Copying vs aliasing lists: b = a compared with a[:] or list(a)

In Python, a variable does not contain a list; it holds a reference to a list object stored somewhere in memory. Think of the name as a label tied to the object rather than a box holding it. This small fact explains one of the most tested behaviours on the PCEP (Certified Entry-Level Python Programmer) exam. When you write b = a where a is a list, Python copies nothing. It ties a second label, b, to the very same list object. Now a and b are two names (aliases) for one list, and a change made through either name is visible through the other.