negative numbers are not equal...
dlenski at gmail.com
Fri Aug 15 15:56:41 CEST 2008
On Thu, 14 Aug 2008 17:18:55 -0300, ariel ledesma wrote:
> hello guys
> i just ran into this when comparing negative numbers, they start
> returning False from -6 down, but only when comparing with 'is'
> >>> m = -5
> >>> a = -5
> >>> m is a
> >>> m = -6
> >>> a = -6
> >>> m is a
> >>> m == a
> i read that 'is' compares if they are really the same object, but i
> don't that's it because then why does -5 return True? of course i could
> only use == to compare, but still, what am i missing here? thanks in
They also return False for positive numbers > 256. Try this:
>>> print [x for x in range(-10,260) if x is not x+1-1]
[-10, -9, -8, -7, -6, 257, 258, 259]
There is a good explanation for this: the "is" operator checks for object
identity, not equality. Basically "a is m" asks, does the variable name
"a" reference the same memory location as the variable name "m"?
For integers in the range of -5<=x<=256, Python pre-caches all of these
values, and whenever a variable takes on one of those values, it uses the
cached value. This is an optimization: it prevents the need to allocate
a new Python object for these very very common small integer values.
For integer literals outside this range, a new Python object gets created
when they are assigned to variables, so a=500 followed by m=500 will
create new objects.
The "is" operator just shows the effect of this caching. It's
unimportant for real code since you never care whether two numeric
variables refer to the same object (only important for complex data
structures where their storage may overlap)... only whether they are
equal or not.
(PS- The small integer pre-caching is described somewhere in the C API
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