On 25.05.2014 00:14, Robert Kern wrote:
On 2014-05-24 23:05, Luis José Novoa wrote:
Hi All,

Hope you're doing great. One quick question. I am defining an array of
sets using numpy as:


Has nothing to do with numpy, but the problem is exclusively with your innermost expression [set([])]*3.

Now, if I want to add an element to the set in, lets say, a[0], and I
use the .add(4) operation, which results in:

with .add you are modifying the *existing* set.

array([set([4]), set([4]), set([4])], dtype=object)

which I do not want. If I use the union operator

a[0] = a[0] | set([4])

here you are forming a *new* set and put it in a[0] replacing the old set at this position.

then I obtain what I want:

array([set([4]), set([]), set([])], dtype=object)

Can anyone explain whay this happens?

Same reason why you shouldn't make a list of lists like so: [[]]*3


The above link explains the underlying problem.


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