Python-IO&EE.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "## Python File I/O\n",
    "https://docs.python.org/3/tutorial/inputoutput.html"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "### Reading and Writing Files"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Help on built-in function open in module io:\n",
      "\n",
      "open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)\n",
      "    Open file and return a stream.  Raise IOError upon failure.\n",
      "    \n",
      "    file is either a text or byte string giving the name (and the path\n",
      "    if the file isn't in the current working directory) of the file to\n",
      "    be opened or an integer file descriptor of the file to be\n",
      "    wrapped. (If a file descriptor is given, it is closed when the\n",
      "    returned I/O object is closed, unless closefd is set to False.)\n",
      "    \n",
      "    mode is an optional string that specifies the mode in which the file\n",
      "    is opened. It defaults to 'r' which means open for reading in text\n",
      "    mode.  Other common values are 'w' for writing (truncating the file if\n",
      "    it already exists), 'x' for creating and writing to a new file, and\n",
      "    'a' for appending (which on some Unix systems, means that all writes\n",
      "    append to the end of the file regardless of the current seek position).\n",
      "    In text mode, if encoding is not specified the encoding used is platform\n",
      "    dependent: locale.getpreferredencoding(False) is called to get the\n",
      "    current locale encoding. (For reading and writing raw bytes use binary\n",
      "    mode and leave encoding unspecified.) The available modes are:\n",
      "    \n",
      "    ========= ===============================================================\n",
      "    Character Meaning\n",
      "    --------- ---------------------------------------------------------------\n",
      "    'r'       open for reading (default)\n",
      "    'w'       open for writing, truncating the file first\n",
      "    'x'       create a new file and open it for writing\n",
      "    'a'       open for writing, appending to the end of the file if it exists\n",
      "    'b'       binary mode\n",
      "    't'       text mode (default)\n",
      "    '+'       open a disk file for updating (reading and writing)\n",
      "    'U'       universal newline mode (deprecated)\n",
      "    ========= ===============================================================\n",
      "    \n",
      "    The default mode is 'rt' (open for reading text). For binary random\n",
      "    access, the mode 'w+b' opens and truncates the file to 0 bytes, while\n",
      "    'r+b' opens the file without truncation. The 'x' mode implies 'w' and\n",
      "    raises an `FileExistsError` if the file already exists.\n",
      "    \n",
      "    Python distinguishes between files opened in binary and text modes,\n",
      "    even when the underlying operating system doesn't. Files opened in\n",
      "    binary mode (appending 'b' to the mode argument) return contents as\n",
      "    bytes objects without any decoding. In text mode (the default, or when\n",
      "    't' is appended to the mode argument), the contents of the file are\n",
      "    returned as strings, the bytes having been first decoded using a\n",
      "    platform-dependent encoding or using the specified encoding if given.\n",
      "    \n",
      "    'U' mode is deprecated and will raise an exception in future versions\n",
      "    of Python.  It has no effect in Python 3.  Use newline to control\n",
      "    universal newlines mode.\n",
      "    \n",
      "    buffering is an optional integer used to set the buffering policy.\n",
      "    Pass 0 to switch buffering off (only allowed in binary mode), 1 to select\n",
      "    line buffering (only usable in text mode), and an integer > 1 to indicate\n",
      "    the size of a fixed-size chunk buffer.  When no buffering argument is\n",
      "    given, the default buffering policy works as follows:\n",
      "    \n",
      "    * Binary files are buffered in fixed-size chunks; the size of the buffer\n",
      "      is chosen using a heuristic trying to determine the underlying device's\n",
      "      \"block size\" and falling back on `io.DEFAULT_BUFFER_SIZE`.\n",
      "      On many systems, the buffer will typically be 4096 or 8192 bytes long.\n",
      "    \n",
      "    * \"Interactive\" text files (files for which isatty() returns True)\n",
      "      use line buffering.  Other text files use the policy described above\n",
