On 05/24/2018 11:17 AM, 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.
Thoughts?
I think the same people making that mistake now would still do so
because they didn't know they needed the special operator.
[[] for _ in range(5)] works just as well without adding more syntax.
--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com
Email address domain is currently out of order. See above to fix.
--
https://mail.python.org/mailman/listinfo/python-list