Simple example of using Tornado to build a "Hello, World" web application in a Jupyter Notebook.

Tornado - Hello, World.ipynb

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    "# Imports\n",
    "\n",
    "To utilize [Tornado], we need to import or load certain packages from the framework.\n",
    "\n",
    "[Tornado]: http://www.tornadoweb.org/en/stable/"
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    "import tornado.ioloop\n",
    "import tornado.web"
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   "source": [
    "# Constants\n",
    "\n",
    "Our web application needs to listen on a **port**, which is a number associated with our program that acts as our \"mailbox\" number.  That is if another machine wishes to communicate with our application, then the other machine will send a message to our machine's **address** and with a **port** number associated with our application.\n",
    "\n",
    "Normally, web servers use **port** `80`, which considered a priviledged **port**.  Since we are running as a regular user (rather than an administrator), we should use a higher-number port such as `8080`, `8888`, or `9999`."
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    "PORT = 9999"
   ]
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    "# Handlers\n",
    "\n",
    "To define what happens when we get a request from the user, we define **handlers** that are associated with different URL **endpoints** or **resource paths**.\n",
    "\n",
    "**Endpoints** are the paths after the **hostname** and **port** in a **URL**.  For instance, if the **URL** is `http://www.nd.edu:80/courses/`. then the **hostname** would be `www.nd.edu`, the **port** would be `80`, and the **endpoint** would be `/courses/`."
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   "source": [
    "class HelloHandler(tornado.web.RequestHandler):\n",
    "    def get(self):\n",
    "        self.write(\"Hello, World\")"
   ]
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    "# Application\n",
    "\n",
    "Once we have our **handlers** defined, we need to associate them with specific URL **endpoints** and then we can **listen** on our configured **port**.\n",
    "\n",
    "Each **handler** is a **sub-class** of the `tornado.web.RequestHandler` and defines a `get` **method** that will be called whenever a **HTTP** **GET** request is made on the web application."
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    "Application = tornado.web.Application([\n",
    "    (r\"/\", HelloHandler),\n",
    "])\n",
    "\n",
    "Application.listen(PORT)"
   ]
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   "source": [
    "# Testing\n",
    "\n",
    "Once you have executed all the code above, you can test your web application by going to http://hostname:port (where **hostname** is the name of your machine and **port** is the configured port).\n",
    "\n",
    "For instance, if you are running the web application on your laptop and the **port** is `9999`, then you can access the application on the same machine via http://localhost:9999.\n",
    "\n",
    "If you need to make modifications and restart the application, then you will need to **interrupt** and **restart** the notebook kernel."
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