{
  "cells": [
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "# Python Introduction: Names and Values"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "Define and reference a variable:"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 22,
      "metadata": {
        "collapsed": false
      },
      "outputs": [],
      "source": [
        "a = 3*2 + 5"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 23,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "11"
            ]
          },
          "execution_count": 23,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 24,
      "metadata": {
        "collapsed": false
      },
      "outputs": [],
      "source": [
        "a = \"interesting\"*3"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 25,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "'interestinginterestinginteresting'"
            ]
          },
          "execution_count": 25,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "No type declaration needed!\n",
        "\n",
        "(But values still have types--let's check.)"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 26,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "str"
            ]
          },
          "execution_count": 26,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "type(a)"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "Python variables are like *pointers*.\n",
        "\n",
        "(if that word makes sense)"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 27,
      "metadata": {
        "collapsed": false
      },
      "outputs": [],
      "source": [
        "a = [1,2,3]"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 28,
      "metadata": {
        "collapsed": false
      },
      "outputs": [],
      "source": [
        "b = a"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 29,
      "metadata": {
        "collapsed": false
      },
      "outputs": [],
      "source": [
        "b.append(4)"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 30,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "[1, 2, 3, 4]"
            ]
          },
          "execution_count": 30,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "b"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 31,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "[1, 2, 3, 4]"
            ]
          },
          "execution_count": 31,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "You can see this pointer with `id()`."
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 33,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": [
            "140441837907336 140441837907336\n"
          ]
        }
      ],
      "source": [
        "print(id(a), id(b))"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "The `is` operator tests for object sameness."
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 34,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "True"
            ]
          },
          "execution_count": 34,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a is b"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "This is a **stronger** condition than being equal!"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 36,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": [
            "IS    False\n",
            "EQUAL True\n"
          ]
        }
      ],
      "source": [
        "a = [1,2,3]\n",
        "b = [1,2,3]\n",
        "print(\"IS   \", a is b)\n",
        "print(\"EQUAL\", a == b)"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "What do you think the following prints?"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 38,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": [
            "[1, 2, 3]\n"
          ]
        }
      ],
      "source": [
        "a = [1,2,3]\n",
        "b = a\n",
        "a = a + [4]\n",
        "print(b)"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 39,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "False"
            ]
          },
          "execution_count": 39,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a is b"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "Why is that?"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "-----\n",
        "How could this lead to bugs?"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "----------\n",
        "* To help manage this risk, Python provides **immutable** types.\n",
        "\n",
        "* Immutable types cannot be changed in-place, only by creating a new object.\n",
        "\n",
        "* A `tuple` is an immutable `list`."
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 40,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "list"
            ]
          },
          "execution_count": 40,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a = [1,2,3]\n",
        "type(a)"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 41,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "tuple"
            ]
          },
          "execution_count": 41,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a = (1,2,3)\n",
        "\n",
        "type(a)"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "Let's try to change that tuple."
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 42,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "ename": "TypeError",
          "evalue": "'tuple' object does not support item assignment",
          "output_type": "error",
          "traceback": [
            "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
            "\u001b[0;32m<ipython-input-42-ec18b7a325e0>\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[0;36m2\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;36m0\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": [
        "a[2] = 0"
      ]
    },
    {
      "cell_type": "markdown",
      "metadata": {},
      "source": [
        "*Bonus question:* How do you spell a single-element tuple?"
      ]
    },
    {
      "cell_type": "code",
      "execution_count": 43,
      "metadata": {
        "collapsed": false
      },
      "outputs": [
        {
          "data": {
            "text/plain": [
              "tuple"
            ]
          },
          "execution_count": 43,
          "metadata": {},
          "output_type": "execute_result"
        }
      ],
      "source": [
        "a = (3,)\n",
        "\n",
        "type(a)"
      ]
    }
  ],
  "metadata": {},
  "nbformat": 4,
  "nbformat_minor": 0
}