Semantics of ==

Isaac To kkto at csis.hku.hk
Fri Mar 26 07:26:18 CET 2004


>>>>> "Raymond" == Raymond Hettinger <python at rcn.com> writes:

    Raymond> [Axel Boldt]
    >> > >>> l=[1] > >>> s=l > >>> l.append(s) > >>> w=[1] > >>> r=[1,w] >
    >> >>> w.append(r) > >>> s [1, [...]]  > >>> w [1, [1, [...]]]  > >>>
    >> s==w > True
    >> >
    >> > Note that they're equal, yet are printed differently.
    >> >
    >> > >>> s[0]=2 > >>> w[0]=2 > >>> s==w > False
    >> >
    >> > All of a sudden they have become unequal.

    Raymond> [John Roth]
    >> You've got a recursive structure! I originally thought that the [...]
    >> was something you'd done to the printout, but it isn't.
    >> 
    >> I think the original True is a bug. It's getting confused by the
    >> recursion.

    Raymond> Armin Rigo found and fixed this for Py2.4:

    Raymond> Python 2.4a0 (#46, Mar 23 2004, 01:55:44) [MSC v.1200 32 bit
    Raymond> (Intel)] on win32 <snipped>
    >>>> s
    Raymond> [1, [...]]
    >>>> w
    Raymond> [1, [1, [...]]]
    >>>> s==w

    Raymond> Traceback (most recent call last): File "<pyshell#9>", line 1,
    Raymond> in -toplevel- s==w RuntimeError: maximum recursion depth
    Raymond> exceeded in cmp

I'm confused... what makes the new behaviour more "correct" than the original?

Regards,
Isaac.



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