      "      for binary files.\n",
      "    \n",
      "    encoding is the name of the encoding used to decode or encode the\n",
      "    file. This should only be used in text mode. The default encoding is\n",
      "    platform dependent, but any encoding supported by Python can be\n",
      "    passed.  See the codecs module for the list of supported encodings.\n",
      "    \n",
      "    errors is an optional string that specifies how encoding errors are to\n",
      "    be handled---this argument should not be used in binary mode. Pass\n",
      "    'strict' to raise a ValueError exception if there is an encoding error\n",
      "    (the default of None has the same effect), or pass 'ignore' to ignore\n",
      "    errors. (Note that ignoring encoding errors can lead to data loss.)\n",
      "    See the documentation for codecs.register or run 'help(codecs.Codec)'\n",
      "    for a list of the permitted encoding error strings.\n",
      "    \n",
      "    newline controls how universal newlines works (it only applies to text\n",
      "    mode). It can be None, '', '\\n', '\\r', and '\\r\\n'.  It works as\n",
      "    follows:\n",
      "    \n",
      "    * On input, if newline is None, universal newlines mode is\n",
      "      enabled. Lines in the input can end in '\\n', '\\r', or '\\r\\n', and\n",
      "      these are translated into '\\n' before being returned to the\n",
      "      caller. If it is '', universal newline mode is enabled, but line\n",
      "      endings are returned to the caller untranslated. If it has any of\n",
      "      the other legal values, input lines are only terminated by the given\n",
      "      string, and the line ending is returned to the caller untranslated.\n",
      "    \n",
      "    * On output, if newline is None, any '\\n' characters written are\n",
      "      translated to the system default line separator, os.linesep. If\n",
      "      newline is '' or '\\n', no translation takes place. If newline is any\n",
      "      of the other legal values, any '\\n' characters written are translated\n",
      "      to the given string.\n",
      "    \n",
      "    If closefd is False, the underlying file descriptor will be kept open\n",
      "    when the file is closed. This does not work when a file name is given\n",
      "    and must be True in that case.\n",
      "    \n",
      "    A custom opener can be used by passing a callable as *opener*. The\n",
      "    underlying file descriptor for the file object is then obtained by\n",
      "    calling *opener* with (*file*, *flags*). *opener* must return an open\n",
      "    file descriptor (passing os.open as *opener* results in functionality\n",
      "    similar to passing None).\n",
      "    \n",
      "    open() returns a file object whose type depends on the mode, and\n",
      "    through which the standard file operations such as reading and writing\n",
      "    are performed. When open() is used to open a file in a text mode ('w',\n",
      "    'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open\n",
      "    a file in a binary mode, the returned class varies: in read binary\n",
      "    mode, it returns a BufferedReader; in write binary and append binary\n",
      "    modes, it returns a BufferedWriter, and in read/write mode, it returns\n",
      "    a BufferedRandom.\n",
      "    \n",
      "    It is also possible to use a string or bytearray as a file for both\n",
      "    reading and writing. For strings StringIO can be used like a file\n",
      "    opened in a text mode, and for bytes a BytesIO can be used like a file\n",
      "    opened in a binary mode.\n",
      "\n"
     ]
    }
   ],
   "source": [
    "help(open)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": true,
    "deletable": true,
    "editable": true
   },
   "outputs": [],
   "source": [
    "f = open('workfile.txt', 'w') # r, w, a; b\n",
    "\"\"\"\"\n",
    "Normally, files are opened in text mode, that means, \n",
    "you read and write strings from and to the file,\n",
    "which are encoded in a specific encoding.\n",
    "'b' appended to the mode opens the file in binary mode,\n",
    "now the data is read and written in the form of bytes objects.\n",
    "\"\"\"\n",
    "f.write('L1 1234567890\\n')\n",
    "f.write('L2 1234567890\\n')\n",
    "f.close()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": true,
    "deletable": true,
    "editable": true
   },
   "outputs": [],
   "source": [
    "# Note that assert is a statement, not a function.\n",
