Assignment Versus Equality

Rustom Mody rustompmody at gmail.com
Wed Jun 29 02:37:41 EDT 2016


On Wednesday, June 29, 2016 at 12:03:30 PM UTC+5:30, Rustom Mody wrote:
> On Wednesday, June 29, 2016 at 11:57:03 AM UTC+5:30, Chris Angelico wrote:
> > On Wed, Jun 29, 2016 at 3:55 PM, Rustom Mody wrote:
> > >> The transparent shift from machine-word to bignum is what no longer
> > >> exists. Both Py2 and Py3 will store large integers as bignums; Py2 has
> > >> two separate data types (int and long), with ints generally
> > >> outperforming longs, but Py3 simply has one (called int, but
> > >> functionally like Py2's long), and does everything with bignums.
> > >> There's no longer a boundary - instead, everything gets the "bignum
> > >> tax". How steep is that tax? I'm not sure, but microbenchmarking shows
> > >> that there definitely is one. How bad is it in real-world code? No
> > >> idea.
> > >>
> > >> ChrisA
> > >
> > > New to me -- thanks.
> > > I thought it did an FSR type covert machine word → BigInt conversion under the hood.
> > > Tax is one question
> > > Justification for this change is another
> > 
> > CPython doesn't currently do anything like that, but it would be
> > perfectly possible to do it invisibly, and thus stay entirely within
> > the language spec. I'm not aware of any Python implementation that
> > does this, but it wouldn't surprise me if PyPy has some magic like
> > that. It's PyPy's kind of thing.
> > 
> > It's also entirely possible that a future CPython will have this kind
> > of optimization too. It all depends on someone doing the
> > implementation work and then proving that it's worth the complexity.
> > 
> > ChrisA
> 
> Huh? I though I was just describing python2's behavior:
> 
> $ python
> Python 2.7.11+ (default, Apr 17 2016, 14:00:29) 
> [GCC 5.3.1 20160413] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
> >>> x=2
> >>> type(x)
> <type 'int'>
> >>> y=x ** 80
> >>> y
> 1208925819614629174706176L
> >>> type(y)
> <type 'long'>

Um ok I see the ...L there at the end of 2 ** 80
So its not exactly 'under the hood'


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