lexical closure，f1的主要目的为f2提供运行环境

In [22]: def f1(x):
   ....:     def f2(y):
   ....:         return x*y
   ....:     return f2
   ....: 
In [23]: f = f1(3)
In [24]: f(4)
Out[24]: 12
In [25]: f(5)
Out[25]: 15
In [26]: f(6)
Out[26]: 18