unexpected behaviour of nested lists in python

Question

I have a nested list called basic and I want to change one of its entries. I had assumed the following behaviour:

expected = [ [9],[0] ]
unexpected         


        

Answer 1

In your first example:

basic = [ [0],[0] ]

you have created a list object containing two different list objects. You can see that they are different objects via id() or identity:

assert id(basic[0]) != id(basic[1])

assert basic[0] is not basic[1]

In your second example:

l = [0]
modified = [ l, l ]

you have placed the same list object into another list two times. Both list indicies refer to the same object:

assert id(basic[0]) == id(basic[1])

assert basic[0] is basic[1]

So, yes. This is how variables (and the objects they point to) work in Python.

To get the expected behavior, you need to create separate list objects:

modified = [ l.copy(), l.copy() ]