[Edu-sig] A Descriptor (demonstrating the concept)

[Kirby Urner]

"""
Just for Fun

A quick sketch of a Descriptor construct.

A descriptor is a type instance with a
__get __ and __set__ protocol, also __delete__,
and designed to satisfy a common need to manage
setting and getting with mediating methods.

http://docs.python.org/3.4/howto/descriptor.html

(cc) Kirby Urner, MIT License
O'Reilly School of Technology

"""

class Guardian:
    """
    Takes a field name and a function for screening
    out banned values for said field name
    """

    def __init__(self, fn, banned):
        self.field = fn
        self.banned = banned

    def __set__(self, obj, val):
        # print("Setting {} on {}".format(self.field, obj))
        if self.banned(val):
            raise AttributeError("Can't be {}".format(val))
        obj.__dict__[self.field] = val

    def __get__(self, obj, objtype):
        objtype.ringding()  # calls class method
        return obj.__dict__[self.field]

class Guest:

    # data descriptors
    age  = Guardian("age",  banned = lambda a: a in range(1,21)) # 21 or
older
    fave_color = Guardian("fave_color", banned = lambda c: c in ["Orange",
"Black"])

    def __init__(self, n, age=0, fc=None):
        self.name = n
        # data descriptors always override instance dictionaries.
        self.age = age
        self.fave_color = fc

    @classmethod
    def ringding(cls):  # called by __get__ in data descriptors
        print("Ring Ding!")

    def __repr__(self):
        return "Guest of name {}".format(self.name)



t1 = Guest('Helga', 40)
t2 = Guest('Vlad', 30)

print("Age of t2:", t2.age)

try:
    t1.age = 11  # comes up against banned()
except AttributeError:
    print("No can do:  11")

try:
    t1.fave_color = "Orange"  # ditto banned()
except AttributeError:
    print("No can do:  Orange")

t1.age = 22
t1.fave_color = "Green"

# attributes stick to their respective instances if legal
print(t1.age)
print(t1.fave_color)
print(t2.age)
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