gistfile1.py

class Foo(object):
 
    def __init__(self, a_value):
        self._a = None # Create private instance variable: _a
        self.a = a_value # Re-use the setter method (in the form of a property) to do validation and set the actual value

    @property
    def a(self): # used to be called "get_a"
        return self._a

    @a.setter
    def a(self, new_value): # used to be called "set_a"
        if new_value < 0:
            raise ValueError("a must be non-negative")
        else:
            self._a = new_value
 
f = Foo(123)
print(f.a)
f.a = 1
f.a = -1