Python Quiz

Grant Edwards grante at visi.com
Thu Jul 17 21:31:20 CEST 2003


In article <X7ydnRCQKq4rdIuiXTWJjQ at comcast.com>, Terry Reedy wrote:

>> Yup. I found a lot of things in FORTRAN surprising.  Python is
>> so much nicer in that respect.  Though I was recently surprised
>> that the remove() method for lists uses "==" and not "is" to
>> determine what to remove.  It's documented that it works that
>> way. But, it wasn't what I excpected, and it took me a while to
>> figure out that it was using my class's __cmp__ method rather
>> than the object ID.
> 
> Given ints = [9, 99, 999, 9999] would it not surprise you even more if
> ints.remove(99) worked and ints.remove(999) did not, or even worse, if
> the behavior of ints.remove(99) depended on the implementation?

Yes, it would.

> (Similar examples apply to strings, where the implementation of
> interning and hence behavior based on identity *has* changed!)

I'm not claiming it's right, or even rational, but for
user-defined objects, I expected it to use "is".  Now I know
better, and am more careful when I impliment __cmp__.  I had
expected to be able to have two different objects that compared
equal.  And I also expected to be able to use list.remove() to
remove a specified object.  I had to give up one or the other.

-- 
Grant Edwards                   grante             Yow!  ... he dominates the
                                  at               DECADENT SUBWAY SCENE.
                               visi.com            




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