Mutable vs. Immutable.ipynb

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   "source": "# Mutable vs. Immutable Objects \n<br />\n### DesertPy Lightning Talk, June 2015\n<br />\n### Ashley Anderson\n[@aganders3](twitter.com/aganders3) on Twitter or [email protected]"
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   "metadata": {
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   "cell_type": "markdown",
   "source": "## Everything is an object"
  },
  {
   "metadata": {
    "slideshow": {
     "slide_type": "-"
    }
   },
   "cell_type": "markdown",
   "source": "Every Python object has a type, identity, and *value*.\n\nThe builtin function `type()` returns a `type` object.  \nThe builtin function `id()` returns the object's identity (an integer).  \nValue is kind of vague, though."
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   "cell_type": "code",
   "source": "print 1, type(1), id(1)\n\nfoo = range(10)\nprint foo, type(foo), id(foo)\n\nimport sys   \nprint sys, type(sys), id(sys)\n\ndef bar():\n    pass\n\nprint bar, type(bar), id(bar)",
   "execution_count": 169,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "1 <type 'int'> 4299197592\n[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] <type 'list'> 4415085312\n<module 'sys' (built-in)> <type 'module'> 4297509808\n<function bar at 0x10728b320> <type 'function'> 4415075104\n"
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   "cell_type": "markdown",
   "source": "## Mutability is about *values*\n\nThink of an object's value as any and all of its  attributes. For example, anything you'd type `self.` to get at from inside if you wrote the class yourself.\n\nMost custom classes are mutable. To create a simple immutable class, try using a [`namedtuple`](https://docs.python.org/2/library/collections.html#collections.namedtuple). Otherwise you can raise a `TypeError` from `__setattr__` to make your own immutable type."
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   "cell_type": "markdown",
   "source": "### Immutable objects\nYou can't change their values.  \nThe value of `x` is the same before and after `foo` is called.\n\n    x = some_immutable\n    foo(x)\n    print x\n\nThis is *kind of* like pass-by-value.\n\nImmutable objects include **numbers**, **strings**, **tuples**."
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   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
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    "trusted": true
   },
   "cell_type": "code",
   "source": "def add_one(bar):\n    bar += 1\n\nb = 5\nprint b, id(b)\nadd_one(b)\nprint b, id(b)",
   "execution_count": 120,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "5 4299197496\n5 4299197496\n"
    }
   ]
  },
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   "source": "### Mutable objects\nYou can change their values.  \nThe value of `y` *may be different* after `foo` is called.\n\n    y = some_mutable\n    foo(y)\n    print y\n    \nThis is *kind of* like pass-by-reference.\n\nMutable objects include **lists**, **dictionaries**, etc."
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    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "def reverse_list(foo):\n    foo.reverse()\n\na = range(10)\nprint a, id(a)\nreverse_list(a)\nprint a, id(a)",
   "execution_count": 121,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 4415096664\n[9, 8, 7, 6, 5, 4, 3, 2, 1, 0] 4415096664\n"
    }
   ]
  },
  {
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    "slideshow": {
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   },
   "cell_type": "markdown",
   "source": "## But, wait...\nIt's pretty easy to change the value of a number variable..."
  },
  {
   "metadata": {
    "collapsed": false,
    "slideshow": {
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    "trusted": true
   },
   "cell_type": "code",
   "source": "foo = 42\nprint foo\n\nfoo += 1\nprint foo\n\nfoo = 42\nprint foo",
   "execution_count": 151,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "42\n43\n42\n"
    }
   ]
  },
  {
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    "slideshow": {
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   "cell_type": "markdown",
   "source": "## Nah...\nThe assignment operator doesn't change the value of an object, it changes its identity."
