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])
>
>
> The first two work as expected but the 3rd seems to leak some
> internal abstraction. It seems to work if I force content-copying:
>
> >>> m[:],n[:] = n[:],m[:]
>
> or even just
>
> >>> m[:],n[:] = n,m[:]
>
> but not
>
> >>> m[:],n[:] = n[:],m
>
> Is this a bug, something Python should smack the programmer for
> trying, or just me pushing the wrong edges? :)
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
After the third line, m == n, giving the behaviour you see. 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
Tim Delaney
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