    "# closed is an attribute of f and it is a boolean.\n",
    "\n",
    "assert f.closed"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "L1 1234567890\n",
      "L2 1234567890\n",
      "\n",
      "28\n"
     ]
    }
   ],
   "source": [
    "f = open('workfile.txt', 'r')\n",
    "data = f.read() # read all data in f, may need lots memory\n",
    "f.close()\n",
    "\n",
    "print(data)\n",
    "print(len(data))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "L1 1234567890\n",
      "\n",
      "14\n"
     ]
    }
   ],
   "source": [
    "f = open('workfile.txt', 'r')\n",
    "line = f.readline() # read to \\n or end of file\n",
    "f.close()\n",
    "\n",
    "print(line)\n",
    "print(len(line))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['L1 1234567890\\n', 'L2 1234567890\\n']\n"
     ]
    }
   ],
   "source": [
    "\"\"\"\n",
    "It is good practice to use the with keyword when dealing with file objects.\n",
    "This has the advantage that the file is properly closed after its suite finishes,\n",
    "even if an exception is raised on the way.\n",
    "\"\"\"\n",
    "\n",
    "with open('workfile.txt', 'r') as f:\n",
    "    li = f.readlines()\n",
    "\n",
    "assert f.closed\n",
    "print(li)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "L1 1234567890\n",
      "\n",
      "L2 1234567890\n",
      "\n"
     ]
    }
   ],
   "source": [
    "with open('workfile.txt', 'r') as f:\n",
    "    for line in f:  # memory efficient, fast, and leads to simple code\n",
    "        print(line)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['L1 1234567890\\n', 'L2 1234567890\\n']\n"
     ]
    }
   ],
   "source": [
    "with open('workfile.txt', 'r') as f:\n",
    "    li = list(f)\n",
    "\n",
    "print(li)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['L1 1234567890\\n', 'L2 1234567890\\n']\n"
     ]
    }
   ],
   "source": [
    "with open('workfile.txt', 'r') as f:\n",
    "    li = list(f)\n",
    "\n",
    "print(li)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2\n",
      "5 1\n"
     ]
    }
   ],
   "source": [
    "# seek and tell\n",
    "f = open('workfile.txt', 'r+')\n",
    "f.write('0123456789abcdef')\n",
    "\n",
    "f.seek(5)      # Go to the 6th byte in the file\n",
    "a = f.read(1)\n",
    "\n",
    "f.seek(1)  # Go to the 1st byte\n",
    "b = f.read(1)\n",
    "\n",
    "print(f.tell()) # tell me where I am in this text file\n",
    "\n",
    "f.close()\n",
    "\n",
    "print(a, b)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "### json"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'[1, \"simple\", \"list\"]'"
      ]
     },
     "execution_count": 11,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "import json\n",
    "json.dumps([1, 'simple', 'list'])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'[1, \"simple\", [2, 3, 4], {\"one\": 1, \"two\": 2, \"three\": 3}]'"
      ]
     },
     "execution_count": 12,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x = [1, 'simple', [2, 3, 4], {\"one\":1, \"two\":2, \"three\":3}]\n",
    "json.dumps(x)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, \"simple\", [2, 3, 4], {\"one\": 1, \"two\": 2, \"three\": 3}]\n",
      "<class 'str'>\n"
     ]
    }
   ],
   "source": [
    "# save x in a file by json's format\n",
    "with open(\"workfile.txt\", \"w\") as f:\n",
    "    json.dump(x, f)\n",
    "\n",
    "# read data from json file\n",
    "with open(\"workfile.txt\", \"r\") as f:\n",
    "    data = f.read()\n",
    "\n",
    "print(data)\n",
    "print(type(data)) # but x is a list"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "[1, 'simple', [2, 3, 4], {'one': 1, 'two': 2, 'three': 3}]\n",
      "<class 'list'>\n"
     ]
    }
   ],
   "source": [
    "with open(\"workfile.txt\", \"r\") as f:\n",
    "    data = json.load(f)\n",
    "print(data)\n",
    "print(type(data))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "### csv\n",
    "\n",
    "Note that built-in cvs functions are not as convenient as pandas's cvs functions."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {
    "collapsed": true,
    "deletable": true,
    "editable": true
   },
   "outputs": [],
   "source": [
    "import csv\n",
    "\n",
    "it = [\"123\", \"456\", \"789\"]\n",
    "\n",
    "with open('workfile.csv', 'w', newline='') as f:\n",
    "    writer = csv.writer(f)\n",
    "    writer.writerow(it)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {
    "collapsed": true,
    "deletable": true,
    "editable": true
   },
   "outputs": [],
   "source": [
    "with open('workfile.csv', 'w', newline='') as f:\n",
    "    writer = csv.writer(f)\n",
