flatten.ipynb

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   "source": [
    "## My understanding\n",
    "In Python, you can concatinate lists by using `+`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 2, 3, 4, 5]"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
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   "source": [
    "[1, 2] + [3, 4, 5]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['a', 'b', ['c', 'd'], 'e', 'f']"
      ]
     },
     "execution_count": 2,
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   "source": [
    "['a', 'b', ['c', 'd']] + ['e', 'f']"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "When I saw `sum(texts, [])` I automatically imagined `texts + []` "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[['a'], ['b']]"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "texts = [['a'], ['b']]\n",
    "texts + []"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "which would not change dimentionality in anyway. Then I looked at:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [],
   "source": [
    "?? sum"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "```\n",
    "Signature: sum(iterable, start=0, /)\n",
    "Docstring:\n",
    "Return the sum of a 'start' value (default: 0) plus an iterable of numbers\n",
    "\n",
    "When the iterable is empty, return the start value.\n",
    "This function is intended specifically for use with numeric values and may\n",
    "reject non-numeric types.\n",
    "Type:      builtin_function_or_method\n",
    "```\n",
    "\n",
    "\n",
    "And experimented with `sum`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "6"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
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   ],
   "source": [
    "sum([1, 2], 3)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "ename": "TypeError",
     "evalue": "sum() can't sum strings [use ''.join(seq) instead]",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0mTraceback (most recent call last)",
      "\u001b[0;32m<ipython-input-6-558d097dd53c>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0msum\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;34m'a'\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'b'\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m'c'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m: sum() can't sum strings [use ''.join(seq) instead]"
     ]
    }
   ],
   "source": [
    "sum(['a', 'b'], 'c')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### I believe `sum` does the following:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "6"
      ]
     },
     "execution_count": 7,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "iterable = [1, 2]\n",
    "start = 3\n",
    "\n",
    "for i in iterable: start += i\n",
    "    \n",
    "start"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "So when you do:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['a', 'b', ['c']]"
      ]
     },
     "execution_count": 8,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "sum([['a'], ['b'], [['c']]], [])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "It is equivalent to:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "['a', 'b', ['c']]"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "[] + ['a'] + ['b'] + [['c']]"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "and dimentionality is reduced by 1"
   ]
  }
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