On 2018-05-24 20:12, Ned Batchelder wrote:
On 5/24/18 2:17 PM, Steven D'Aprano wrote:
Python has a sequence replication operator:
py> [1, 2]*3
[1, 2, 1, 2, 1, 2]
Unfortunately, it is prone to a common "gotcha":
py> x = [[]]*5 # make a multi-dimensional list
py> x
[[], [], [], [], []]
py> x[0].append(1)
py> x
[[1], [1], [1], [1], [1]]
The reason for this behaviour is that * does not copy the original list's
items, it simply replicates the references to the items. So we end up
with a new list containing five references to the same inner list.
This is not a bug and changing the behaviour is not an option.
But what do people think about proposing a new list replication with copy
operator?
[[]]**5
would return a new list consisting of five shallow copies of the inner
list.
Why "**"? Why not "@"?
[[]] @ 5
"shallow" will be the next problem. Do we also need this?:
[[[]]]***5 # j/k
I suppose the choice should be limited to 2 options: shallow copy and
deep copy.
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