Peculiar swap behavior

andrew cooke andrew at acooke.org
Tue Feb 24 00:41:55 CET 2009


andrew cooke wrote:
> Delaney, Timothy (Tim) wrote:
>> Tim Chase wrote:
>>> # swap list contents...not so much...
>>>  >>> m,n = [1,2,3],[4,5,6]
>>>  >>> m[:],n[:] = n,m
>>>  >>> m,n
>>> ([4, 5, 6], [4, 5, 6])
> [...]
>> For these types of things, it's best to expand the code out. The
>> appropriate expansion of:
>>     m,n = [1,2,3],[4,5,6]
>>     m[:],n[:] = n,m
>> is:
>>     m = [1,2,3]
>>     n = [4,5,6]
>>     m[:] = n
>>     n[:] = m
>> [...] OTOH, for:
>>     m,n = [1,2,3],[4,5,6]
>>     m[:],n[:] = n[:],m[:]
>> the expansion is more like:
>>     m = [1,2,3]
>>     n = [4,5,6]
>>     rhs1 = n[:]
>>     rhs2 = m[:]
>>     m[:] = rhs1
>>     n[:] = rhs2
>
> Maybe I'm just being stupid, but you don't seem to have explained
> anything.  Isn't the question: Why is the expansion different for the two
> cases?  Why don't both expand to have the intermediate rhs variables?

To answer my own question - you can rewrite all cases to use rhs1, rhs2.

The point is that when you do that: one case (m, n = n, m) reassigns
variables; one case (m[:], n[:] = n, m) mutates one list to be equal to
the other; one case (m[:], n[:] = n[:], m[:]) avoids seeing the effects of
the mutation because a copy is generated.

Which is what other people said, I guess.

Andrew





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