Traits Early Experiments 2

Traitlets2.ipynb

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   "source": [
    "### A Quick Introduction to Traits\n",
    "\n",
    "Traits aren't very well-documented.\n",
    "Since they are ptentially extremely useful for \"software plumbing\"\n",
    "this short notebook in intended to familiarise you with their behaviour.\n",
    "\n",
    "First let me show you how up-to-date I am:"
   ]
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   "source": [
    "import sys; sys.version"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Since the Jupyter Notebook (and the IPython system generally) use\n",
    "the `traitlets` module, a lightweight implementation, we will too.\n",
    "\n",
    "We being by making the whole module available, then we import a few specific names."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "source": [
    "import traitlets as tts\n",
    "from traitlets import HasTraits, Unicode, Integer, Float, observe, All"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "To declare a class with traits, it must inherit from `traitlets.HasTraits`.\n",
    "Adding a trait to a class is simply a matter of binding a new Trait\n",
    "object to a class variable·\n"
   ]
  },
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   "cell_type": "code",
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   "source": [
    "class MyClass1(HasTraits):\n",
    "    i = Integer(0)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This ensures that _all_ instances of the `MyClass1` class have an `i` attribute, initially 0."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   "outputs": [],
   "source": [
    "myc1_1 = MyClass1()\n",
    "myc1_1.i"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now look what happens if we try to assign it a string value.\n",
    "I'm catching the exception to save you having to go through the traceback."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "try:\n",
    "    myc1_1.i = \"banana\"\n",
    "except tts.TraitError as e:\n",
    "    print(\"Trait error:\", e)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "So traits give us a simple but effective _typing_ capability.\n",
    "Naturally there are mechanisms available for you to declare\n",
    "your own traits to support the types you use in your code.\n",
    "We'll get to that later."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Furthermore, a class can monitor its traits and take action on changes of value.\n",
    "To monitor a trait from within its class, use the `@observe` decorator to\n",
    "decorate a method"
   ]
  },
  {
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   "execution_count": null,
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   "outputs": [],
   "source": [
    "class MyClass2(MyClass1):\n",
    "    f = Float(32.0)\n",
    "    u = Unicode(\"Python!\")\n",
    "    @observe(\"f\")\n",
    "    def _f_changed(self, c):\n",
    "        print(\"f changed:\", c)\n",
    "        print(\"traits now - i:\", self.i, 'f:', self.f, 'u:', self.u)\n",
    "\n",
    "myc2_1 = MyClass2()\n",
    "print(myc2_1.f)\n",
    "myc2_1.f = 44.0\n",
    "print(\"*\", myc2_1._trait_notifiers)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "You can also observe changes to the traits on an instance externally."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def report_change(s):\n",
    "    def r_c(c):\n",
    "        print(s, \"changes:\", c)\n",
    "    return r_c\n",
    "_i_changed = report_change(\"INSTANCE\")\n",
    "print(\"*\", myc2_1._trait_notifiers)\n",
    "myc2_1.observe(_i_changed, \"i\")\n",
    "print(\"*\", myc2_1._trait_notifiers)\n",
    "myc2_1.observe(_i_changed, \"i\")\n",
    "print(\"*\", myc2_1._trait_notifiers)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Note that an observer cannot be attached to the same trait twice."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_1._trait_notifiers"
   ]
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   "outputs": [],
   "source": [
    "myc2_1.i = 342"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "You can also remove observers by calling `unobserve` with the same arguments."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "myc2_1.unobserve(_i_changed, 'i')\n",
    "myc2_1.i = 341"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "myc2_2 = MyClass2(i=100)\n",
    "myc2_2.i = 0"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "myc2_2.f = 1.1412\n",
    "myc2_2.u = \"Perl\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
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   },
   "outputs": [],
   "source": [
    "myc2_3 = MyClass2(i=99)\n",
    "myc2_3.i = 21"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "When you instantiate a class that has traits, you can set the\n",
    "trait values as keyword arguments to the instantiation call.\n",
    "\n",
    "The assignment of these values will be recorded as a change from the default value if the trait is being observed.\n",
    "Multiple observations will be correctly handled."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_4 = MyClass2(i=100, u=\"Steve\")\n",
    "def changes(c):\n",
    "    print(\"Changes:\", c)\n",
    "\n",
    "myc2_4.observe(changes, All)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_4.u = \"Some value\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_4.f = 3.14159"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_4.i = 4"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Classes (and, for that matter, instances) can report on the traits they have.\n",
    "These methods return a dict in which trait names are the keys, and the traits are the values."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "MyClass2.class_traits()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_1.traits()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Note that the instance traits are the same as the class traits."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_1.f = 3.33333"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This ability to implement the _observer_ pattern is valuable to provide\n",
    "all kinds of standard plumbing in software systems."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "myc2_4.trait_metadata(\"i\", 1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "class MyCounter(HasTraits):\n",
    "    i = Integer()\n",
    "\n",
    "from threading import Thread\n",
    "from time import sleep\n",
    "\n",
    "class MyThread(Thread):\n",
    "    def __init__(self, counter):\n",
    "        Thread.__init__(self)\n",
    "        self.counter = MyCounter()\n",
    "    def run(self):\n",
    "        while True:\n",
    "            sleep(1)\n",
    "            self.counter.i += 1\n",
    "\n",
    "thread = MyThread(MyCounter())\n",
    "thread.start()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "thread.counter.i"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def report_count(c):\n",
    "    print(\"Counter now\", c['new'])\n",
    "thread.counter.observe(report_count, \"i\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "thread.counter.unobserve(report_count, \"i\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "thread.counter.i = 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "help(observe)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "help(thread.counter.observe)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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