Python "is" behavior

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  • michalis.avraam@gmail.com
    #1

    Python "is" behavior

    I am not certain why this is the case, but...
    >>a = 256
    >>b = 256
    >>a is b
    True
    >>a = 257
    >>b = 257
    >>a is b
    False

    Can anyone explain this further? Why does it happen? 8-bit integer
    differences?
    Tags: None
    • George Sakkis
      #2
      Re: Python "is&quo t; behavior

      On Jun 20, 12:31 pm, michalis.avr... @gmail.com wrote:
      I am not certain why this is the case, but...
      >
      >a = 256
      >b = 256
      >a is b
      >
      True
      >
      >a = 257
      >b = 257
      >a is b
      >
      False
      >
      Can anyone explain this further? Why does it happen? 8-bit integer
      differences?
      No, implementation-dependent optimization (caching). For all we know,
      the next python version may cache up to 1024 or it may turn off
      caching completely; do not rely on it. More generally, do not use 'is'
      when you really mean '=='.

      George

        Comment

        • michalis.avraam@gmail.com
          #3
          Re: Python "is&quo t; behavior

          On Jun 20, 9:42 am, George Sakkis <george.sak...@ gmail.comwrote:
          On Jun 20, 12:31 pm, michalis.avr... @gmail.com wrote:
          >
          >
          >
          I am not certain why this is the case, but...
          >
          >>a = 256
          >>b = 256
          >>a is b
          >
          True
          >
          >>a = 257
          >>b = 257
          >>a is b
          >
          False
          >
          Can anyone explain this further? Why does it happen? 8-bit integer
          differences?
          >
          No, implementation-dependent optimization (caching). For all we know,
          the next python version may cache up to 1024 or it may turn off
          caching completely; do not rely on it. More generally, do not use 'is'
          when you really mean '=='.
          >
          George
          Thank you George. I am very curious about some of these internal
          Python things that I keep stumbling upon through friends. And thank
          you for all the help!

            Comment

            • George Sakkis
              #4
              Re: Python &quot;is&quo t; behavior

              On Jun 20, 12:45 pm, michalis.avr... @gmail.com wrote:
              On Jun 20, 9:42 am, George Sakkis <george.sak...@ gmail.comwrote:
              >
              >
              >
              On Jun 20, 12:31 pm, michalis.avr... @gmail.com wrote:
              >
              I am not certain why this is the case, but...
              >
              >a = 256
              >b = 256
              >a is b
              >
              True
              >
              >a = 257
              >b = 257
              >a is b
              >
              False
              >
              Can anyone explain this further? Why does it happen? 8-bit integer
              differences?
              >
              No, implementation-dependent optimization (caching). For all we know,
              the next python version may cache up to 1024 or it may turn off
              caching completely; do not rely on it. More generally, do not use 'is'
              when you really mean '=='.
              >
              George
              >
              Thank you George. I am very curious about some of these internal
              Python things that I keep stumbling upon through friends. And thank
              you for all the help!
              As far it's plain curiosity it's ok, but it's a small implementation
              detail you shouldn't rely on. There's nothing magic about 256, just
              the size decided for 2.5. If you tried it on 2.4 you'd get:

              Python 2.4.2 (#1, Mar 8 2006, 13:24:00)
              [GCC 3.4.4 20050721 (Red Hat 3.4.4-2)] on linux2
              Type "help", "copyright" , "credits" or "license" for more information.
              >>a=99
              >>b=99
              >>a is b
              True
              >>a=100
              >>b=100
              >>a is b
              False

              I was more surprised by the following:

              Python 2.5.1 (r251:54863, May 8 2007, 14:46:30)
              [GCC 3.4.6 20060404 (Red Hat 3.4.6-3)] on linux2
              Type "help", "copyright" , "credits" or "license" for more information.
              >>a= 123456; b=123456; a is b
              True

              For some reason, stacking multiple statements reuses the same object.

              George

                Comment

                • =?ISO-8859-1?Q?=22Martin_v=2E_L=F6wis=22?=
                  #5
                  Re: Python &quot;is&quo t; behavior

                  For some reason, stacking multiple statements reuses the same object.

                  Each code object has a co_consts tuple referring to all constants used
                  in the code. The compiler interns duplicate constants for a single
                  compiler run, resulting in the same object being used when the code
                  is put into a single line (or in a function, or module).

                  When the code is split into multiple interactive statements, the
                  compiler runs multiple times, and doesn't know anything about past
                  runs.

                  Regards,
                  Martin

                    Comment

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