Appending items to a list of lists in python

Question

I'm getting mad with list indexes, and can't explain what I'm doing wrong.

I have this piece of code in which I want to create a list of lists, each one containing values of the same circuit parameter (voltage, current etc..) that I'm reading from a csv file that looks like this:

Sample, V1, I1, V2, I2
0, 3, 0.01, 3, 0.02
1, 3, 0.01, 3, 0.03

And so on. What I want is to create a list that for example contains V1 and I1 (but I want to chose interactively) in the form [[V1], [I1]], so:

[[3,3], [0.01, 0.01]]

The code that I'm using is this:

plot_data = [[]]*len(positions)    
for row in reader:
    for place in range(len(positions)):
        value = float(row[positions[place]])
        plot_data[place].append(value)

plot_data is the list that contains all the values, while positions is a list with the indexes of the columns that I want to copy from the .csv file. The problem is that if I try the commands in the shell, seems to work, but if I run the script instead of appending each value to the proper sub-list, it appends all values to all lists, so I obtain 2 (or more) identical lists.

Answer 1

Python lists are mutable objects and here:

plot_data = [[]] * len(positions) 

you are repeating the same list len(positions) times.

>>> plot_data = [[]] * 3
>>> plot_data
[[], [], []]
>>> plot_data[0].append(1)
>>> plot_data
[[1], [1], [1]]
>>> 

Each list in your list is a reference to the same object. You modify one, you see the modification in all of them.

If you want different lists, you can do this way:

plot_data = [[] for _ in positions]

for example:

>>> pd = [[] for _ in range(3)]
>>> pd
[[], [], []]
>>> pd[0].append(1)
>>> pd
[[1], [], []]