1. 导入Fraction
from fractions import Fraction
fractions 位于python/python36/fractions.py下面，学会查看源码，并且通熟一遍。
2. Fractions 定义
class Fraction(numbers.Rational):
"""This class implements rational numbers. 实现有理数
In the two-argument form of the constructor, Fraction(8, 6) will
produce a rational number equivalent to 4/3（自动约分）. Both arguments must
be Rational. The numerator defaults to 0 and the denominator
defaults to 1 so that Fraction(3) == 3 and Fraction() == 0. （分子默认是0，而分母默认是1）
Fractions can also be constructed from:
- numeric strings similar to those accepted by the
float constructor (for example, '-2.3' or '1e10')
- strings of the form '123/456'
- float and Decimal instances
- other Rational instances (including integers)
"""
具体实例运行一下，就可以看出这个函数的功能，
具体使用可以直接查看lib下面的源码，功能太多。
fractions.py里面还有一个gcd函数，这个函数在Python3.5之后就废弃了，官方建议使用math.gcd()
In [55]: gcd(2,1)
C:\Users\79432\AppData\Local\Programs\Python\Python36\Scripts\ipython:1: DeprecationWarning: fractions.gcd() is deprecated. Use math.gcd() instead.
Out[55]: 1