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
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