Add nbval

Author: Tania Allard

04_Testing.ipynb

@@ -24,7 +24,7 @@
    "metadata": {},
    "source": [
     "There are various approaches to tests software:\n",
-    "- Assertions\n",
+    "- Assertions: 🦄 == 🦄\n",
     "- Exceptions: within the code serve as ⚠️\n",
     "- Unit tests: investigate the behaviour of units of code (e.g functions)\n",
     "- Regression tests: defends against 🐛\n",
@@ -179,7 +179,75 @@
     "we did *unit testing*!\n",
     "Notice something in the functions we just wrote? \n",
     "- Set-up: `mean = country.get_mean(interim_data)`\n",
-    "- Assertions: `assert mean_price == 20.786`"
+    "- Assertions: `assert mean_price == 20.786`\n",
+    "\n",
+    "Now don't forget to commit your code:\n",
+    "```\n",
+    "$ git add .\n",
+    "$ git commit -m \"Add unit test suite\"\n",
+    "```"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Past as Truth\n",
+    "\n",
+    "Regression tests assume that the past is “correct.” They are great for letting developers know when and how a code base has changed. They are not great for letting anyone know why the change occurred. The change between what a code produces now and what it computed before is called a regression.\n",
+    "\n",
+    "** How many times have you tried to run a script or a notebook you found online just to realize it is broken?**\n",
+    "\n",
+    "Let's do some regression testing on the Jupyter notebook using *nbval*"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# nbval\n",
+    "\n",
+    "We first need to understand how a Jupyter notebook works. \n",
+    "All the data is stored in a .json like format (organised key, data values)... this includes the results, code, and markdown.\n",
+    "\n",
+    "![json](assets/json.jpg)\n",
+    "\n",
+    "Nbval checks the stored values while doing a *mock run* on the notebook and compares the saved version of the notebook vs the results obtained from the mock run \n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Try it on your shell \n",
+    "\n",
+    "```\n",
+    "$ pytest --nbval src/data/00_explore-data.ipynb\n",
+    "```\n",
+    "\n",
+    "What would happen if you were to have a cell like this one?\n",
+    "```python\n",
+    "import time\n",
+    "print('This notebook was last run on: ' + time.strftime('%d/%m/%y') + ' at: ' + time.strftime('%H:%M:%S'))\n",
+    "```"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Provenance\n",
+    "\n",
+    "Image you created a beautiful graph and some results that makes your research Nobel worthy. Of course you ran the workflow multiple times doing minimal changes every single time. But now, 6 months later you need that **one** plot for you Nobel!!\n",
+    "\n",
+    "We can use the package [recipy](https://github.com/recipy/recipy) to log each run of your code to a database, keeping track of the input files, output files and the version of your code, and then let you query this database to find out how you actually did create graph.png"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Make sure everything is commited to git before carrying on.\n"
    ]
   },
   {