通过索引分配列表列表的元素可设置列表的所有首个元素

Question

I have a list of 3 values per month that a initialized like this:

v=[[0.0,0.0,0.0]]*12

In 2 nested loops I want to add a value to an element:

v[month-1][pos] += row[0]

At one point in the execution of the code month-1 = 8 and pos = 0 . row[0] which I get from a query to a database is 1.0 .

Strangely as result I get [[1.0, 0.0, 0.0], [1.0, 0.0, 0.0], [1.0,....
So the first element of every sub-list is set to 1.0 . How can this happen? Even more strange is if I debug the code and manually set the value v[8][0]=9 or so it works as intended.

Answer 1

When you do this v=[[0.0,0.0,0.0]]*12 you are getting a list with 12 references to the same list, to fix that use a comprehension instead:

v = [[0.0,0.0,0.0] for _ in range(12)]

Usually the * used like this should not be wanted to use over mutable elements.