    "    writer.writerows(it)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": false,
    "deletable": true,
    "editable": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['1', '2', '3']\n",
      "['4', '5', '6']\n",
      "['7', '8', '9']\n"
     ]
    }
   ],
   "source": [
    "with open('workfile.csv', newline='') as f:\n",
    "    reader = csv.reader(f)\n",
    "    for row in reader:\n",
    "        print(row)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "###  pickle\n",
    "The pickle module implements binary protocols for serializing and de-serializing a Python object structure.\n",
    "* JSON is a text serialization format (it outputs unicode text, although most of the time it is then encoded to utf-8), while pickle is a binary serialization format;\n",
    "* JSON is human-readable, while pickle is not;\n",
    "* JSON is interoperable and widely used outside of the Python ecosystem, while pickle is Python-specific;\n",
    "* JSON, by default, can only represent a subset of the Python built-in types, and no custom classes; pickle can represent an extremely large number of Python types (many of them automatically, by clever usage of Python’s introspection facilities; complex cases can be tackled by implementing specific object APIs)."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import pickle\n",
    "\n",
    "# An arbitrary collection of objects supported by pickle.\n",
    "data = {\n",
    "    'a': [1, 2.0, 3, 4+6j],\n",
    "    'b': (\"character string\", b\"byte string\"),\n",
    "    'c': {None, True, False}\n",
    "}\n",
    "\n",
    "with open('test.pickle', 'wb') as f:\n",
    "    pickle.dump(data, f)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{'a': [1, 2.0, 3, (4+6j)], 'b': ('character string', b'byte string'), 'c': {False, True, None}}\n"
     ]
    }
   ],
   "source": [
    "import pickle\n",
    "\n",
    "with open('test.pickle', 'rb') as f:\n",
    "    data = pickle.load(f)\n",
    "    \n",
    "print(data)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 20,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{'a': [1, 2.0, 3, (4+6j)],\n",
      " 'b': ('character string', b'byte string'),\n",
      " 'c': {False, True, None}}\n"
     ]
    }
   ],
   "source": [
    "import pprint\n",
    "pprint.pprint(data)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['aaaaaaaaaa',\n",
      " ('spam',\n",
      "  ('eggs',\n",
      "   ('lumberjack',\n",
      "    ('knights', ('ni', ('dead', ('parrot', ('fresh fruit',)))))))),\n",
      " ['aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'],\n",
      " ['cccccccccccccccccccc', 'dddddddddddddddddddd']]\n"
     ]
    }
   ],
   "source": [
    "import pprint\n",
    "tup = ('spam', ('eggs', ('lumberjack', ('knights', ('ni', ('dead', ('parrot', ('fresh fruit',))))))))\n",
    "stuff = ['a' * 10, tup, ['a' * 30, 'b' * 30], ['c' * 20, 'd' * 20]]\n",
    "pprint.pprint(stuff)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 22,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['aaaaaaaaaa',\n",
      " ('spam', ('eggs', (...))),\n",
      " ['aaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'],\n",
      " ['cccccccccccccccccccc', 'dddddddddddddddddddd']]\n"
     ]
    }
   ],
   "source": [
    "pprint.pprint(stuff, depth=3)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "## Python Error and Exception"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "### Syntax Errors"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 23,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "SyntaxError",
     "evalue": "invalid syntax (<ipython-input-23-614901b0e5ee>, line 1)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;36m  File \u001b[1;32m\"<ipython-input-23-614901b0e5ee>\"\u001b[1;36m, line \u001b[1;32m1\u001b[0m\n\u001b[1;33m    while True print('Hello world')\u001b[0m\n\u001b[1;37m                   ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m invalid syntax\n"
     ]
    }
   ],
   "source": [
    "while True print('Hello world')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Exceptions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "ZeroDivisionError",
     "evalue": "division by zero",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mZeroDivisionError\u001b[0m                         Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-24-9ce172bd90a7>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;36m10\u001b[0m \u001b[1;33m*\u001b[0m \u001b[1;33m(\u001b[0m\u001b[1;36m1\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[1;31mZeroDivisionError\u001b[0m: division by zero"
     ]
