Descriptors pyug

gistfile1.txt

{
 "metadata": {
  "celltoolbar": "Slideshow",
  "name": "",
  "signature": "sha256:b392c503122c5ac4b5ed7cb8d3ce0d2ba9fce4f4d8596d0ed399df0e06bc778a"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "Descriptors\n",
      "-----------\n",
      "\n",
      "A pythonic alternative to getters and setters"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "First some background knowledge\n",
      "-------------------------------"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Circle:\n",
      "     PI = 3.14\n",
      "     def __init__(self, radius):\n",
      "         self.radius = radius\n",
      "\n",
      "mycircle = Circle(2)\n",
      "mycircle.radius"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "-"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 1,
       "text": [
        "2"
       ]
      }
     ],
     "prompt_number": 1
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mycircle.PI"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "-"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 2,
       "text": [
        "3.14"
       ]
      }
     ],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "In python, the attribute objects are stored in a special attribute called `__dict__`"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mycircle.__dict__"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 3,
       "text": [
        "{'radius': 2}"
       ]
      }
     ],
     "prompt_number": 3
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "source": [
      "This is equivalent:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mycircle.__dict__['radius'] is mycircle.radius"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 4,
       "text": [
        "True"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "mycircle.__dict__['radius'] = 3\n",
      "mycircle.radius"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 5,
       "text": [
        "3"
       ]
      }
     ],
     "prompt_number": 5
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "If you try to access an attribute object, which doesn't exist, the class level attributes are used as fallback"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Circle:\n",
      "     PI = 3.14\n",
      "     def __init__(self, radius):\n",
      "         self.radius = radius\n",
      "\n",
      "mycircle = Circle(2)\n",
      "mycircle.PI is Circle.__dict__['PI']"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 6,
       "text": [
        "True"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "Circle.__dict__"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 7,
       "text": [
        "mappingproxy({'PI': 3.14, '__dict__': <attribute '__dict__' of 'Circle' objects>, '__init__': <function Circle.__init__ at 0x7f4c201617a0>, '__weakref__': <attribute '__weakref__' of 'Circle' objects>, '__module__': '__main__', '__doc__': None})"
       ]
      }
     ],
     "prompt_number": 7
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "source": [
      "dict_proxy is used by Python where you need a dict but don\u2019t want to allow modifications. "
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "Rules for accessing an attribute on an object like `obj.foo` gets\n",
      "\n",
      "1. Value in `obj.__dict__` if it exists\n",
      "\n",
      "2. Value in `type(obj).__dict__`\n",
      "\n",
      "\n"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "An assignment always creates an entry in `obj.__dict__`"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "fragment"
      }
     },
     "source": [
      "But this rules are incomplete... \n"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "What is a descriptor?\n",
      "---------------------\n",
      "A descriptor is any object that implements at least one of methods named `__get__()`, `__set__()`, and `__delete__()`\n",
      "\n",
      "The signature of `__get__`, `__set__` and `__del__` are fixed:\n",
      "\n",
      "`def __get__(self, obj, type=None) --> value`\n",
      "\n",
      "`def __set__(self, obj, value) --> None`\n",
      "\n",
      "`def __delete__(self, obj) --> None`\n",
      "\n",
      "\n",
      "data descriptor vs non-data descriptor\n",
      "--------------------------------------\n",
      "\n",
      "A **data descriptor** implements both `__get__()` and `__set__()`. \n",
      "Implementing only `__get__()` makes you a **non-data descriptor**.\n",
      "\n",
      "\n",
      "See https://docs.python.org/3.4/howto/descriptor.html#descriptor-protocol\n",
      "\n"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "Rules with descriptors\n",
      "----------------------\n",
      "\n",
