import math


class Circle:
    def __init__(self, radius):
        if radius < 0:
            raise ValueError("positive radius expected")
        self.radius = radius

    def area(self):
        assert self.radius >= 0, "positive radius expected"
        return math.pi * self.radius ** 2


c = Circle(3)
print(c.area())


c = Circle(-3)
print(c.area())