python assignment

Steven Taschuk staschuk at
Fri Jul 25 05:59:12 CEST 2003

Quoth dan:
> but again the +[] looks funky in the morning light.  Can I always
> assume that an operation of this sort will return a new object, even
> if it has no effect on one of the operands?

Afaik it's not documented, but I think you may safely assume that
arithmetic on lists will always produce a new list, even in such
cases as
    a_list + []
Having such expressions possibly return the existing object is a
harmless optimization for immutable types, but would be madness
for mutable objects such as lists.  Guido is not mad.  (A little
eccentric sometimes, occasionally silly, and noticeably Dutch, but
not mad.)

> I suppose a clearer fix would be v[x] = copy.copy(temp), eh?

Yes, that, or
    v[x] = list(temp)
    v[x] = temp[:]
according to taste.  The latter is most common, I think.

Steven Taschuk                               staschuk at
"[T]rue greatness is when your name is like ampere, watt, and fourier
 -- when it's spelled with a lower case letter."      -- R.W. Hamming

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