Advanced Python Workshop - Class Notes

4 - Context Managers (Advanced Python Workshop).ipynb

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  "name": "4 - Context Managers (Advanced Python Workshop)"
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   "cells": [
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     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "# Context Managers"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## The `with` statement"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "f = open(\"a.txt\", \"w\")\n",
      "f.write(\"hello\")\n",
      "f.close()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 3
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "with open(\"a.txt\", \"w\") as f:\n",
      "    f.write(\"hello\")"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 4
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "f = open(\"a.txt\")\n",
      "dir(f)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "pyout",
       "prompt_number": 7,
       "text": [
        "['__class__',\n",
        " '__delattr__',\n",
        " '__doc__',\n",
        " '__enter__',\n",
        " '__exit__',\n",
        " '__format__',\n",
        " '__getattribute__',\n",
        " '__hash__',\n",
        " '__init__',\n",
        " '__iter__',\n",
        " '__new__',\n",
        " '__reduce__',\n",
        " '__reduce_ex__',\n",
        " '__repr__',\n",
        " '__setattr__',\n",
        " '__sizeof__',\n",
        " '__str__',\n",
        " '__subclasshook__',\n",
        " 'close',\n",
        " 'closed',\n",
        " 'encoding',\n",
        " 'errors',\n",
        " 'fileno',\n",
        " 'flush',\n",
        " 'isatty',\n",
        " 'mode',\n",
        " 'name',\n",
        " 'newlines',\n",
        " 'next',\n",
        " 'read',\n",
        " 'readinto',\n",
        " 'readline',\n",
        " 'readlines',\n",
        " 'seek',\n",
        " 'softspace',\n",
        " 'tell',\n",
        " 'truncate',\n",
        " 'write',\n",
        " 'writelines',\n",
        " 'xreadlines']"
       ]
      }
     ],
     "prompt_number": 7
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Example: chdir"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import os\n",
      "class chdir:\n",
      "    def __init__(self, dir):\n",
      "        self.dir = dir\n",
      "        print \"__init__\"\n",
      "        \n",
      "    def __enter__(self):\n",
      "        print \"__enter__\"\n",
      "        self.olddir = os.getcwd()\n",
      "        os.chdir(self.dir)\n",
      "        return self\n",
      "        \n",
      "    def __exit__(self, *a):\n",
      "        print \"__exit__\"\n",
      "        os.chdir(self.olddir)\n",
      "\n",
      "print os.getcwd()\n",
      "print \"before with\"\n",
      "with chdir(\"/tmp\") as x:\n",
      "    print os.getcwd()\n",
      "print \"after with\"\n",
      "print os.getcwd()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "/Users/anand/Dropbox/Trainings/2013/advancedpython-may2013\n",
        "before with\n",
        "__init__\n",
        "__enter__\n",
        "/private/tmp\n",
        "__exit__\n",
        "after with\n",
        "/Users/anand/Dropbox/Trainings/2013/advancedpython-may2013\n"
       ]
      }
     ],
     "prompt_number": 21
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "**Problem** Write a context manager `capture_output` to capture stdout inside a with block.\n",
      "\n",
      "    with capture_output() as buf:\n",
      "        print \"hello\"\n",
      "\n",
      "    out = buf.getvalue()\n",
      "    print \"captured\", repr(out)\n",
      "\n",
      "Hint: See `StringIO.StringIO` and `sys.stdout`."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Lets try to understand how to capture output without context managers."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import sys\n",
      "from StringIO import StringIO\n",
      "\n",
      "oldstdout = sys.stdout\n",
      "buf = StringIO()\n",
      "sys.stdout = buf\n",
      "print \"hello\"\n",
      "print \"world\"\n",
      "sys.stdout = oldstdout\n",
      "print \"captured\", repr(buf.getvalue())"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "captured 'hello\\nworld\\n'\n"
       ]
      }
     ],
     "prompt_number": 8
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "# solution\n",
      "\n",
      "from StringIO import StringIO\n",
      "import sys\n",
      "\n",
      "class capture_output:\n",
      "    def __init__(self):\n",
      "        self.buf = StringIO()\n",
      "\n",
      "    def __enter__(self):\n",
      "        self.oldstdout = sys.stdout\n",
      "        sys.stdout = self.buf\n",
      "        return self.buf\n",
      "        \n",
      "    def __exit__(self, type, exc, traceback):\n",
      "        sys.stdout = self.oldstdout\n",
      "    \n",
      "#with capture_output() as buf:\n",
      "#    print \"hello\"\n",
      "\n",
      "capture_output()\n",
      "\n",
      "print \"hello\"\n",
      "\n",
      "#out = buf.getvalue()\n",
      "#print \"captured\", repr(out)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "hello\n"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "### Example: ignore_exception"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "class ignore_exception:\n",
      "    def __init__(self):\n",
      "        pass\n",
      "\n",
      "    def __enter__(self):\n",
      "        pass\n",
      "        \n",
      "    def __exit__(self, type, exc, traceback):\n",
      "        print type, exc, traceback\n",
      "        return True\n",
      "\n",
      "with ignore_exception():\n",
      "    print \"begin\"\n",
      "    raise IOError(\"fo\")\n",
      "    \n",
      "    "
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "begin\n",
        "<type 'exceptions.IOError'> fo <traceback object at 0x102f48998>\n"
       ]
      }
     ],
     "prompt_number": 14
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "**Problem** Improve the above `ignore_exception` class to ignore only one particular exception. \n",
      "\n",
      "    with ignore_exception(IOError):\n",
      "        raise IOError(\"foo\")\n"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [],
     "language": "python",
     "metadata": {},
     "outputs": []
    }
   ],
   "metadata": {}
  }
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