英文文档：
id(object)
return the “identity” of an object. this is an integer which is guaranteed to be unique and constant for this object during its lifetime. two objects with non-overlapping lifetimes may have the sameid() value.
cpython implementation detail: this is the address of the object in memory.
说明：
1. 返回对象的唯一标识符，用整数表示。在程序生命周期内，这个标识符常量是唯一的。
>>> a = 1
>>> id(a)
>>> id(1.0)
>>> a = ‘some text’
>>> id(a)