3. Errors and Exceptions.ipynb

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   "source": [
    "# Errors and Exceptions"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {
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     "slide_type": "fragment"
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   "source": [
    "### When are errors and exceptions being raised, how to do this yourself, and how to catch them"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   "source": [
    "# Syntax errors"
   ]
  },
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   "execution_count": 42,
   "metadata": {
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   "outputs": [
    {
     "ename": "SyntaxError",
     "evalue": "invalid syntax (<ipython-input-42-65fb81d76d74>, line 1)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;36m  File \u001b[1;32m\"<ipython-input-42-65fb81d76d74>\"\u001b[1;36m, line \u001b[1;32m1\u001b[0m\n\u001b[1;33m    x = 1 +\u001b[0m\n\u001b[1;37m            ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m invalid syntax\n"
     ]
    }
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   "source": [
    "x = 1 + "
   ]
  },
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   "execution_count": 4,
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   "outputs": [
    {
     "ename": "SyntaxError",
     "evalue": "invalid syntax (<ipython-input-4-ef435a983973>, line 1)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;36m  File \u001b[1;32m\"<ipython-input-4-ef435a983973>\"\u001b[1;36m, line \u001b[1;32m1\u001b[0m\n\u001b[1;33m    if 1==1 print('One equals one!')\u001b[0m\n\u001b[1;37m                ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m invalid syntax\n"
     ]
    }
   ],
   "source": [
    "if 1==1 print('One equals one!')"
   ]
  },
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   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
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   },
   "outputs": [
    {
     "ename": "IndentationError",
     "evalue": "expected an indented block (<ipython-input-5-0b2670b59603>, line 2)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31mIndentationError\u001b[0m\u001b[1;31m:\u001b[0m expected an indented block\n"
     ]
    }
   ],
   "source": [
    "if 1==1:\n",
    "print('One equals one!')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   },
   "source": [
    "# Exceptions"
   ]
  },
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   "execution_count": 1,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
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   },
   "outputs": [
    {
     "ename": "ZeroDivisionError",
     "evalue": "division by zero",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[1;31mZeroDivisionError\u001b[0m                         Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-1-05c9758a9c21>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;36m1\u001b[0m\u001b[1;33m/\u001b[0m\u001b[1;36m0\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mZeroDivisionError\u001b[0m: division by zero"
     ]
    }
   ],
   "source": [
    "1/0"
   ]
  },
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   "execution_count": 2,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
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   "outputs": [
    {
     "ename": "IOError",
     "evalue": "[Errno 2] No such file or directory: 'nonexistingfile'",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[1;31mIOError\u001b[0m                                   Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-2-64a2d6c790ac>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0mopen\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'nonexistingfile'\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;34m'r'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mIOError\u001b[0m: [Errno 2] No such file or directory: 'nonexistingfile'"
     ]
    }
   ],
   "source": [
    "open('nonexistingfile', 'r')"
   ]
  },
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   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
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    "slideshow": {
     "slide_type": "subslide"
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   "outputs": [
    {
     "ename": "KeyError",
     "evalue": "3",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[1;31mKeyError\u001b[0m                                  Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-3-a993ef2fb6a5>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m      1\u001b[0m \u001b[0mmydict\u001b[0m \u001b[1;33m=\u001b[0m \u001b[1;33m{\u001b[0m\u001b[1;36m1\u001b[0m\u001b[1;33m:\u001b[0m \u001b[1;34m'one'\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m2\u001b[0m\u001b[1;33m:\u001b[0m \u001b[1;34m'two'\u001b[0m\u001b[1;33m}\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[1;32mdel\u001b[0m \u001b[0mmydict\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;36m3\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mKeyError\u001b[0m: 3"
     ]
    }
   ],
   "source": [
    "mydict = {1: 'one', 2: 'two'}\n",
    "del mydict[3]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   },
   "source": [
    "# Try, except syntax"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "fragment"
    }
   },
   "source": [
    "```\n",
    "try:\n",
    "    <statements>\n",
    "except <name/type of exception>:\n",
    "    <statements>\n",
    "except <another name/type of exception>:\n",
    "    <statements>\n",
    "else:                                     # Optional\n",
    "    <statements>\n",
    "```"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 37,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
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   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "An exception occurred!\n"
     ]
    }
   ],
   "source": [
    "try:\n",
    "    1/0\n",
    "except: # Using except without any specific exception name will catch all possible exceptions:\n",
    "    print('An exception occurred!')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "fragment"
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "No problems encountered for this item, result: 10.0\n",
      "No problems encountered for this item, result: 5.0\n",
      "No problems encountered for this item, result: 3.3333333333333335\n",
      "Error: cannot divide by something else than a number!\n",
      "Error: cannot divide by zero!\n"
     ]
    }
   ],
   "source": [
    "mylist = [1, 2, 3, '4', 0]\n",
    "for number in mylist:\n",
    "    try:\n",
    "        result = 10 / number\n",
    "    except ZeroDivisionError:\n",
    "        print('Error: cannot divide by zero!')\n",
    "    except TypeError:\n",
    "        print('Error: cannot divide by something else than a number!')\n",
    "    else:\n",
    "        print('No problems encountered for this item, result:', result)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "slideshow": {
     "slide_type": "slide"
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   },
   "source": [
    "# Raising Exceptions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 35,
   "metadata": {
    "collapsed": false,
    "slideshow": {
     "slide_type": "subslide"
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   },
   "outputs": [
    {
     "ename": "TypeError",
     "evalue": "The input values x and y should both be even numbers.",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[1;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-35-284ea5990bf6>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m      5\u001b[0m \u001b[1;31m# Check input\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      6\u001b[0m \u001b[1;32mif\u001b[0m \u001b[0mx\u001b[0m \u001b[1;33m%\u001b[0m \u001b[1;36m2\u001b[0m \u001b[1;33m!=\u001b[0m \u001b[1;36m0\u001b[0m \u001b[1;32mor\u001b[0m \u001b[0my\u001b[0m \u001b[1;33m%\u001b[0m \u001b[1;36m2\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[1;32m----> 7\u001b[1;33m     \u001b[1;32mraise\u001b[0m \u001b[0mTypeError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'The input values x and y should both be even numbers.'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m      8\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m+\u001b[0m\u001b[0my\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
      "\u001b[1;31mTypeError\u001b[0m: The input values x and y should both be even numbers."
     ]
    }
   ],
   "source": [
    "# Specify two even numbers\n",
    "x = 0\n",
    "y = 1\n",
    "\n",
    "# Check input\n",
    "if x % 2 != 0 or y % 2 != 0:\n",
    "    raise TypeError('The input values x and y should both be even numbers.')\n",
    "print(x+y)"
   ]
  }
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