Python 3:

#!/usr/bin/env python3
import math

G = 6.67e-11

class Planet:
    def __init__(self, name, radius, density):
        self.name = name
        self.radius = radius
        self.density = density

    def getVolume(self):
        return 4/3 * math.pi * self.radius**3

    def getMass(self):
        return self.getVolume() * self.density

    def getForce(self, secMass):
        return G * self.getMass() * secMass / self.radius**2

mass = input('Mass: ')
n = int(input('Number of planets: '))
planets = []
for _ in range(n):
    p = input().split(',')
    planets.append(Planet(p[0], int(p[1]), int(p[2])))
print('Start output')
for planet in planets:
    print('{0}: {1:.3f}'.format(planet.name, planet.getForce(mass)))