to understand the idea of mutability

python-mutability.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### python variables are memory references"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10919712\n",
      "0xa69f20\n",
      "140043181835248\n",
      "0x7f5e581b43f0\n"
     ]
    }
   ],
   "source": [
    "var = 10\n",
    "print(id(var))\n",
    "print(hex(id(var)))\n",
    "string = 'hello world!'\n",
    "print(id(string))\n",
    "print(hex(id(string)))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### reference counting"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "var = 140616357063792\n",
    "var_common = 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2  <- create an extra reference, affect reference count\n",
      "3365\n"
     ]
    }
   ],
   "source": [
    "from sys import getrefcount\n",
    "print(getrefcount(var), \" <- create an extra reference, affect reference count\") \n",
    "print(getrefcount(var_common))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1  <- does not affect reference count\n"
     ]
    }
   ],
   "source": [
    "from ctypes import c_long\n",
    "print(c_long.from_address(id(var)).value, \" <- does not affect reference count\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### gargabe collection"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 96,
   "metadata": {},
   "outputs": [],
   "source": [
    "import ctypes\n",
    "import gc\n",
    "\n",
    "def in_gc(object_id):\n",
    "    return object_id in [id(obj) for obj in gc.get_objects()]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 97,
   "metadata": {},
   "outputs": [],
   "source": [
    "class O:\n",
    "    def refer(self, other):\n",
    "        self.other = other\n",
    "        \n",
    "    @property\n",
    "    def address(self):\n",
    "        return 'self: {0}, other: {1}'.format(hex(id(self)), hex(id(self.other)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 98,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "self: 0x7f5e580f36d8, other: 0x7f5e580f3908\n",
      "self: 0x7f5e580f3908, other: 0x7f5e580f36d8\n"
     ]
    }
   ],
   "source": [
    "gc.disable()\n",
    "# create circular reference\n",
    "o1 = O()\n",
    "o2 = O()\n",
    "o1.other = o2\n",
    "o2.other = o1\n",
    "print(o1.address)\n",
    "print(o2.address)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 99,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 99,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "o1.other.other == o1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 100,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n"
     ]
    }
   ],
   "source": [
    "o2_object_id = id(o2)\n",
    "# drop o2\n",
    "o2 = None\n",
    "print(c_long.from_address(o2_object_id).value)\n",
    "\n",
    "o1_object_id = id(o1)\n",
    "# drop o1\n",
    "o1 = None\n",
    "print(c_long.from_address(o1_object_id).value)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 101,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "True\n",
      "True\n"
     ]
    }
   ],
   "source": [
    "print(in_gc(o1_object_id))\n",
    "print(in_gc(o2_object_id))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 102,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "False\n",
      "False\n"
     ]
    }
   ],
   "source": [
    "gc.collect()\n",
    "print(in_gc(o1_object_id))\n",
    "print(in_gc(o2_object_id))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### variable re-assignment"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 46,
   "metadata": {},
   "outputs": [],
   "source": [
    "from ctypes import c_long\n",
    "\n",
    "a = 10\n",
    "a_id = id(a)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 47,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0xa69f20\n",
      "165\n"
     ]
    }
   ],
   "source": [
    "print(hex(a_id))\n",
    "print(c_long.from_address(a_id).value)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 48,
   "metadata": {},
   "outputs": [],
   "source": [
    "a = a + 10"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 49,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0xa69f20\n",
      "164\n"
     ]
    }
   ],
   "source": [
    "# old address\n",
    "print(hex(a_id))\n",
    "print(c_long.from_address(a_id).value)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 50,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "0xa6a060\n"
     ]
    }
   ],
   "source": [
    "# new address\n",
    "print(hex(id(a)))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### shared reference"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "a shared reference with b: True\n"
     ]
    }
   ],
   "source": [
    "# immutable\n",
    "a = 10\n",
    "b = 10\n",
    "print('a shared reference with b:',hex(id(a)) == hex(id(b)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "a shared reference with b: False\n"
     ]
    }
   ],
   "source": [
    "b = 20\n",
    "print('a shared reference with b:',hex(id(a)) == hex(id(b)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "l1 shared reference with l2: False\n",
      "l1 shared reference with l3: True\n"
     ]
    }
   ],
   "source": [
    "# mutable\n",
    "l1 = [1, 2, 3]\n",
    "l2 = [1, 2, 3]\n",
    "l3 = l1\n",
    "print('l1 shared reference with l2:', hex(id(l1)) == hex(id(l2)))\n",
    "print('l1 shared reference with l3:', hex(id(l1)) == hex(id(l3)))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "l1 shared reference with l3: True\n"
     ]
    }
   ],
   "source": [
    "l3.append(4)\n",
    "print('l1 shared reference with l3:', hex(id(l1)) == hex(id(l3)))"
   ]
  }
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