python_reference/python_true_false.ipynb

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  {
   "cells": [
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Sebastian Raschka, 03/2014  \n",
      "Code was executed in Python 3.4.0"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "###`True` and `False` in the `datetime` module\n",
      "\n",
      "Pointed out in a nice article **\"A false midnight\"** at [http://lwn.net/SubscriberLink/590299/bf73fe823974acea/](http://lwn.net/SubscriberLink/590299/bf73fe823974acea/):\n",
      "\n",
      "*\"it often comes as a big surprise for programmers to find (sometimes by way of a hard-to-reproduce bug) that,   \n",
      "unlike any other time value, midnight (i.e. datetime.time(0,0,0)) is False.  \n",
      "A long discussion on the python-ideas mailing list shows that, while surprising,  \n",
      "that behavior is desirable\u2014at least in some quarters.\"*"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import datetime\n",
      "\n",
      "print(datetime.time(0,0,0), '(Midnight) evaluates to ', end=\"\")\n",
      "if not datetime.time(0,0,0):\n",
      "    print(False)\n",
      "else:\n",
      "    print(True)\n",
      "    \n",
      "print(datetime.time(1,0,0), '(1 am) evaluates to ', end=\"\")\n",
      "if not datetime.time(1,0,0):\n",
      "    print(False)\n",
      "else:\n",
      "    print(True)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "00:00:00 (Midnight) evaluates to False\n",
        "01:00:00 (1 am) evaluates to True\n"
       ]
      }
     ],
     "prompt_number": 12
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Boolean `True`"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_true_val = True\n",
      "\n",
      "\n",
      "print('my_true_val == True:', my_true_val == True)\n",
      "print('my_true_val is True:', my_true_val is True)\n",
      "\n",
      "print('my_true_val == None:', my_true_val == None)\n",
      "print('my_true_val is None:', my_true_val is None)\n",
      "\n",
      "print('my_true_val == False:', my_true_val == False)\n",
      "print('my_true_val is False:', my_true_val is False)\n",
      "\n",
      "\n",
      "if my_true_val:\n",
      "    print('\"if my_true_val:\" is True')\n",
      "else:\n",
      "    print('\"if my_true_val:\" is False')\n",
      "    \n",
      "if not my_true_val:\n",
      "    print('\"if not my_true_val:\" is True')\n",
      "else:\n",
      "    print('\"if not my_true_val:\" is False')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Boolean `False`"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_false_val = False\n",
      "\n",
      "\n",
      "print('my_false_val == True:', my_false_val == True)\n",
      "print('my_false_val is True:', my_false_val is True)\n",
      "\n",
      "print('my_false_val == None:', my_false_val == None)\n",
      "print('my_false_val is None:', my_false_val is None)\n",
      "\n",
      "print('my_false_val == False:', my_false_val == False)\n",
      "print('my_false_val is False:', my_false_val is False)\n",
      "\n",
      "\n",
      "if my_false_val:\n",
      "    print('\"if my_false_val:\" is True')\n",
      "else:\n",
      "    print('\"if my_false_val:\" is False')\n",
      "    \n",
      "if not my_false_val:\n",
      "    print('\"if not my_false_val:\" is True')\n",
      "else:\n",
      "    print('\"if not my_false_val:\" is False')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_false_val == True: False\n",
        "my_false_val is True: False\n",
        "my_false_val == None: False\n",
        "my_false_val is None: False\n",
        "my_false_val == False: True\n",
        "my_false_val is False: True\n",
        "\"if my_false_val:\" is False\n",
        "\"if not my_false_val:\" is True\n"
       ]
      }
     ],
     "prompt_number": 76
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## `None` 'value'"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_none_var = None\n",
      "\n",
      "print('my_none_var == True:', my_none_var == True)\n",
      "print('my_none_var is True:', my_none_var is True)\n",
      "\n",
      "print('my_none_var == None:', my_none_var == None)\n",
      "print('my_none_var is None:', my_none_var is None)\n",
      "\n",
      "print('my_none_var == False:', my_none_var == False)\n",
      "print('my_none_var is False:', my_none_var is False)\n",
      "\n",
      "\n",
      "if my_none_var:\n",
      "    print('\"if my_none_var:\" is True')\n",
      "else:\n",
      "    print('\"if my_none_var:\" is False')\n",
      "\n",
      "if not my_none_var:\n",
      "    print('\"if not my_none_var:\" is True')\n",
      "else:\n",
      "    print('\"if not my_none_var:\" is False')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_none_var == True: False\n",
        "my_none_var is True: False\n",
        "my_none_var == None: True\n",
        "my_none_var is None: True\n",
        "my_none_var == False: False\n",
        "my_none_var is False: False\n",
        "\"if my_none_var:\" is False\n",
        "\"if not my_none_var:\" is True\n"
       ]
      }
     ],
     "prompt_number": 62
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Empty String"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_empty_string = \"\"\n",
      "\n",
      "print('my_empty_string == True:', my_empty_string == True)\n",
      "print('my_empty_string is True:', my_empty_string is True)\n",
      "\n",
      "print('my_empty_string == None:', my_empty_string == None)\n",
      "print('my_empty_string is None:', my_empty_string is None)\n",
      "\n",
      "print('my_empty_string == False:', my_empty_string == False)\n",
