On 11/05/2012 10:07 PM, Chris Angelico wrote:
On Tue, Nov 6, 2012 at 4:51 PM, Andrew Robinson
<andr...@r3dsolutions.com> wrote:
I really don't think doing a shallow copy of lists would break anyone's
program.
Well, it's a change, a semantic change. It's almost certainly going to
break _something_. But for the sake of argument, we can suppose that
the change could be made. Would it be the right thing to do?
Shallow copying by default would result in extremely weird behaviour.
All the same confusion would result, only instead of comparing
[None]*4 with [[None]]*4, there'd be confusion over the difference
between [[None]]*4 and [[[None]]]*4.
I don't think it would help anything, and it'd result in a lot more
work for no benefit.
ChrisA
I don't follow.
a=[ None ]*4 would give a=[ None, None, None, None ] as usual.
All four None's would be the same object, but there are automatically 4
different pointers to it.
Hence,
a[0]=1 would give a=[ 1, None, None, None ] as usual.
a=[ [None] ]*4 would give a=[ [None], [None], [None], [None] ] as usual
BUT:
a[0][0] = 1 would no longer give a=[ [1],[1],[1],[1] ] *Rather* it would
give
a=[ [1].[None].[None],[None] ]
The None objects are all still the same one, BUT the lists themselves
are different.
Again, a=[ ["alpha","beta"] * 4 ] would give:
a=[ ["alpha","beta"], ["alpha","beta"], ["alpha","beta"], ["alpha","beta"] ]
All four strings, "alpha", are the same object -- but there are 5
different lists; The pointers inside the initial list are copied four
times -- not the string objects;
But the *lists* themselves are created new for each replication.
If you nest it another time;
[[[None]]]*4, the same would happen; all lists would be independent --
but the objects which aren't lists would be refrenced-- not copied.
a=[[["alpha","beta"]]]*4 would yield:
a=[[['alpha', 'beta']], [['alpha', 'beta']], [['alpha', 'beta']],
[['alpha', 'beta']]]
and a[0][0]=1 would give [[1],[['alpha', 'beta']], [['alpha', 'beta']],
[['alpha', 'beta']]]]
rather than a=[[1], [1], [1], [1]]
Or at another level down: a[0][0][0]=1 would give: a=[[[1, 'beta']],
[['alpha', 'beta']], [['alpha', 'beta']], [['alpha', 'beta']] ]
rather than a=[[[1, 'beta']], [[1, 'beta']], [[1, 'beta']], [[1, 'beta']]]
The point is, there would be no difference at all noticed in what data
is found where in the array;
the *only* thing that would change is that replacing an item by
assignment would only affect the *location* assigned to -- all other
locations would not be affected.
That really is what people *generally* want.
If the entire list is meant to be read only -- the change would affect
*nothing* at all.
See if you can find *any* python program where people desired the
multiplication to have the die effect that changing an object in one of
the sub lists -- changes all the objects in the other sub lists.
I'm sure you're not going to find it -- and even if you do, it's going
to be 1 program in 1000's.
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