On 2009-02-23 16:17, 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?
n[:] on the RHS makes a copy of the list.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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