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