recursion in __cmp__

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  • Victor Safronovich
    #1

    recursion in __cmp__

    please comment this
    Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on win32[color=blue][color=green][color=darkred]
    >>> class A:[/color][/color][/color]
    def __cmp__(self, other):
    print '%s|%s' %(`self`, `other`)
    return cmp(self, other)
    [color=blue][color=green][color=darkred]
    >>> a = A()
    >>> cmp(a,A())[/color][/color][/color]
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
    0[color=blue][color=green][color=darkred]
    >>> cmp(a,5)[/color][/color][/color]
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    <__main__.A instance at 0x00989838>|5
    0
    Tags: None
    • Alex Martelli
      #2
      Re: recursion in __cmp__

      Victor Safronovich <vsafronovich@n aumen.ru> wrote:
      [color=blue]
      > please comment this
      > Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on win32[color=green][color=darkred]
      > >>> class A:[/color][/color]
      > def __cmp__(self, other):
      > print '%s|%s' %(`self`, `other`)
      > return cmp(self, other)[/color]

      What is there to comment? This is obviously and blatantly recursive,
      since cmp(a, b) delegates back to a.__cmp__(b) [[for classic classes;
      for a newstyle class it would be type(a).__cmp__ (a, b) with the same
      recursive effect in this case]].

      Just like the simpler example: def __len__(self): return len(self)


      Alex

        Comment

        • John Doe
          #3
          Re: recursion in __cmp__

          Really Victor,
          no one comments on old versions. Try it on 2.3.

          Dan

          On Fri, 05 Nov 2004 00:45:28 -0800, Victor Safronovich wrote:
          [color=blue]
          > please comment this
          > Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on win32[color=green][color=darkred]
          >>>> class A:[/color][/color]
          > def __cmp__(self, other):
          > print '%s|%s' %(`self`, `other`)
          > return cmp(self, other)
          >[color=green][color=darkred]
          >>>> a = A()
          >>>> cmp(a,A())[/color][/color]
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > <__main__.A instance at 0x00989838>|<__ main__.A instance at 0x0097D120>
          > 0[color=green][color=darkred]
          >>>> cmp(a,5)[/color][/color]
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > <__main__.A instance at 0x00989838>|5
          > 0[/color]

            Comment

            • Peter Otten
              #4
              Re: recursion in __cmp__

              Victor Safronovich wrote:
              [color=blue]
              > please comment this
              > Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on win32[color=green][color=darkred]
              >>>> class A:[/color][/color]
              > def __cmp__(self, other):
              > print '%s|%s' %(`self`, `other`)
              > return cmp(self, other)
              >[color=green][color=darkred]
              >>>> a = A()
              >>>> cmp(a,A())[/color][/color][/color]

              Does it get clearer with the following reimplementatio n of cmp()?
              [color=blue][color=green][color=darkred]
              >>> class A:[/color][/color][/color]
              .... def __cmp__(self, other):
              .... return mycompare(self, other)
              ....[color=blue][color=green][color=darkred]
              >>> def mycompare(a, b):[/color][/color][/color]
              .... try:
              .... return a.__cmp__(b)
              .... except AttributeError:
              .... return mycompare(id(a) , id(b))
              ....[color=blue][color=green][color=darkred]
              >>> mycompare(A(), A())[/color][/color][/color]
              Traceback (most recent call last):
              File "<stdin>", line 1, in ?
              File "<stdin>", line 3, in mycompare
              File "<stdin>", line 3, in __cmp__
              File "<stdin>", line 3, in mycompare
              File "<stdin>", line 3, in __cmp__
              [snip]
              File "<stdin>", line 3, in __cmp__
              File "<stdin>", line 3, in mycompare
              RuntimeError: maximum recursion depth exceeded[color=blue][color=green][color=darkred]
              >>>[/color][/color][/color]

              Peter

                Comment

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