Python_is_and_euqal_operator.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This gist is a summary of the [Is there a difference between “==” and “is”?\n",
    "](https://stackoverflow.com/questions/132988/is-there-a-difference-between-and-is) thread on Stackoverflow."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Use `is` to **check identifies**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "False"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "m = 1234567\n",
    "n = 1234567\n",
    "m is n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Think of `is` as:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "id(m) == id(n)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Implementation Exception\n",
    "\n",
    "The official implementation of python is CPython, which [optimizes the storage of integers in range(-5 to 256) by making them singletons](https://docs.python.org/2/c-api/int.html#c.PyInt_FromLong). This is also true for short strings."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "m = 123\n",
    "n = 123\n",
    "m is n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "m = 'abc'\n",
    "n = 'abc'\n",
    "m is n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The code above happens due to optimization. It is NOT guaranteed to happen in other implementations. Thus, one should not expect this to happen and should not use `is` to compare values."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Use `==` to **check equality**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "m = 1234567\n",
    "n = 1234567\n",
    "m == n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Comparing to None\n",
    "\n",
    "> Comparisons to singletons like None should always be done with is or  is not, never the equality operators.\n",
    "\n",
    "> Also, beware of writing if x when you really mean if x is not None -- e.g. when testing whether a variable or argument that defaults to None was set to some other value. The other value might have a type (such as a container) that could be false in a boolean context!\n",
    "\n",
    "-- [PEP8](https://www.python.org/dev/peps/pep-0008/#programming-recommendations)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is None\n"
     ]
    }
   ],
   "source": [
    "m = None\n",
    "\n",
    "if m is None: # always use is when comparing with None\n",
    "    print('m is None')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Although the following code works, it backfires in the next block. Thus, `==` should never be used to compare with `None`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is None\n"
     ]
    }
   ],
   "source": [
    "m = None\n",
    "\n",
    "if m == None: # Althought it works, don't do it (see the sectin below)\n",
    "    print('m is None')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Checking variable is None (The WRONG WAY)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 26,
   "metadata": {
    "scrolled": true
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is None\n"
     ]
    }
   ],
   "source": [
    "m = list() # empty container\n",
    "\n",
    "if m: # will always evaluate to false in an if statement\n",
    "    print('m is not None')\n",
    "else:\n",
    "    print('m is None')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 27,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is None\n"
     ]
    }
   ],
   "source": [
    "m = dict() # empty container\n",
    "\n",
    "if m: # will always evaluate to false in an if statement\n",
    "    print('m is not None')\n",
    "else:\n",
    "    print('m is None')"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 28,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is None\n"
     ]
    }
   ],
   "source": [
    "m = set() # empty container\n",
    "\n",
    "if m: # will always evaluate to false in an if statement\n",
    "    print('m is not None')\n",
    "else:\n",
    "    print('m is None')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "When the variable is an empty container (any object that holds an arbitrary number of other objects), and it is used in a `if` statement, it always evaluates to `False`."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### The Correct way to check for None (always spell out the is not None part, don't be lazy)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "m is not None\n"
     ]
    }
   ],
   "source": [
    "m = [] # m is not None, it is just an empty list\n",
    "\n",
    "if m is not None:\n",
    "    print('m is not None')\n",
    "else:\n",
    "    print('m is None')"
   ]
  }
 ],
 "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": "3.7.3"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 2
}