[Tutor] Python cashes low integers? How? Where?

[Kalle Svensson]

Hi!

[Dick Moores]
> >>> a = 4
> >>> b = 1 + 3
> >>> a is b
> True
...
> OK. Now, this intrigued me. Where does Python do this cashing? Not
> in my computer's memory, because rebooting doesn't change the above
> results. So where?

Indeed in your computer's memory, but the cache is not persistent
between reboots or even Python invocations.

The line 'a = 4' creates an integer object with the value 4 (let's
call it <int 4>), and since 4 is less than some limit, the object is
cached.  'a' now points to <int 4> and the integer cache contains one
object, the same <int 4>.

Next, the line 'b = 1 + 3' creates two integer objects, <int 1> and
<int 3>.  They are also cached.  They are then added together.  Since
the result is 4, no new integer object is created but a reference to
the cached <int 4> is returned and 'b' is made to point to it.

Thus 'a is b' comes to be true.  The integer cache now contains
<int 1>, <int 3> and <int 4>.  The next time you start Python, the
integer cache is empty again.

Hope this helps!

Peace,
  Kalle
-- 
http://juckapan.org/~kalle/
http://laxmasteriet.se/04-05/