Decimal Precision.ipynb

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   "source": [
    "Decimals show up with a large precision because ... computers. 🙈 If you're interested in the background, watch [this video](https://www.youtube.com/watch?v=8afbTaA-gOQ)."
   ]
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   "execution_count": 1,
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     "data": {
      "text/plain": [
       "Decimal('1.5500000000000000444089209850062616169452667236328125')"
      ]
     },
     "execution_count": 1,
     "metadata": {},
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   ],
   "source": [
    "import decimal\n",
    "decimal.Decimal(1.55)"
   ]
  },
  {
   "attachments": {},
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   "metadata": {
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   "source": [
    "Adding two Decimals will give you a result with high precision."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "Decimal('3.100000000000000088817841970')"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decimal.Decimal(1.55) + decimal.Decimal(1.55)"
   ]
  },
  {
   "attachments": {},
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   "source": [
    "However you can set a global (or local) precision in the decimal context:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
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   "outputs": [],
   "source": [
    "context = decimal.getcontext()\n",
    "context.prec = 5\n",
    "\n",
    "decimal.setcontext(context)"
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "Which does not have effect on the display of the number. That display is just the value in memory."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "Decimal('1.5500000000000000444089209850062616169452667236328125')"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decimal.Decimal(1.55)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "But it does have impact on caluclations with decimals. They will honor the new precision."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "Decimal('3.1000')"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decimal.Decimal(1.55) + decimal.Decimal(1.55)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "Decimal('0.52632')"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "\n",
    "decimal.Decimal(1.23456789) / decimal.Decimal(2.34567890)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
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   },
   "source": [
    "Quantize Operations work too"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "Decimal('1.24')"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "decimal.Decimal(1.23456789).quantize(decimal.Decimal(\"0.01\"), rounding=decimal.ROUND_UP)"
   ]
  },
  {
   "attachments": {},
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   "source": [
    "But their exponent length must not exceed the context precision."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "ename": "InvalidOperation",
     "evalue": "[<class 'decimal.InvalidOperation'>]",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mInvalidOperation\u001b[0m                          Traceback (most recent call last)",
      "Cell \u001b[0;32mIn [8], line 1\u001b[0m\n\u001b[0;32m----> 1\u001b[0m decimal\u001b[39m.\u001b[39;49mDecimal(\u001b[39m1.23456789\u001b[39;49m)\u001b[39m.\u001b[39;49mquantize(decimal\u001b[39m.\u001b[39;49mDecimal(\u001b[39m\"\u001b[39;49m\u001b[39m0.00001\u001b[39;49m\u001b[39m\"\u001b[39;49m), rounding\u001b[39m=\u001b[39;49mdecimal\u001b[39m.\u001b[39;49mROUND_UP)\n",
      "\u001b[0;31mInvalidOperation\u001b[0m: [<class 'decimal.InvalidOperation'>]"
     ]
    }
   ],
   "source": [
    "decimal.Decimal(1.23456789).quantize(decimal.Decimal(\"0.00001\"), rounding=decimal.ROUND_UP)"
   ]
  }
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