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   "cell_type": "markdown",
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   "source": [
    "# Names\n",
    "\n",
    "Names are given to values in Python using an *assignment* statement. In an assignment, a name is followed by `=`, which is followed by any expression. The value of the expression to the right of `=` is *assigned* to the name. Once a name has a value assigned to it, the value will be substituted for that name in future expressions."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "30"
      ]
     },
     "execution_count": 1,
     "metadata": {},
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   ],
   "source": [
    "a = 10\n",
    "b = 20\n",
    "a + b"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A previously assigned name can be used in the expression to the right of `=`. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "0.5"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "quarter = 1/4\n",
    "half = 2 * quarter\n",
    "half"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "However, only the current value of an expression is assigned to a name. If that value changes later, names that were defined in terms of that value will not change automatically."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "0.5"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
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   ],
   "source": [
    "quarter = 4\n",
    "half"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Names must start with a letter, but can contain both letters and numbers. A name cannot contain a space; instead, it is common to use an underscore character `_` to replace each space. Names are only as useful as you make them; it's up to the programmer to choose names that are easy to interpret. Typically, more meaningful names can be invented than `a` and `b`. For example, to describe the sales tax on a $5 purchase in Berkeley, CA, the following names clarify the meaning of the various quantities involved."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "0.475"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "purchase_price = 5\n",
    "state_tax_rate = 0.075\n",
    "county_tax_rate = 0.02\n",
    "city_tax_rate = 0\n",
    "sales_tax_rate = state_tax_rate + county_tax_rate + city_tax_rate\n",
    "sales_tax = purchase_price * sales_tax_rate\n",
    "sales_tax"
   ]
  }
 ],
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