attributes.py

class PositiveInt(object):
    __slots__ = 'value'
    
    def __init__(self, value):
        self.__set__(instance=None, value=value)
    
    def __get__(self, instance, owner):
        return self.value

    def __set__(self, instance, value):
        if not value > 0 or not isinstance(value, int):
            raise ValueError("Not a positive integer")
        self.value = value

class Object(object):
    number = PositiveInt(1)

myobject = Object()

myobject.number = 5

try:
    myobject.number = -1
except ValueError:
    pass

class TypeSafeAttribute(object):
    def __init__(self, type, value):
        self.type = type
        self.__set__(instance=None, value=value)

    def __get__(self, instance, owner):
        return self.value

    def __set__(self, instance, value):
        if not isinstance(value, self.type):
            raise ValueError("{value} is not an instance of {type}".format(
                value=value, type=self.type))
        self.value = value

class Object(object):
    attribute = TypeSafeAttribute(int, 0)

myobject = Object()

myobject.attribute = 5

myobject.attribute = "hello"