02-mutable-caveats.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Caveats with mutable attributes and arguments"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## HauntedBus\n",
    "\n",
    "A simple class to illustrate the danger of a mutable class attribute used as a default value for an instance attribute. Based on _Example 8-12_ of [Fluent Python, 1e](https://www.amazon.com/Fluent-Python-Concise-Effective-Programming/dp/1491946008)."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class HauntedBus:\n",
    "    \"\"\"A bus haunted by ghost passengers\"\"\"\n",
    "    \n",
    "    passengers = []  # 🐛\n",
    "    \n",
    "    def pick(self, name):\n",
    "        self.passengers.append(name)\n",
    "        \n",
    "    def drop(self, name):\n",
    "        self.passengers.remove(name)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[]"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus1 = HauntedBus()\n",
    "bus1.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Ann', 'Bob']"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus1.pick('Ann')\n",
    "bus1.pick('Bob')\n",
    "bus1.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Ann', 'Bob']"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus2 = HauntedBus()\n",
    "bus2.passengers"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Ghost passengers!\n",
    "\n",
    "The `.pick` and `.drop` methods were changing the `HauntedBus.passengers` class attribute."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## HauntedBus_v2"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [],
   "source": [
    "class HauntedBus_v2:\n",
    "    \"\"\"Another bus haunted by ghost passengers\"\"\"\n",
    "    \n",
    "    def __init__(self, passengers=[]):  # 🐛\n",
    "        self.passengers = passengers\n",
    "    \n",
    "    def pick(self, name):\n",
    "        self.passengers.append(name)\n",
    "        \n",
    "    def drop(self, name):\n",
    "        self.passengers.remove(name)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[]"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus3 = HauntedBus_v2()\n",
    "bus3.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Charlie', 'Debbie']"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus3.pick('Charlie')\n",
    "bus3.pick('Debbie')\n",
    "bus3.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Charlie', 'Debbie']"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus4 = HauntedBus_v2()\n",
    "bus4.passengers"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Ghost passengers!!\n",
    "\n",
    "The `.pick` and `.drop` methods were changing the default value for the passengers argument in the `__init__` method.\n",
    "\n",
    "The argument defaults are also class attributes (indirectly, because `__init__` is a class attribute).\n",
    "\n",
    "Check it out:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "(['Charlie', 'Debbie'],)"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "HauntedBus_v2.__init__.__defaults__"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## TwilightBus"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [],
   "source": [
    "class TwilightBus:\n",
    "    \"\"\"A bus model that makes passengers vanish\"\"\"\n",
    "\n",
    "    def __init__(self, passengers=None):\n",
    "        if passengers is None:\n",
    "            self.passengers = []\n",
    "        else:\n",
    "            self.passengers = list(passengers)\n",
    "\n",
    "    def pick(self, name):\n",
    "        self.passengers.append(name)\n",
    "        \n",
    "    def drop(self, name):\n",
    "        self.passengers.remove(name)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "hockey_team = ['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']\n",
    "bus5 = TwilightBus(hockey_team)\n",
    "bus5.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Tina', 'Maya', 'Diana', 'Alice']"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus5.drop('Sue')\n",
    "bus5.drop('Pat')\n",
    "bus5.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "hockey_team"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The assignment on line 8, `self.passengers = passengers`, creates an _alias_ to the `hockey_team` list.\n",
    "\n",
    "Therefore, the `.drop` method removes names from the `hockey_team` list."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Bus"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Bus:\n",
    "    \"\"\"The bus we wanted all along\"\"\"\n",
    "\n",
    "    def __init__(self, passengers=None):\n",
    "        self.passengers = list(passengers) if passengers else []\n",
    "\n",
    "    def pick(self, name):\n",
    "        self.passengers.append(name)\n",
    "        \n",
    "    def drop(self, name):\n",
    "        self.passengers.remove(name)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']"
      ]
     },
     "execution_count": 15,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "hockey_team = ['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']\n",
    "bus6 = Bus(hockey_team)\n",
    "bus6.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Tina', 'Maya', 'Diana', 'Alice']"
      ]
     },
     "execution_count": 16,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "bus6.drop('Sue')\n",
    "bus6.drop('Pat')\n",
    "bus6.passengers"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['Sue', 'Tina', 'Maya', 'Diana', 'Pat', 'Alice']"
      ]
     },
     "execution_count": 17,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "hockey_team"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "On line 5, the expression `list(passengers)` builds a new list from the `passengers` argument.\n",
    "\n",
    "If `passengers` is a list, `list(passengers)` makes a shallow copy of it.\n",
    "\n",
    "If `passengers` is an other iterable object (`tuple`, `set`, generator, etc...), then `list(passengers)` builds a new list from it.\n",
    "\n",
    "> Be conservative in what you send, be liberal in what you accept — _Postel's Law_"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<img src=\"img/thoughtworks.png\" width=\"300\" title=\"ThoughtWorks, Inc. logo\">"
   ]
  }
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