[Python]alter or assign an object

  To become an effective Python programmer, it is crucial that you learn to draw the distinction between altering an object and assigning to a name, which previously  happened to refer to the object. These two kinds of operations have nothing to do with each other. A statement such as a = [] rebinds name a but performs no alteration at all on the object that was previously bound to name a. Therefore, the issue of references versus copies just doesn't arise in this case: the issue is meaningful only when you alter some object.