gistfile1.py

class Foo(object):

    def __init__(self, a_value):
        self._a = None # Create private instance variable: _a
        self.set_a(a_value) # Re-use the setter method so do validation and set the actual value

    def get_a(self):
        return self._a

    def set_a(self, new_value):
        if new_value < 0:
            raise ValueError("a must be non-negative")
        else:
            self._a = new_value

f = Foo(123)
print(f.get_a())
f.set_a(1)
f.set_a(-1)