Strings are unmutable, lists are mutable (2)
Lists are mutable, i.e. you can directly change an element:
aList = [0.12, 23.4, 12.5]
# change the first element (0.12) to 2.34
aList[0]=2.34
print aList
prints:
[2.3399999999999999, 23.399999999999999, 12.5]