Question

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Python : how to append new elements in a list of list?

I came up with this very strange (for me) behaviour in Python.

I have an empty 2D list (or array if you prefer), and when I add an element to one of it’s columns, all the other columns get the same value added.

Here is the code:

row = [1, 2, 3, 4]
yChannel = 4*[[]]

sectorPlace = 0
for sector in yChannel:
    sector.append(row[sectorPlace])
    sectorPlace += 1

print yChannel

And the output:

[[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]]

Answer 1

The line

yChannel = 4*[[]]

creates a list with four times the same list object. Modifying this single list object will seemingly modify all for sublists, since they are actually all the same list object. You should use

yChannel = [[] for dummy in range(4)]

to create a list of four independent sublists.