When you assign an object to a variable, Python copies the reference to the object.

In this case a and b refer to the same list.

If you want to copy the list (not just a reference to it), you can use the slice operator:

Making a slice of a creates a new list. in this case the slice contains all of the elements from the original list.

Another way to make a copy is the copy function from the copy module:

But is more idiomatic to use the slice operator.

from Thinking in Python