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   "source": [
    "# f-strings\n",
    "\n",
    "[f-strings](https://www.python.org/dev/peps/pep-0498/)\n",
    "are\n",
    "[new in Python 3.6](https://docs.python.org/3.6/whatsnew/3.6.html#pep-498-formatted-string-literals).\n",
    "\n",
    "f-strings look like an ordinary strings with an f in front.\n",
    "They look like an ordinary (immutable) string that does not change,\n",
    "but they are not immutable strings.\n",
    "\n",
    "#    **They are dynamic expressions!!!**\n",
    "\n",
    "One can put arbitrary expressions within curly braces in an f-string,\n",
    "and the arbitrary expressions will be evaluated each time,\n",
    "and then converted to a string.\n",
    "The most common expressions used are just variable names.\n",
    "\n",
    "It is a more\n",
    "[DRY](https://en.wikipedia.org/wiki/Don't_repeat_yourself)\n",
    "way of doing formatting than str.format."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "---\n",
    "\n",
    "First let's look at how format is often done with the str.format method."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "stuff = {\n",
    "    'apple': 1.97,\n",
    "    'banana': 2.99,\n",
    "    'cherry': 3.99,\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The price of apple is 1.97\n",
      "The price of banana is 2.99\n",
      "The price of cherry is 3.99\n"
     ]
    }
   ],
   "source": [
    "# Common pattern of .format use:\n",
    "\n",
    "for name, price in stuff.items():\n",
    "    print(\n",
    "        'The price of {name} is {price}'.\n",
    "        format(name=name, price=price))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Something that sucks about the above print,\n",
    "is that name and price appear three times each.\n",
    "I.e., It is not\n",
    "[DRY](https://en.wikipedia.org/wiki/Don't_repeat_yourself).\n",
    "\n",
    "With f-strings, name and price only have to appear once,\n",
    "making the code easier to read and maintain\n",
    "as in the following example."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The price of apple is 1.97\n",
      "The price of banana is 2.99\n",
      "The price of cherry is 3.99\n"
     ]
    }
   ],
   "source": [
    "for name, price in stuff.items():\n",
    "    print(f'The price of {name} is {price}')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "It reminds me of shell syntax. For example,\n",
    "\n",
    "    echo \"The price of ${name} is ${price}\"\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "It gets better. One may put arbitrary expressions\n",
    "within the curly braces in an f-string.\n",
    "One is not limited to using just variable names."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "The total price of apple is 2.96\n",
      "The total price of banana is 4.49\n",
      "The total price of cherry is 5.99\n"
     ]
    }
   ],
   "source": [
    "tax = 0.50\n",
    "\n",
    "for name, price in stuff.items():\n",
    "    print(f'The total price of {name} is {round(price * (1+tax), 2)}')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Then some crazy whacko guy named\n",
    "[zak](https://github.com/y2kbugger)\n",
    "showed a crazy whacko lambda with no arguments\n",
    "that had an f-string for the body.\n",
    "\n",
    "This makes my head wobble.\n",
    "I am wondering what good uses (if any) there are for this."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "<function __main__.<lambda>>"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "template = lambda: f'tes{k}'\n",
    "template"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'tes9'"
      ]
     },
     "execution_count": 6,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "k = 9\n",
    "template()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'tes8'"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "k = 8\n",
    "template()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "2017-02-25 afterthought:\n",
    "\n",
    "Since it is saved in a variable,\n",
    "the lambda above is a poor use of a lambda.\n",
    "It would be better to just make it a regular (named) function\n",
    "as shown below.\n",
    "\n",
    "I still do not know what good uses (if any) there are for the f-string below."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def template():\n",
    "    return f'tes{k}'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'tes9'"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "k = 9\n",
    "template()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "'tes8'"
      ]
     },
     "execution_count": 10,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "k = 8\n",
    "template()"
   ]
  }
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