      "1. Result of the `__get__` method of the **data descriptor** of the same name attached to the class if it exists\n",
      "\n",
      "2. Value in `obj.__dict__` if it exists\n",
      "\n",
      "3. Result of the `__get__` method of the **non-data descriptor** of the same name on the class\n",
      "\n",
      "3. Value in `type(obj).__dict__`\n",
      "\n"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "An assignment always creates an entry in `obj.__dict__`\n",
      "\n",
      "Unless the **descriptor** has a `__set__` method, in which case you\u2019re calling a function"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Decimal:    \n",
      "    def __get__(self, obj, type=None):\n",
      "        print ('__get__')        \n",
      "\n",
      "    def __set__(self, obj, value):\n",
      "        print ('__set__ value {}'.format(value))\n",
      "        \n",
      "        \n",
      "class Circle:\n",
      "    PI = 3.14\n",
      "    radius = Decimal()\n",
      "        \n",
      "    def __init__(self, radius):\n",
      "        self.radius = radius\n",
      "            \n",
      "\n",
      "mycircle = Circle(2)\n",
      "mycircle.radius\n"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "__set__ value 2\n",
        "__get__\n"
       ]
      }
     ],
     "prompt_number": 8
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Decimal:\n",
      "    def __init__(self, name):\n",
      "        self.name = name\n",
      "    \n",
      "    def __get__(self, obj, type=None):\n",
      "        return obj.__dict__[self.name]\n",
      "\n",
      "    def __set__(self, obj, value):    \n",
      "        obj.__dict__[self.name] = value\n",
      "        \n",
      "        \n",
      "class Circle:\n",
      "     PI = 3.14\n",
      "     radius = Decimal('radius')\n",
      "     def __init__(self, radius):\n",
      "         self.radius = radius\n",
      "            \n",
      "\n",
      "mycircle = Circle(2)\n",
      "mycircle.radius"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 9,
       "text": [
        "2"
       ]
      }
     ],
     "prompt_number": 9
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class Decimal:\n",
      "    def __init__(self, name):\n",
      "        self.name = name\n",
      "    \n",
      "    def __get__(self, obj, type=None):\n",
      "        return round(obj.__dict__[self.name], 2)\n",
      "\n",
      "    def __set__(self, obj, value):    \n",
      "        obj.__dict__[self.name] = float(value)\n",
      "        \n",
      "        \n",
      "class Circle:\n",
      "     PI = 3.14\n",
      "     radius = Decimal('radius')\n",
      "     def __init__(self, radius):\n",
      "         self.radius = radius\n",
      "            \n",
      "\n",
      "mycircle = Circle('2.12133')\n",
      "mycircle.radius"
     ],
     "language": "python",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "outputs": [
      {
       "metadata": {},
       "output_type": "pyout",
       "prompt_number": 10,
       "text": [
        "2.12"
       ]
      }
     ],
     "prompt_number": 10
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "Non-data descriptor example\n",
      "---------------------------"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class cached_property(object):\n",
      "    def __init__(self, func):\n",
      "        self._func = func\n",
      "        self.__name__ = func.__name__\n",
      "\n",
      "    def __get__(self, obj, klass):\n",
      "        print (\"Called the func\")\n",
      "        result = self._func(obj)\n",
      "        obj.__dict__[self.__name__] = result\n",
      "        return result\n",
      "\n",
      "class MyClass(object):\n",
      "    @cached_property\n",
      "    def x(self):\n",
      "        return 42\n",
      "    \n",
      "obj = MyClass()\n",
      "\n",
      "print(obj.x)\n",
      "print(obj.x)\n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "Called the func\n",
        "42\n",
        "42\n"
       ]
      }
     ],
     "prompt_number": 11
    },
    {
     "cell_type": "markdown",
     "metadata": {
      "slideshow": {
       "slide_type": "slide"
      }
     },
     "source": [
      "References\n",
      "----------\n",
      "\n",
      "https://docs.python.org/3.4/howto/descriptor.html\n",
      "\n",
      "http://simeonfranklin.com/blog/2013/nov/17/descriptors-talk-sf-python/\n",
      "\n",
      "Luciano Ramalho at Pycon 2013 on descriptors - http://pyvideo.org/video/1760/encapsulation-with-descriptors\n",
      "\n",
      "David Beazley on cool advanced OOP stuff in Python 3 - http://pyvideo.org/video/1716/python-3-metaprogramming\n"
     ]
    }
   ],
   "metadata": {}
  }
 ]
}