understanding list scope

[Alex]

Hi all!

I have a problem understanding the behaviour of this snippet:

data_set = ({"param":"a"},{"param":"b"},{"param":"c"})

for i in range(len(data_set)):
    ds = data_set[:]
    data = ds[i]
    if i == 1: data['param'] = "y"
    if i == 2: data['param'] = "x"

print data_set


This script print out:
({'param': 'a'}, {'param': 'y'}, {'param': 'x'})

Why? I'm coping data_set in ds so why data_set is changed?

Thanks in advance.

Alex