{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Exceptions\n",
    "- dealing with bugs is normal part of programming\n",
    "- debugging is a very handy programming skill\n",
    "\n",
    "## category of bugs\n",
    "- syntax errors\n",
    "- logical/semantic errros\n",
    "- runtime errors/exceptions"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## exceptions\n",
    "- when runtime error occurs, it creats an exception object\n",
    "- program halts; Python prints out the traceback with error message\n",
    "- https://docs.python.org/3/tutorial/errors.html"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "ename": "ZeroDivisionError",
     "evalue": "division by zero",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mZeroDivisionError\u001b[0m                         Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-1-ef255b193978>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m55\u001b[0m\u001b[0;34m/\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mZeroDivisionError\u001b[0m: division by zero"
     ]
    }
   ],
   "source": [
    "print(55/0)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "ename": "IndexError",
     "evalue": "list index out of range",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mIndexError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-2-d994a3a20fe2>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0malist\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0malist\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mIndexError\u001b[0m: list index out of range"
     ]
    }
   ],
   "source": [
    "alist = []\n",
    "print(alist[0])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "ename": "TypeError",
     "evalue": "'tuple' object does not support item assignment",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-3-8aeda75553d6>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0matup\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0;34m'a'\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'b'\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'c'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0matup\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'A'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
     ]
    }
   ],
   "source": [
    "atup = ('a', 'b', 'c')\n",
    "atup[0] = 'A'"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- each exception has two parts- Name: description"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## catching exceptions\n",
    "- use try and except blocks\n",
    "- try statement has several separate clauses/parts\n",
    "<pre>\n",
    "    try:\n",
    "        # statement(s) thay may potentially throw(s) exception\n",
    "    except ExceptionName1:\n",
    "        # catch specific exception ExceptionName1\n",
    "        # statement(s) to handle the exception\n",
    "    [except ExceptionName2 as err:\n",
    "        # statements\n",
    "    ]\n",
    "    [except:\n",
    "        # catch any error not caught by previous except blocks\n",
    "    ]\n",
    "    [else:\n",
    "        # follows all except clause\n",
    "        # executes if try clause does NOT raise an exception\n",
    "    ]\n",
    "    [finally:\n",
    "        # clean-up actions that must be executed under all circumstances; \n",
    "        # exectued on the way out when try block is left via a break, continue, or return statement\n",
    "    ]\n",
    "</pre>\n",
    "- <em>[ ] optional </em> "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### example 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Enter dividend: 10\n",
      "Enter divisor: 2\n",
      "quotient= 5.0\n",
      "remainder= 0\n",
      "executing finally clause\n"
     ]
    }
   ],
   "source": [
    "try:\n",
    "    x = int(input(\"Enter dividend: \"))\n",
    "    y = int(input(\"Enter divisor: \"))\n",
    "    quotient = x/y\n",
    "    remainder = x%y\n",
    "except ZeroDivisionError as ex:\n",
    "    print('Exception occured:', ex)\n",
    "    print('arguments:', ex.args)\n",
    "except:\n",
    "    print('Some exception occured...')\n",
    "else:\n",
    "    print(\"quotient=\", quotient)\n",
    "    print(\"remainder=\", remainder)\n",
    "finally:\n",
    "    print(\"executing finally clause\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### example 2\n",
    "- input validation"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Please enter a number: f\n",
      "Oops! That was not a valid number. Try again...\n",
      "Please enter a number: dsaf\n",
      "Oops! That was not a valid number. Try again...\n",
      "Please enter a number: adsf\n",
      "Oops! That was not a valid number. Try again...\n",
      "Please enter a number: asdf\n",
      "Oops! That was not a valid number. Try again...\n",
      "Please enter a number: 10\n"
     ]
    }
   ],
   "source": [
    "while True:\n",
    "    try:\n",
    "        x = int(input(\"Please enter a number: \"))\n",
    "        break\n",
    "    except ValueError:\n",
    "        print(\"Oops! That was not a valid number. Try again...\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## raising exceptions\n",
    " - raise statement allows programer to throw their own exceptions"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### example 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "ename": "NameError",
     "evalue": "MyException",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mNameError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-8-290333e3086c>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0;32mraise\u001b[0m \u001b[0mNameError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"MyException\"\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mNameError\u001b[0m: MyException"
     ]
    }
   ],
   "source": [
    "raise NameError(\"MyException\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "An exception flew by...\n"
     ]
    },
    {
     "ename": "NameError",
     "evalue": "My Exception",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mNameError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-9-9b6ca7775e88>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      1\u001b[0m \u001b[0;32mtry\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m     \u001b[0;32mraise\u001b[0m \u001b[0mNameError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'My Exception'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      3\u001b[0m \u001b[0;32mexcept\u001b[0m \u001b[0mNameError\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      4\u001b[0m     \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'An exception flew by...'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      5\u001b[0m     \u001b[0;32mraise\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;31mNameError\u001b[0m: My Exception"
