{
 "cells": [
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   "metadata": {},
   "source": [
    "# Effective Python - 59 Specific Ways to Write Better Python. \n",
    "# *Chapter 4 - Metaclasses and Attributes*\n",
    "Book by Brett Slatkin. \n",
    "Summary notes by Tyler Banks."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* Metaclasses let you intercept Python's class statement and provide special behavior each time a class is defined"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Item 29: Use Plain Attributes Instead of Get and Set Methods"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* Conventionally other programming languages use getters and setters"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Before: 50000.0\n",
      "After: 10000.0\n"
     ]
    }
   ],
   "source": [
    "class OldResistor(object):\n",
    "    def __init__(self, ohms):\n",
    "        self._ohms = ohms\n",
    "    def get_ohms(self):\n",
    "        return self._ohms\n",
    "    def set_ohms(self, ohms):\n",
    "        self._ohms = ohms\n",
    "        \n",
    "r0 = OldResistor(50e3)\n",
    "print('Before: %5r' % r0.get_ohms())\n",
    "r0.set_ohms(10e3)\n",
    "print('After: %5r' % r0.get_ohms())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* Python has a more pythonic way of doing this\n",
    "* You'll implement the `@property` decorator and `setter` attribute"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Before:     0 amps\n",
      "After:  0.01 amps\n"
     ]
    }
   ],
   "source": [
    "class Resistor(object):\n",
    "    def __init__(self, ohms):\n",
    "        self.ohms = ohms\n",
    "        self.voltage = 0\n",
    "        self.current = 0\n",
    "\n",
    "class VoltageResistance(Resistor):\n",
    "    def __init__(self, ohms):\n",
    "        super().__init__(ohms)\n",
    "        self._voltage = 0\n",
    "    @property\n",
    "    def voltage(self):\n",
    "        return self._voltage\n",
    "    @voltage.setter\n",
    "    def voltage(self, voltage):\n",
    "        self._voltage = voltage\n",
    "        self.current = self._voltage / self.ohms\n",
    "        \n",
    "r2 = VoltageResistance(1e3)\n",
    "print('Before: %5r amps' % r2.current)\n",
    "r2.voltage = 10\n",
    "print('After: %5r amps' % r2.current)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "ename": "ValueError",
     "evalue": "0.000000 ohms must be > 0",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mValueError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-9-16fe06f3ea7a>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m     13\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     14\u001b[0m \u001b[0mr3\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mBoundedResistance\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m1e3\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 15\u001b[1;33m \u001b[0mr3\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mohms\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;36m0\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-9-16fe06f3ea7a>\u001b[0m in \u001b[0;36mohms\u001b[1;34m(self, ohms)\u001b[0m\n\u001b[0;32m      9\u001b[0m     \u001b[1;32mdef\u001b[0m \u001b[0mohms\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mself\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0mohms\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     10\u001b[0m         \u001b[1;32mif\u001b[0m \u001b[0mohms\u001b[0m \u001b[1;33m<=\u001b[0m \u001b[1;36m0\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m---> 11\u001b[1;33m             \u001b[1;32mraise\u001b[0m \u001b[0mValueError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'%f ohms must be > 0'\u001b[0m \u001b[1;33m%\u001b[0m \u001b[0mohms\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m     12\u001b[0m         \u001b[0mself\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0m_ohms\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mohms\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m     13\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mValueError\u001b[0m: 0.000000 ohms must be > 0"
     ]
    }
   ],
   "source": [
    "# Using setters to check values and throw exceptions\n",
    "class BoundedResistance(Resistor):\n",
    "    def __init__(self, ohms):\n",
    "        super().__init__(ohms)\n",
    "    @property\n",
    "    def ohms(self):\n",
    "        return self._ohms\n",
    "    @ohms.setter\n",
    "    def ohms(self, ohms):\n",
    "        if ohms <= 0:\n",
    "            raise ValueError('%f ohms must be > 0' % ohms)\n",
    "        self._ohms = ohms\n",
    "        \n",
    "r3 = BoundedResistance(1e3)\n",
    "r3.ohms = 0"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* You could also use `@property` to make attributes immutable\n",
    "* Define new class interfaces using simple public attributes instead of setters and getters"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Item 30: Consider `@property` Instead of Refactoring Attributes"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* Use @property to give existing instance attributes new functionallity\n",
    "* Make incremental progress toward better data models by using `@property`\n",
    "* Consider refactoring a class and all call sites when you find yourself using `@property` too heavily"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Item 31: Use Descriptors for Reusable `@property` Methods"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "* Problem with `@property` is that the methods it decorates can't be reused for multiple attributes of the same class. They also can't be reused by unrelated classes\n",
    "* Ex: "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Homework(object):\n",
    "    def __init__(self):\n",
    "        self._grade = 0\n",
    "    @property\n",
    "    def grade(self):\n",
    "        return self._grade\n",
    "    @grade.setter\n",
    "    def grade(self, value):\n",
    "        if not (0 <= value <= 100):\n",
    "            raise ValueError('Grade must be between 0 and 100')\n",
    "        self._grade = value"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [],
   "source": [
    "person = Homework()\n",
    "person.grade = 100"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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