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    "trusted": true
   },
   "cell_type": "code",
   "source": "foo = 42\nprint foo, id(foo)\nfoo += 1\nprint foo, id(foo)\nfoo = \"bar\"\nprint foo, id(foo)",
   "execution_count": 148,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "42 4299198584\n43 4299198560\nbar 4381029040\n"
    }
   ]
  },
  {
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    "slideshow": {
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   },
   "cell_type": "markdown",
   "source": "## Values get confusing when we have containers\n\nIf you have an immutable container (e.g. a tuple) that holds mutable objects (e.g. lists), you can still mutate the contained objects without mutating the container."
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     "slide_type": "-"
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    "trusted": true
   },
   "cell_type": "code",
   "source": "A = ([],[],[])\nprint A, id(A[0]), id(A)\nA[0].append(5)\nprint A, id(A[0]), id(A)\nA[1] = [5]",
   "execution_count": 171,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "([], [], []) 4414903504 4414893984\n([5], [], []) 4414903504 4414893984\n"
    },
    {
     "ename": "TypeError",
     "evalue": "'tuple' object does not support item assignment",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-171-868c79a8f9fc>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      3\u001b[0m \u001b[0mA\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mappend\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m5\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      4\u001b[0m \u001b[0;32mprint\u001b[0m \u001b[0mA\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mid\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mA\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mid\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mA\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 5\u001b[0;31m \u001b[0mA\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
     ],
     "output_type": "error"
    }
   ]
  },
  {
   "metadata": {
    "slideshow": {
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   },
   "cell_type": "markdown",
   "source": "## Gotcha!\nMostly you don't have to think about this stuff, but sometimes you do."
  },
  {
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "def my_appending_function(element, initial_list=[]):\n    # do some stuff\n    initial_list.append(element)\n    return initial_list",
   "execution_count": 1,
   "outputs": []
  },
  {
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "print my_appending_function((1,2,3,5), range(2))\nprint my_appending_function(10)\nprint my_appending_function(5)",
   "execution_count": 2,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "[0, 1, (1, 2, 3, 5)]\n[10]\n[10, 5]\n"
    }
   ]
  },
  {
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "cell_type": "markdown",
   "source": "## Mutable objects as default arguments\nThese are defined *once* when the function is defined. This is a result of [`def` being an *executable statement* that binds the name of the function to the function object](http://effbot.org/zone/default-values.htm). All this has something to do with Python functions being \"first class\" objects."
  },
  {
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "cell_type": "markdown",
   "source": "## Mutable objects as deafault arguments\nSo...instead of this:"
  },
  {
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "-"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "def my_appending_function(element, initial_list=[]):\n    # do some stuff\n    initial_list.append(element)\n    return initial_list",
   "execution_count": 165,
   "outputs": []
  },
  {
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "cell_type": "markdown",
   "source": "Do this:"
  },
  {
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "-"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "def my_appending_function(element, initial_list=None):\n    if initial_list is None:\n        initial_list = []\n    # do some stuff\n    initial_list.append(element)\n    return initial_list",
   "execution_count": 166,
   "outputs": []
  },
  {
   "metadata": {
    "slideshow": {
     "slide_type": "subslide"
    }
   },
   "cell_type": "markdown",
   "source": "## Mutable objects as default arguments\n\nNow it does what you (may have) expected:"
  },
  {
   "metadata": {
    "collapsed": true,
    "slideshow": {
     "slide_type": "fragment"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "def my_appending_function(number, initial_list=None):\n    if initial_list is None:\n        initial_list = []\n    # do some stuff\n    initial_list.append(number)\n    return initial_list",
   "execution_count": 77,
   "outputs": []
  },
  {
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
    },
    "trusted": true
   },
   "cell_type": "code",
   "source": "print my_appending_function(10)\nprint my_appending_function(5)\nprint my_appending_function(10)\nprint my_appending_function(5)",
   "execution_count": 167,
   "outputs": [
    {
     "output_type": "stream",
     "name": "stdout",
     "text": "[10]\n[5]\n[10]\n[5]\n"
    }
   ]
  }
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