前拷贝 与 赋值
>>> a = {1:‘one‘,2:‘two‘,3:‘three‘}
>>> b = a.copy()
>>> c = a
>>> id(a)
2217313747232
>>> id(b)
2217313747304
>>> id(c)
2217313747232
赋值贴一个新标签在相同数据上
>>> c[4] = ‘four‘
>>> c
{1: ‘one‘, 2: ‘two‘, 3: ‘three‘, 4: ‘four‘}
>>> a
{1: ‘one‘, 2: ‘two‘, 3: ‘three‘, 4: ‘four‘}
>>> b
{1: ‘one‘, 2: ‘two‘, 3: ‘three‘}