      "print('my_empty_string is False:', my_empty_string is False)\n",
      "\n",
      "\n",
      "if my_empty_string:\n",
      "    print('\"if my_empty_string:\" is True')\n",
      "else:\n",
      "    print('\"if my_empty_string:\" is False')\n",
      "    \n",
      "if not my_empty_string:\n",
      "    print('\"if not my_empty_string:\" is True')\n",
      "else:\n",
      "    print('\"if not my_empty_string:\" is False')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_empty_string == True: False\n",
        "my_empty_string is True: False\n",
        "my_empty_string == None: False\n",
        "my_empty_string is None: False\n",
        "my_empty_string == False: False\n",
        "my_empty_string is False: False\n",
        "\"if my_empty_string:\" is False\n",
        "\"if my_empty_string:\" is True\n"
       ]
      }
     ],
     "prompt_number": 61
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## Empty List\n",
      "It is generally not a good idea to use the `==` to check for empty lists. "
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_empty_list = []\n",
      "\n",
      "\n",
      "print('my_empty_list == True:', my_empty_list == True)\n",
      "print('my_empty_list is True:', my_empty_list is True)\n",
      "\n",
      "print('my_empty_list == None:', my_empty_list == None)\n",
      "print('my_empty_list is None:', my_empty_list is None)\n",
      "\n",
      "print('my_empty_list == False:', my_empty_list == False)\n",
      "print('my_empty_list is False:', my_empty_list is False)\n",
      "\n",
      "\n",
      "if my_empty_list:\n",
      "    print('\"if my_empty_list:\" is True')\n",
      "else:\n",
      "    print('\"if my_empty_list:\" is False')\n",
      "    \n",
      "if not my_empty_list:\n",
      "    print('\"if not my_empty_list:\" is True')\n",
      "else:\n",
      "    print('\"if not my_empty_list:\" is False')\n",
      "\n",
      "\n",
      "    \n"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_empty_list == True: False\n",
        "my_empty_list is True: False\n",
        "my_empty_list == None: False\n",
        "my_empty_list is None: False\n",
        "my_empty_list == False: False\n",
        "my_empty_list is False: False\n",
        "\"if my_empty_list:\" is False\n",
        "\"if not my_empty_list:\" is True\n"
       ]
      }
     ],
     "prompt_number": 67
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## [0]-List"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_zero_list = [0]\n",
      "\n",
      "\n",
      "print('my_zero_list == True:', my_zero_list == True)\n",
      "print('my_zero_list is True:', my_zero_list is True)\n",
      "\n",
      "print('my_zero_list == None:', my_zero_list == None)\n",
      "print('my_zero_list is None:', my_zero_list is None)\n",
      "\n",
      "print('my_zero_list == False:', my_zero_list == False)\n",
      "print('my_zero_list is False:', my_zero_list is False)\n",
      "\n",
      "\n",
      "if my_zero_list:\n",
      "    print('\"if my_zero_list:\" is True')\n",
      "else:\n",
      "    print('\"if my_zero_list:\" is False')\n",
      "    \n",
      "if not my_zero_list:\n",
      "    print('\"if not my_zero_list:\" is True')\n",
      "else:\n",
      "    print('\"if not my_zero_list:\" is False')"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_zero_list == True: False\n",
        "my_zero_list is True: False\n",
        "my_zero_list == None: False\n",
        "my_zero_list is None: False\n",
        "my_zero_list == False: False\n",
        "my_zero_list is False: False\n",
        "\"if my_zero_list:\" is True\n",
        "\"if not my_zero_list:\" is False\n"
       ]
      }
     ],
     "prompt_number": 70
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "## List comparison  \n",
      "List comparisons are a handy way to show the difference between `==` and `is`.  \n",
      "While `==` is rather evaluating the equality of the value, `is` is checking if two objects are equal.\n",
      "The examples below show that we can assign a pointer to the same list object by using `=`, e.g., `list1 = list2`. If we want to make a copy of the list values, we have to make a little "
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "my_first_list = [1]\n",
      "my_second_list = [1]\n",
      "print('my_first_list == my_second_list:', my_first_list == my_second_list)\n",
      "print('my_first_list is my_second_list:', my_first_list is my_second_list)\n",
      "\n",
      "\n",
      "my_third_list = my_first_list\n",
      "print('my_first_list == my_third_list:', my_first_list == my_third_list)\n",
      "print('my_first_list is my_third_list:', my_first_list is my_third_list)\n",
      "\n",
      "my_fourth_list = my_first_list[:]\n",
      "print('my_first_list == my_fourth_list:', my_first_list == my_fourth_list)\n",
      "print('my_first_list is my_fourth_list:', my_first_list is my_fourth_list)"
     ],
     "language": "python",
     "metadata": {},
     "outputs": [
      {
       "output_type": "stream",
       "stream": "stdout",
       "text": [
        "my_first_list == my_second_list: True\n",
        "my_first_list is my_second_list: False\n",
        "my_first_list == my_third_list: True\n",
        "my_first_list is my_third_list: True\n",
        "my_first_list == my_fourth_list: True\n",
        "my_first_list is my_fourth_list: False\n"
       ]
      }
     ],
     "prompt_number": 82
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [],
     "language": "python",
     "metadata": {},
     "outputs": []
    }
   ],
   "metadata": {}
  }
 ]
}