Multiply operator applied to list(data structure)

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EVERYTHING in python are objects, and python never makes copies unless explicity asked to do so.

When you do 

innerList = [0] * 10

you create a list with 10 elements, all of them refering to the same int object 0.

Since integer objects are immutable, when you do

innerList[1] = 15

You are changing the second element of the list so that it refers to another integer 15. That always works because of int objects immutability.

That’s why

outerList = innerList * 5

Will create a list object with 5 elements, each one is a reference to the same innerList just as above. But since list objects are mutable:

outerList[2].append('something')

Is the same as:

innerList.append('something')

Because they are two references to the same list object. So the element ends up in that single list. It appears to be duplicated, but the fact is that there is only one list object, and many references to it.

By contrast if you do

outerList[1] = outerList[1] + ['something']

Here you are creating another list object (using + with lists is an explicit copy), and assigning a reference to it into the second position of outerList. If you “append” the element this way (not really appending, but creating another list), innerList will be unaffected.

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