7-5 sdut-oop-6 计算各种图形的周长（多态）-- python

from abc import ABCMeta, abstractmethod
import math
from webbrowser import BaseBrowser

class Shape:
    __metaclass__=ABCMeta

    @abstractmethod
    def length(self):
        pass

class Triangle(Shape):
    def __init__(self ,a,b,c):
        if a<=0 or b<=0 or c<=0:
            a=0
            b=0
            c=0
        if a+b<=c or c+b<=a or c+a<=b:
            a=0
            b=0
            c=0
        self.a=a
        self.b=b
        self.c=c
    def length(self):
        return self.a+self.b+self.c

class Rectangle(Shape):
    def __init__(self ,a,b):
        if a<=0 or b<=0:
            a=0
            b=0
        self.a=a
        self.b=b
    def length(self):
        return 2.0*(self.a+self.b)

class Circle(Shape):
    def __init__(self ,r):
        if r<=0:
            r=0
        self.r=r
    def length(self):
        return (self.r)*2.0*3.14

while True:
    try:
        nums=[eval(x) for x in input().split()]
        n=len(nums)
        ans=0.0
        if n==1:
            ans=(Circle(nums[0]).length())
        elif n==2:
           ans=(Rectangle(nums[0],nums[1]).length())
        else: 
            ans=(Triangle(nums[0],nums[1],nums[2]).length())
        print("{:.2f}".format(ans))
    except:
        break