     ]
    }
   ],
   "source": [
    "try:\n",
    "    raise NameError('My Exception')\n",
    "except NameError:\n",
    "    print('An exception flew by...')\n",
    "    raise"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## user-defined exceptions\n",
    "- one can define their own exceptions and raise them as needed\n",
    "- should typically derive from the Exception class, either directly or indirectly\n",
    "\n",
    "### example 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "class InputError(Exception):\n",
    "    \"\"\"\n",
    "    Exception raised for errors in the input.\n",
    "    \n",
    "    Attributes:\n",
    "    expression -- input expression in which the error occured\n",
    "    message -- explaination of the error\n",
    "    \"\"\"\n",
    "    def __init__(self, expression, message):\n",
    "        self.expression = expression\n",
    "        self.message = message\n",
    "    "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Help on class InputError in module __main__:\n",
      "\n",
      "class InputError(builtins.Exception)\n",
      " |  Exception raised for errors in the input.\n",
      " |  \n",
      " |  Attributes:\n",
      " |  expression -- input expression in which the error occured\n",
      " |  message -- explaination of the error\n",
      " |  \n",
      " |  Method resolution order:\n",
      " |      InputError\n",
      " |      builtins.Exception\n",
      " |      builtins.BaseException\n",
      " |      builtins.object\n",
      " |  \n",
      " |  Methods defined here:\n",
      " |  \n",
      " |  __init__(self, expression, message)\n",
      " |      Initialize self.  See help(type(self)) for accurate signature.\n",
      " |  \n",
      " |  ----------------------------------------------------------------------\n",
      " |  Data descriptors defined here:\n",
      " |  \n",
      " |  __weakref__\n",
      " |      list of weak references to the object (if defined)\n",
      " |  \n",
      " |  ----------------------------------------------------------------------\n",
      " |  Methods inherited from builtins.Exception:\n",
      " |  \n",
      " |  __new__(*args, **kwargs) from builtins.type\n",
      " |      Create and return a new object.  See help(type) for accurate signature.\n",
      " |  \n",
      " |  ----------------------------------------------------------------------\n",
      " |  Methods inherited from builtins.BaseException:\n",
      " |  \n",
      " |  __delattr__(self, name, /)\n",
      " |      Implement delattr(self, name).\n",
      " |  \n",
      " |  __getattribute__(self, name, /)\n",
      " |      Return getattr(self, name).\n",
      " |  \n",
      " |  __reduce__(...)\n",
      " |      helper for pickle\n",
      " |  \n",
      " |  __repr__(self, /)\n",
      " |      Return repr(self).\n",
      " |  \n",
      " |  __setattr__(self, name, value, /)\n",
      " |      Implement setattr(self, name, value).\n",
      " |  \n",
      " |  __setstate__(...)\n",
      " |  \n",
      " |  __str__(self, /)\n",
      " |      Return str(self).\n",
      " |  \n",
      " |  with_traceback(...)\n",
      " |      Exception.with_traceback(tb) --\n",
      " |      set self.__traceback__ to tb and return self.\n",
      " |  \n",
      " |  ----------------------------------------------------------------------\n",
      " |  Data descriptors inherited from builtins.BaseException:\n",
      " |  \n",
      " |  __cause__\n",
      " |      exception cause\n",
      " |  \n",
      " |  __context__\n",
      " |      exception context\n",
      " |  \n",
      " |  __dict__\n",
      " |  \n",
      " |  __suppress_context__\n",
      " |  \n",
      " |  __traceback__\n",
      " |  \n",
      " |  args\n",
      "\n"
     ]
    }
   ],
   "source": [
    "help(InputError)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def getInteger():\n",
    "    x = input('Enter an integer number: ')\n",
    "    if not x.isdigit():\n",
    "        raise InputError(x, 'That is not an integer!')\n",
    "    return int(x)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Enter an integer number: dsaf\n"
     ]
    },
    {
     "ename": "InputError",
     "evalue": "('dsaf', 'That is not an integer!')",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mInputError\u001b[0m                                Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-15-f90a077ee9cc>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mx\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mgetInteger\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      2\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mx\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;32m<ipython-input-14-6a80b90df6da>\u001b[0m in \u001b[0;36mgetInteger\u001b[0;34m()\u001b[0m\n\u001b[1;32m      2\u001b[0m     \u001b[0mx\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0minput\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'Enter an integer number: '\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      3\u001b[0m     \u001b[0;32mif\u001b[0m \u001b[0;32mnot\u001b[0m \u001b[0mx\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0misdigit\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 4\u001b[0;31m         \u001b[0;32mraise\u001b[0m \u001b[0mInputError\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mx\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'That is not an integer!'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      5\u001b[0m     \u001b[0;32mreturn\u001b[0m \u001b[0mint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mx\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
      "\u001b[0;31mInputError\u001b[0m: ('dsaf', 'That is not an integer!')"
     ]
    }
   ],
   "source": [
    "x = getInteger()\n",
    "print(x)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "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": "3.6.2"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}