A simple Circle module with radius setter.

circle_setter.py

class Circle(object):
    def __init__(self, value):
        self.radius = None
        self.set_radius(value)

    def set_radius(self, value):
        if value < 0:
            raise ValueError('radius must be positive: {0}'.format(value))
        self.radius = value
    
circle = Circle(3)
print circle.radius
circle.set_radius(10)  # ugly usage