model_property.py

# -*- coding: utf-8 -*-
"""
Created on Wed Dec  7, 2016

@author: Kirby Urner

This class comes directly from the Python documentation.

Property emulates, in pure Python, what the __builtin__ 
property type does.  

Note use of Descriptor special names __set__, __get__, plus 
__delete__, which will in turn trigger the methods saved as 
fget, fset, fdel. 

Used with decorator syntax, with helper methods e.g.
setter, is most typical.  See prop_circle.py
"""

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):
        # type(self) and Property are synonymous
        return Property(self.fget, self.fset, fdel, self.__doc__)