python3_yield.ipynb

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   "source": [
    "# Yield Generator"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## What 's Generator ?\n",
    "- S generator is a function that returns an object (iterator) which we can iterate over (one value at a time).\n",
    "\n",
    "- Ex: class with __iter()__, __next__(), **yield**\n",
    "\n",
    "## Yield Feature\n",
    "- If a function contains at least one yield statement (it may contain other yield or return statements), it becomes a generator function. Both yield and return will return some value from a function.\n",
    "\n",
    "## Yield vs Return \n",
    "- The difference is that, while a return statement terminates a function entirely, yield statement pauses the function saving all its states and later continues from there on successive calls."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Example : Fibonacci series"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### print"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n"
     ]
    }
   ],
   "source": [
    "def fab(max): \n",
    "    n, a, b = 0, 0, 1 \n",
    "    while n < max: \n",
    "        print(b) \n",
    "        a, b = b, a + b \n",
    "        n = n + 1\n",
    "fab(5)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- Defect\n",
    "    1. Bad for repetitive usage\n",
    "    2. other variable or function cannot access the result"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### return "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n"
     ]
    }
   ],
   "source": [
    "def fab(max): \n",
    "    n, a, b = 0, 0, 1 \n",
    "    L = [] \n",
    "    while n < max: \n",
    "        L.append(b) \n",
    "        a, b = b, a + b \n",
    "        n = n + 1 \n",
    "    return L\n",
    "\n",
    "for n in fab(5): \n",
    "    print(n) "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- Defect\n",
    "    - Memory problem (if the size reach high)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Class and Generator ( implement __iter() and next)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Fab(object): \n",
    "    \n",
    "    def __init__(self, max): \n",
    "        self.max = max \n",
    "        self.n, self.a, self.b = 0, 0, 1 \n",
    "\n",
    "    def __iter__(self): \n",
    "        return self \n",
    "\n",
    "    def next(self): \n",
    "        if self.n < self.max: \n",
    "            r = self.b \n",
    "            self.a, self.b = self.b, self.a + self.b \n",
    "            self.n = self.n + 1 \n",
    "            return r \n",
    "        raise StopIteration()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "- Defect\n",
    "    - Not clear"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Yield"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n"
     ]
    }
   ],
   "source": [
    "def fab(max): \n",
    "    n, a, b = 0, 0, 1 \n",
    "    while n < max: \n",
    "        yield b \n",
    "        a, b = b, a + b \n",
    "        n = n + 1\n",
    "\n",
    "for n in fab(5): \n",
    "    print(n) "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "1. Yield function is not a merely a function, it's generator\n",
    "2. call fab(5) won't execute fab, instead, it return a interable\n",
    "3. when calling the fab, it executes until yield function and return. Then, continues to execute"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1\n",
      "1\n",
      "2\n",
      "3\n",
      "5\n",
      "1\n",
      "1\n",
      "2\n"
     ]
    }
   ],
   "source": [
    "f = fab(5)\n",
    "print(next(f))\n",
    "print(next(f))\n",
    "print(next(f))\n",
    "print(next(f))\n",
    "print(next(f))\n",
    "\n",
    "for i in fab(3):\n",
    "    print(i)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Example read_file memory"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {},
   "outputs": [],
   "source": [
    "def read_file(fpath): \n",
    "   BLOCK_SIZE = 1024 \n",
    "   with open(fpath, 'rb') as f: \n",
    "       while True: \n",
    "           block = f.read(BLOCK_SIZE) \n",
    "           if block: \n",
    "               yield block \n",
    "           else: \n",
    "               return"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "reference :\n",
    "1. https://www.ibm.com/developerworks/cn/opensource/os-cn-python-yield/index.html\n",
    "2. https://www.programiz.com/python-programming/generator"
   ]
  }
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