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Re: Why do these statements evaluate the way they do?

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  Re: Why do these statements evaluate the way they do? Kev Dwyer <kevin.p.dwyer@gmail.com> - 2016-05-07 08:04 +0100

#108257 — Re: Why do these statements evaluate the way they do?

Anthony Papillion wrote:

> I'm trying to figure out why the following statements evaluate the way
> they do and I'm not grasping it for some reason. I'm hoping someone can
> help me.
> 
> 40+2 is 42 #evaluates to True
> But
> 2**32 is 2**32 #evaluates to False
> 
> This is an example taken from a Microsoft blog on the topic. They say the
> reason is because the return is based on identity and not value but, to
> me, these statements are fairly equal.
> 
> Can someone clue me in?
> 
> Anthony

The *is* operator tests for identity, that is whether the objects on either 
side of the operator are the same object.

CPython caches ints in the range -5 to 256 as an optimisation, so ints in 
this range are always the same object, and so the is operator returns True.

Outside this range, a new int is created as required, and comparisons using 
is return False.

This can be seen by looking at the id of the ints:

Python 3.5.1 (default, Dec 29 2015, 10:53:52)                                                                                               
[GCC 4.8.3 20140627 [gcc-4_8-branch revision 212064]] on linux                                                                              
Type "help", "copyright", "credits" or "license" for more information. 
>>> a = 42
>>> b = 42
>>> a is b
True
>>> id(a)
9186720
>>> id(b)
9186720
>>> c = 2 ** 32
>>> d = 2 ** 32
>>> c is d
False
>>> id(c)
140483107705136
>>> id(d)
140483107705168

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