So what are __slots__ and when should I use them?

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  • dan.j.weber@gmail.com
    #1

    So what are __slots__ and when should I use them?

    I see some programs declaring the names of class variables in
    "__slots__" . I've looked this up and the docs say something about old
    and new style classes, whatever that means. Can someone give me a
    simple, brief explanation of what __slots__ are and when I should use
    them? Thanks.

    Tags: None
    • Alex Martelli
      #2
      Re: So what are __slots__ and when should I use them?

      <dan.j.weber@gm ail.com> wrote:
      [color=blue]
      > I see some programs declaring the names of class variables in
      > "__slots__" . I've looked this up and the docs say something about old
      > and new style classes, whatever that means. Can someone give me a
      > simple, brief explanation of what __slots__ are and when I should use
      > them? Thanks.[/color]

      Normally, for flexibility and simplicity, every class instance carries
      around a dictionary -- which is great, and very fast, _but_ does take up
      a little memory per-instance. __slots__ lets you make a class whose
      instances does NOT carry a dictionary, saving tens of bytes, at a price
      in simplicity and flexibility. So, it's useful for classes whose
      instances are little more than holders for a few small pieces of data,
      ideally don't use inheritance (__slots__ and inheritance can mix, but
      not trivially so), AND as long as you plan to have HUGE numbers of
      instances of such classes "alive" at the same time, so that it matters a
      lot to you to save those few bytes per instance. The classes must be
      new-type (inherit from object or some other built-in type).

      Many people try to use one of __slots__'s unfortunate side effects (the
      fact that it removes flexibility from the instances of classes that
      define it) for other purposes (presumably because they're coming from
      languages where class instances don't HAVE that flexibility, and they're
      trying to use Python as if it was a different language, rather than
      using Python as Python). However, I heartily recommend that you
      consider defining __slots__ only as an optimization (memory saving),
      should you ever find yourself in a situation meeting all of the
      requirements in the previous paragraph (if ever).


      Alex

        Comment

        • Alex Martelli
          #3
          Re: So what are __slots__ and when should I use them?

          <dan.j.weber@gm ail.com> wrote:
          [color=blue]
          > I see some programs declaring the names of class variables in
          > "__slots__" . I've looked this up and the docs say something about old
          > and new style classes, whatever that means. Can someone give me a
          > simple, brief explanation of what __slots__ are and when I should use
          > them? Thanks.[/color]

          Normally, for flexibility and simplicity, every class instance carries
          around a dictionary -- which is great, and very fast, _but_ does take up
          a little memory per-instance. __slots__ lets you make a class whose
          instances does NOT carry a dictionary, saving tens of bytes, at a price
          in simplicity and flexibility. So, it's useful for classes whose
          instances are little more than holders for a few small pieces of data,
          ideally don't use inheritance (__slots__ and inheritance can mix, but
          not trivially so), AND as long as you plan to have HUGE numbers of
          instances of such classes "alive" at the same time, so that it matters a
          lot to you to save those few bytes per instance. The classes must be
          new-type (inherit from object or some other built-in type).

          Many people try to use one of __slots__'s unfortunate side effects (the
          fact that it removes flexibility from the instances of classes that
          define it) for other purposes (presumably because they're coming from
          languages where class instances don't HAVE that flexibility, and they're
          trying to use Python as if it was a different language, rather than
          using Python as Python). However, I heartily recommend that you
          consider defining __slots__ only as an optimization (memory saving),
          should you ever find yourself in a situation meeting all of the
          requirements in the previous paragraph (if ever).


          Alex

            Comment

            • MrJean1
              #4
              Re: So what are __slots__ and when should I use them?

              Here is an example of the difference between a class with __slots__ and
              __dict__

              <http://mail.python.org/pipermail/python-list/2004-May/220513.html>

              /Jean Brouwers

                Comment

                • MrJean1
                  #5
                  Re: So what are __slots__ and when should I use them?

                  Here is an example of the difference between a class with __slots__ and
                  __dict__

                  <http://mail.python.org/pipermail/python-list/2004-May/220513.html>

                  /Jean Brouwers

                    Comment

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