 
> 
> CPython 2.7:
> 
> 
> 
> py> id("spam ham"[1:]) == id("foo bar"[1:])
> 
> True
> 
> 
> 
> You *cannot* replace is with id() except when the objects are guaranteed 
> 
> to both be alive at the same time, and even then you *shouldn't* replace 
> 
> is with id() because that is a pessimation (the opposite of an 
> 
> optimization -- something that makes code run slower, not faster).
> 
> 
> 
> 
> 
> > and "a==True" should be automatically changed into memory comparison.
> 
> 
> 
> Absolutely not. That would be a backward-incompatible change that would 
> 
> break existing programs:
> 
> 
> 
> py> 1.0 == True
> 
> True
> 
> py> from decimal import Decimal
> 
> py> Decimal("1.0000") == True
> 
> True
> 
> 
> 
> 
> 
> 
> 
> -- 
> 
> Steven
the is statement could be made into a function