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  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "# Global and local scope of Python variables"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "# - compatibility with Python 3\n",
      "from __future__ import print_function  # print('me') instead of print 'me'\n",
      "from __future__ import division  # 1/2 == 0.5, not 0"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 1
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Variable *scope* refers to the places that you can see or access a variable."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If you define a variable at the top level of your script or module or notebook, this is a global variable:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_var = 3"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 2
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "The variable is *global* because any Python function or class defined in this module or notebook, is able to access this variable.  For example:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def my_first_func():\n",
      "    # my_func can 'see' the global variable\n",
      "    print('I see \"my_var\" = ', my_var, ' from \"my_first_func\"')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 3
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_first_func()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "I see \"my_var\" =  3  from \"my_first_func\"\n"
       ]
      }
     ],
     "prompt_number": 4
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Variables defined inside a function or class, are not global.  Only the function or class can see the variable:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "def my_second_func():\n",
      "    a = 10\n",
      "    print('I see \"a\" = ', a, 'from \"my_second_func\"')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [],
     "prompt_number": 5
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_second_func()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "I see \"a\" =  10 from \"my_second_func\"\n"
       ]
      }
     ],
     "prompt_number": 6
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "But here, down in the top or global level of the notebook, we can't see that variable:"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "a"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "ename": "NameError",
       "evalue": "name 'a' is not defined",
       "output_type": "pyerr",
       "traceback": [
        "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mNameError\u001b[0m                                 Traceback (most recent call last)",
        "\u001b[0;32m<ipython-input-7-60b725f10c9c>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0ma\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
        "\u001b[0;31mNameError\u001b[0m: name 'a' is not defined"
       ]
      }
     ],
     "prompt_number": 7
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "The variable `a` is therefore said to be *local* to the function.  Put another way, the variable `a` has local scope.  Conversely the variable `my_var` has global scope."
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "The rules on variable scope cover many more cases than these very simple examples.\n",
      "\n",
      "See http://nbviewer.ipython.org/github/rasbt/python_reference/blob/master/tutorials/scope_resolution_legb_rule.ipynb for a nice walkthrough."
     ]
    }
   ],
   "metadata": {}
  }
 ]
}