mutable_cached_property.ipynb

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "from django.utils.functional import cached_property"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class Foo(object):\n",
    "    @cached_property\n",
    "    def mutable(self):\n",
    "        return [1,2,3]\n",
    "    \n",
    "    @property\n",
    "    def immutable(self):\n",
    "        return [1,2,3]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 2, 3]"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "instance = Foo()\n",
    "instance.mutable"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 2, 3]"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "instance.immutable"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 2, 3]\n",
      "([1, 2, 3, 4], [1, 2, 3, 4])\n"
     ]
    }
   ],
   "source": [
    "i_can_change_this = instance.mutable\n",
    "print(i_can_change_this)\n",
    "i_can_change_this.append(4)\n",
    "print(i_can_change_this, instance.mutable)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "What's happening above is that it's returning the value in the instance's `__dict__` and this can be changed because it's referenced. This is not what I want to happen."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 2, 3]\n",
      "([1, 2, 3, 4], [1, 2, 3])\n"
     ]
    }
   ],
   "source": [
    "never_changes = instance.immutable\n",
    "print(never_changes)\n",
    "never_changes.append(4)\n",
    "print(never_changes, instance.immutable)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Above with the builtin property is exactly what I want to happen, but with a cached value."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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