python/Using reindex for adding missing columns to a dataframe.ipynb

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   "source": [
    "# Use `reindex` for adding missing columns to a dataframe"
   ]
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   "source": [
    "import pandas as pd"
   ]
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    "df = pd.DataFrame([\n",
    "    {\n",
    "        'a': 1,\n",
    "        'b': 2,\n",
    "        'd': 4\n",
    "    }\n",
    "])"
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       "<div>\n",
       "<table border=\"1\" class=\"dataframe\">\n",
       "  <thead>\n",
       "    <tr style=\"text-align: right;\">\n",
       "      <th></th>\n",
       "      <th>a</th>\n",
       "      <th>b</th>\n",
       "      <th>d</th>\n",
       "    </tr>\n",
       "  </thead>\n",
       "  <tbody>\n",
       "    <tr>\n",
       "      <th>0</th>\n",
       "      <td>1</td>\n",
       "      <td>2</td>\n",
       "      <td>4</td>\n",
       "    </tr>\n",
       "  </tbody>\n",
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      "text/plain": [
       "   a  b  d\n",
       "0  1  2  4"
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   "source": [
    "df"
   ]
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   "cell_type": "markdown",
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   "source": [
    "### Using reindex to add missing columns to a dataframe\n",
    "https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.reindex.html"
   ]
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   "source": [
    "columns = ['a', 'b', 'c', 'd']"
   ]
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   "execution_count": 16,
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       "<div>\n",
       "<table border=\"1\" class=\"dataframe\">\n",
       "  <thead>\n",
       "    <tr style=\"text-align: right;\">\n",
       "      <th></th>\n",
       "      <th>a</th>\n",
       "      <th>b</th>\n",
       "      <th>c</th>\n",
       "      <th>d</th>\n",
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       "  </thead>\n",
       "  <tbody>\n",
       "    <tr>\n",
       "      <th>0</th>\n",
       "      <td>1</td>\n",
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       "   a  b  c  d\n",
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   "source": [
    "df.reindex(columns=columns, fill_value=0)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This can also be used to get a subset of the columns"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 21,
   "metadata": {
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "['a', 'b']"
      ]
     },
     "execution_count": 21,
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   ],
   "source": [
    "columns_subset = columns[:2]\n",
    "columns_subset"
   ]
  },
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   "execution_count": 19,
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       "<div>\n",
       "<table border=\"1\" class=\"dataframe\">\n",
       "  <thead>\n",
       "    <tr style=\"text-align: right;\">\n",
       "      <th></th>\n",
       "      <th>a</th>\n",
       "      <th>b</th>\n",
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    "df.reindex(columns=columns_subset, fill_value=0)"
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   "metadata": {},
   "source": [
    "Which is probably better done this way"
   ]
  },
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   "cell_type": "code",
   "execution_count": 20,
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   "outputs": [
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       "<div>\n",
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       "  <thead>\n",
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       "      <th>a</th>\n",
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   "source": [
    "df[columns_subset]"
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