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Date: Sat, 10 Aug 2013 11:40:40 -0500
From: Tim Chase <python.list@tim.thechases.com>
To: Krishnan Shankar <i.am.songoku@gmail.com>
Subject: Re: Python Basic Doubt
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Xref: csiph.com comp.lang.python:52310

On 2013-08-10 21:03, Krishnan Shankar wrote:
> >>> a=10
> >>> id(a)
> 21665504
> >>> b=a
> >>> id(b)
> 21665504
> >>> c=10
> >>> id(c)
> 21665504
> 
> I am actually assigning new value to c. But from the value of id()
> all three variables take same location. With variables a and b it
> is ok. But why c taking the same location?

As an internal optimization, CPython caches small integer values 

  >>> a = 256
  >>> b = 256
  >>> a is b
  True
  >>> a = 257
  >>> b = 257
  >>> a is b
  False

Because it's an internal implementation detail, you shouldn't count
on this behavior (Jython or Cython or IronPython may differ; or
future versions of Python may cache a different range of numbers).

Generally, if you are using the "is" operator to compare against
anything other than None, you're doing it wrong. There are exceptions
to this, but it takes knowing the particulars.

-tkc