{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "difficult-gather",
   "metadata": {},
   "source": [
    "# Dictionary Merge and Update Operators in Python 3.9\n",
    "\n",
    "When it comes to merging two dictinaries in Python, I find myself googling \"how to merge two dictionaries\" very often. Every time I land on this [StackOverflow answer](https://stackoverflow.com/questions/38987/how-do-i-merge-two-dictionaries-in-a-single-expression-taking-union-of-dictiona). This answers recommends using the **dictionary unpacking** feature:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "705af7b6-2a9c-48b8-997e-0085065f2506",
   "metadata": {},
   "outputs": [],
   "source": [
    "x = {'earth': 5.97, 'mercury': 0.330}\n",
    "y = {'uranus': 86.8, 'mars': 0.642, 'earth': -999}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "id": "e1b0c166-f93f-4b98-812f-1b53d5f19c4c",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{'earth': -999, 'mercury': 0.33, 'uranus': 86.8, 'mars': 0.642}"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "a = {**x, **y}\n",
    "a"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "610a134b-99c6-40a6-b840-61f14e1ab183",
   "metadata": {},
   "source": [
    "Even though I have seen dictionary unpacking used for unpacking keyword arguments in functions, I've **rarely seen it being used as a method for merging dictionaries**, and this makes it a **bit confusing** if you haven't seen it used in this context. I was delighted when I found out that Python 3.9 introduced a dedicated binary operator that permits merging two dictionaries. In Python 3.9, we can use the `|` (bitwise `OR`) to  perform a dictionary merge:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "c8ec51c4-9d85-4acd-b95a-ff9fe39c5142",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{'earth': -999, 'mercury': 0.33, 'uranus': 86.8, 'mars': 0.642}"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "b = x | y\n",
    "b"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "bccd2266-d5dc-4c7b-aa47-166022184e32",
   "metadata": {},
   "source": [
    "As you can see, the resulting dictionary consists of a new dictionary with all the keys of both `x` and `y` dictionaries. Because `x` and `y` have overlapping key `earth`, the new dictionary uses the values from the rightmost dictionary (i.e. `y`)"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "830c0a36-e825-4753-9845-e3a4aa0e47f6",
   "metadata": {},
   "source": [
    "If we prefer to update one of the dictionary inplace (i.e. without needing to create a new dictionary), we can use the `|=` (in-place bitwise `OR`) operator"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "daaf995f-d201-4945-b9a7-e4c17b582054",
   "metadata": {},
   "outputs": [],
   "source": [
    "x |= y"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "id": "fab77b58-cc3d-4ea1-83bd-f6752a993447",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{'earth': -999, 'mercury': 0.33, 'uranus': 86.8, 'mars': 0.642}"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "x"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "727bce58-99fc-45e9-bf14-ca0f7a30b55d",
   "metadata": {},
   "source": [
    "I really dig this new `|` operator. I find the syntax easy to remember and understand compared with the **dictionary unpacking** feature that I used prior to Python 3.9. There's one problem though. This operator is only available for Python 3.9+ and I have to write code for older versions of Python for the time being 😞."
   ]
  }
 ],
 "metadata": {
  "author": "Anderson Banihirwe",
  "date": "2021-06-04",
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  "tags": "python,todayilearned",
  "title": "Dictionary Merge and Update Operators in Python 3.9",
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