    }
   ],
   "source": [
    "10 * (1/0)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "NameError",
     "evalue": "name 'spam' is not defined",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-25-6b1dfe582d2e>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;36m4\u001b[0m \u001b[1;33m+\u001b[0m \u001b[0mspam\u001b[0m\u001b[1;33m*\u001b[0m\u001b[1;36m3\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mNameError\u001b[0m: name 'spam' is not defined"
     ]
    }
   ],
   "source": [
    "4 + spam*3"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 26,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "SyntaxError",
     "evalue": "EOL while scanning string literal (<ipython-input-26-e10b39e5376b>, line 1)",
     "output_type": "error",
     "traceback": [
      "\u001b[1;36m  File \u001b[1;32m\"<ipython-input-26-e10b39e5376b>\"\u001b[1;36m, line \u001b[1;32m1\u001b[0m\n\u001b[1;33m    2' + 2\u001b[0m\n\u001b[1;37m          ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m EOL while scanning string literal\n"
     ]
    }
   ],
   "source": [
    "2' + 2"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Handling Exceptions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 27,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "# https://docs.python.org/3/reference/compound_stmts.html#try\n",
    "# Take a look, and learn BNF (Backus-Naur Form)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 28,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "finally\n"
     ]
    }
   ],
   "source": [
    "a, b = 1, 2\n",
    "try:\n",
    "    if a < b:\n",
    "        print(a)\n",
    "finally:\n",
    "    print(\"finally\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 29,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "42"
      ]
     },
     "execution_count": 29,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "def the_Answer_to_the_Ultimate_Question_of_Life_the_Universe_and_Everything():\n",
    "    try:\n",
    "        1/0\n",
    "    finally:\n",
    "        return 42\n",
    "\n",
    "the_Answer_to_the_Ultimate_Question_of_Life_the_Universe_and_Everything()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 30,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "else except\n",
      "finally\n"
     ]
    }
   ],
   "source": [
    "a = 22\n",
    "b = 33\n",
    " \n",
    "try:\n",
    "    if a > b: # Try to change this!\n",
    "        print(non_exit_var)\n",
    "except:\n",
    "    print(\"except\")\n",
    "else:\n",
    "    print(\"else except\")\n",
    "    \"\"\"\n",
    "    The optional else clause is executed if and when control flows off the end of the try clause.\n",
    "    Exceptions in the else clause are not handled by the preceding except clauses.\n",
    "    The optional else clause, when present, must follow all except clauses.\n",
    "    The use of the else clause is better than adding additional code to the try clause\n",
    "    because it avoids accidentally catching an exception that wasn’t raised by the code\n",
    "    being protected by the try ... except statement.\n",
    "    \"\"\"\n",
    "finally:\n",
    "    print(\"finally\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "try:\n",
    "    if 1/0:\n",
    "        print(\"E\")\n",
    "except (ZeroDivisionError, NameError, SyntaxError):\n",
    "    pass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 32,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "OS error: [Errno 2] No such file or directory: 'myfile.txt'\n"
     ]
    }
   ],
   "source": [
    "import sys\n",
    "\n",
    "try:\n",
    "    f = open('myfile.txt')\n",
    "    s = f.readline()\n",
    "    i = int(s.strip())\n",
    "except OSError as err:\n",
    "    print(\"OS error: {0}\".format(err))\n",
    "except ValueError:\n",
    "    print(\"Could not convert data to an integer.\")\n",
    "except:\n",
    "    print(\"Unexpected error:\", sys.exc_info()[0])\n",
    "    \n",
    "# Try to catch \"IOError\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Raising Exceptions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 33,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Unexpected error: <class 'ZeroDivisionError'>\n"
     ]
    }
   ],
   "source": [
    "import sys\n",
    "def this_fails():\n",
    "    x = 1/0\n",
    "    #raise ValueError(\"ABC\")\n",
    "\n",
    "try:\n",
    "    this_fails()\n",
    "except:\n",
    "    print(\"Unexpected error:\", sys.exc_info()[0]) # Try [1]"
   ]
  }
 ],
 "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": "3.6.0"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}