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   "source": [
    "# How to defer evaluation of f-strings\n",
    "\n",
    "It seems that one solution is to use lambdas, which are explored below.\n",
    "\n",
    "What other solutions are there?"
   ]
  },
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   "cell_type": "markdown",
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   "source": [
    "---\n",
    "\n",
    "Imagine that one wants to format a string\n",
    "selecting the format from several f-strings.\n",
    "Unfortunately, the obvious straightforward way\n",
    "of having the f-strings as values in a dictionary,\n",
    "does not work as desired,\n",
    "because the f-strings are evaluated when creating the dictionary.\n",
    "This unhappy way follows."
   ]
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  {
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   "source": [
    "year, month, day = 'hello', -1, 0\n",
    "date_formats = {\n",
    "    'iso': f'{year}-{month:02d}-{day:02d}',\n",
    "    'us': f'{month}/{day}/{year}',\n",
    "    'other': f'{day} {month} {year}',\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "Notice below that\n",
    "the current values of year, month, and day\n",
    "are ignored when evaluating date_formats['iso']."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
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    "deletable": true,
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2017 3 27\n",
      "hello--1-0\n"
     ]
    }
   ],
   "source": [
    "year, month, day = 2017, 3, 27\n",
    "print(year, month, day)\n",
    "print(date_formats['iso'])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "A solution is to use lambdas in the dictionary, and call them later,\n",
    "as shown below."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
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    "deletable": true,
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   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "iso:\n",
      "2017-03-27\n",
      "2017-04-24\n",
      "2017-05-22\n",
      "us:\n",
      "3/27/2017\n",
      "4/24/2017\n",
      "5/22/2017\n",
      "other:\n",
      "27.3.2017\n",
      "24.4.2017\n",
      "22.5.2017\n"
     ]
    }
   ],
   "source": [
    "year, month, day = 'hello', -1, 0\n",
    "# year, month, and day do not have to be defined when creating dictionary.\n",
    "del year  # Test that with one of them.\n",
    "date_formats = {\n",
    "    'iso': (lambda: f'{year}-{month:02d}-{day:02d}'),\n",
    "    'us': (lambda: f'{month}/{day}/{year}'),\n",
    "    'other': (lambda: f'{day}.{month}.{year}'),\n",
    "}\n",
    "dates = (\n",
    "    (2017, 3, 27),\n",
    "    (2017, 4, 24),\n",
    "    (2017, 5, 22),\n",
    ")\n",
    "\n",
    "for format_name, format in date_formats.items():\n",
    "    print(f'{format_name}:')\n",
    "    for year, month, day in dates:\n",
    "        print(format())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "deletable": true,
    "editable": true
   },
   "source": [
    "This also answers the question about\n",
    "[what good use there is](http://nbviewer.jupyter.org/github/james-prior/cohpy/blob/master/20170130-cohpy-fstrings.ipynb)\n",
    "for\n",
    "[Zak K (aka y2kbugger)](https://github.com/y2kbugger)'s\n",
    "crazy whacko f-string lambdas.\n",
    "Thanks [Zak](https://github.com/y2kbugger)!"
   ]
  }
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