On Wed, Jul 28, 2010 at 1:14 PM, Jason Grout
<jason-s...@creativetrax.com> wrote:
> Someone pointed this out to me recently, and it sounded like a useful gotcha
> to know about python. Apparently small python integers are cached, while
> larger ones are not. Sometimes.
>
> % sage -ipython
> Python 2.6.4 (r264:75706, May 25 2010, 15:42:09)
> Type "copyright", "credits" or "license" for more information.
>
> IPython 0.9.1 -- An enhanced Interactive Python.
> ? -> Introduction and overview of IPython's features.
> %quickref -> Quick reference.
> help -> Python's own help system.
> object? -> Details about 'object'. ?object also works, ?? prints more.
>
> In [1]: a=1
>
> In [2]: b=1
>
> In [3]: a is b
> Out[3]: True
>
> In [4]: a=1000
>
> In [5]: b=1000
>
> In [6]: a is b
> Out[6]: False
>
> In [7]: a=1000; b=1000; a is b
> Out[7]: True
>
>
> Note that these are *python* integers, not Sage integers, in the example
> above. Note also that if the two integers are on the same line, the *are*
> the same object.
> So moral of the story: be *very* careful about where you use "is".
Big +1 ! This is something that always sets of my warning bells when
refereeing code. E.g., I don't like this sort of construction, but saw
it recently in a patch on trac:
def f(n=1):
if n is 1:
...
else:
...
-- William
>
> Thanks,
>
> Jason
>
> --
> To post to this group, send an email to sage-devel@googlegroups.com
> To unsubscribe from this group, send an email to
> sage-devel+unsubscr...@googlegroups.com
> For more options, visit this group at
> http://groups.google.com/group/sage-devel
> URL: http://www.sagemath.org
>
--
William Stein
Professor of Mathematics
University of Washington
http://wstein.org
--
To post to this group, send an email to sage-devel@googlegroups.com
To unsubscribe from this group, send an email to
sage-devel+unsubscr...@googlegroups.com
For more options, visit this group at http://groups.google.com/group/sage-devel
URL: http://www.sagemath.org
