{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "(assert)=\n",
    "# Assert \n",
    "\n",
    "Assert statement allows us to do sanity checks in our code. It is also a very handy tool in preventing bugs that do not throw an `Error`. Consider the following factorial function:\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def fact(n):\n",
    "    result = 1\n",
    "    for i in range(n):\n",
    "        result*=i\n",
    "    return result\n",
    "fact(-1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "What if \\\\(n = 1.5\\\\), \\\\(n = -1\\\\) or \\\\(n \\\\) is not a numeric type? Our function is defined for non-negative integers only. However, for some of those invalid inputs, the function can actually return something! `assert` is a great feature to add here:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "ename": "AssertionError",
     "evalue": "",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mAssertionError\u001b[0m                            Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-9-0ca68cd43ada>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m\u001b[0m\n\u001b[0;32m      5\u001b[0m         \u001b[0mresult\u001b[0m\u001b[1;33m*=\u001b[0m\u001b[0mi\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      6\u001b[0m     \u001b[1;32mreturn\u001b[0m \u001b[0mresult\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 7\u001b[1;33m \u001b[0mfact\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m-\u001b[0m\u001b[1;36m1\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;32m<ipython-input-9-0ca68cd43ada>\u001b[0m in \u001b[0;36mfact\u001b[1;34m(n)\u001b[0m\n\u001b[0;32m      1\u001b[0m \u001b[1;32mdef\u001b[0m \u001b[0mfact\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mn\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m     \u001b[1;32massert\u001b[0m \u001b[0mn\u001b[0m \u001b[1;32mis\u001b[0m \u001b[0mint\u001b[0m \u001b[1;32mand\u001b[0m \u001b[0mn\u001b[0m \u001b[1;33m>=\u001b[0m\u001b[1;36m0\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      3\u001b[0m     \u001b[0mresult\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;36m1\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      4\u001b[0m     \u001b[1;32mfor\u001b[0m \u001b[0mi\u001b[0m \u001b[1;32min\u001b[0m \u001b[0mrange\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mn\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      5\u001b[0m         \u001b[0mresult\u001b[0m\u001b[1;33m*=\u001b[0m\u001b[0mi\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mAssertionError\u001b[0m: "
     ]
    }
   ],
   "source": [
    "def fact(n):\n",
    "    assert n is int and n >=0\n",
    "    result = 1\n",
    "    for i in range(n):\n",
    "        result*=i\n",
    "    return result\n",
    "fact(-1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now our program throws an `AssertionError` and we immediately know that the input is invalid. While in this simple example such precautions might seem to be an overkill, `assert` scales very well. When multiple functions exchange data, this feature might shorten the debugging time **a lot**.\n",
    "If you do not want your program to crash because of the `AssertionError` surround your code with the `try..except` block:   "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "This is an invalid input\n"
     ]
    }
   ],
   "source": [
    "def fact(n):\n",
    "    try:\n",
    "        assert n is int and n >=0\n",
    "        result = 1\n",
    "        for i in range(n):\n",
    "            result*=i\n",
    "        return result\n",
    "    except AssertionError:\n",
    "        print(\"This is an invalid input\")\n",
    "\n",
    "fact(-1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "In some coding regimes, `assert` is considered acceptable only in the development process. It is often `deactivated` in the final version of the software."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "(assert_exercises)=\n",
    "## Exercises\n",
    "\n",
    "* **Perimeter of a circle** Define a function which calculates the perimeter of a circle given its radius.\n",
    "\n",
    "**HINT**: `import math` for the value of \\\\(\\pi\\\\). Do not assume that `r` is of any particular type."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "````{admonition} Answer\n",
    ":class: dropdown\n",
    "\n",
    "```python\n",
    "from math import pi\n",
    "\n",
    "def perimeter(r):\n",
    "    assert r >=0\n",
    "    return 2* math.pi*r\n",
    "    \n",
    "```\n",
    "\n",
    "````"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.7.4"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}