How to implement descriptors in Python

Question

I was working on my own descriptor based on the Property example in the docs: https://docs.python.org/3/howto/descriptor.html#properties

But for some reason all my properties are the same.

class Property(object):
    "Emulate PyProperty_Type() in Objects/descrobject.c"

    def __init__(self, fget=None, fset=None, fdel=None, doc=None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
        if doc is None and fget is not None:
            doc = fget.__doc__
        self.__doc__ = doc

    def __get__(self, obj, objtype=None):
        if obj is None:
            return self
        if self.fget is None:
            raise AttributeError("unreadable attribute")
        return self.fget(obj)

    def __set__(self, obj, value):
        if self.fset is None:
            raise AttributeError("can't set attribute")
        self.fset(obj, value)

    def __delete__(self, obj):
        if self.fdel is None:
            raise AttributeError("can't delete attribute")
        self.fdel(obj)

    def getter(self, fget):
        return type(self)(fget, self.fset, self.fdel, self.__doc__)

    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.__doc__)

    def deleter(self, fdel):
        return type(self)(self.fget, self.fset, fdel, self.__doc__)

class Demo(object):

    def __init__(self, a=None, b=None):
        self._a = a
        self._b = b

    @Property
    def a(self):
        return self._a

    @a.setter
    def a(self, a):
        self._a = a

    @Property
    def b(self):
        return self._b

    @b.setter
    def a(self, b):
        self._b = b

d = Demo('4')
print(d.a, d.b)  # None None   (Should be: '4' None ?)
d.a = '3'
print(d.a, d.b)  # 3 3   (Should be: '3' None ?)

Firstly, why do the values passed to the constructor not get set/accessed as I intended? Why does changing a seem to change b too?

araraonline · Accepted Answer · 2017-10-31 16:55:22Z

2

There's a typo in the code. Fixing it produces the expected output.

@b.setter
def a(self, b):
    self._b = b

Should be:

@b.setter
def b(self, b):
    self._b = b

answered Oct 31, 2017 at 16:55

araraonline

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