learn-python3/notebooks/beginner/functions.ipynb

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   "source": [
    "# Functions"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def my_first_function():\n",
    "    print('Hello world!')\n",
    "\n",
    "print('type: {}'.format(my_first_function))\n",
    "\n",
    "my_first_function()  # Calling a function"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Arguments"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "def greet_us(name1, name2):\n",
    "    print('Hello {} and {}!'.format(name1, name2))\n",
    "\n",
    "greet_us('John Doe', 'Superman')"
   ]
  },
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   "source": [
    "# Function with return value\n",
    "def strip_and_lowercase(original):\n",
    "    modified = original.strip().lower()\n",
    "    return modified\n",
    "\n",
    "uggly_string = '  MixED CaSe '\n",
    "pretty = strip_and_lowercase(uggly_string)\n",
    "print('pretty: {}'.format(pretty))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Keyword arguments"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def my_fancy_calculation(first, second, third):\n",
    "    return first + second - third \n",
    "\n",
    "print(my_fancy_calculation(3, 2, 1))\n",
    "\n",
    "print(my_fancy_calculation(first=3, second=2, third=1))\n",
    "\n",
    "# With keyword arguments you can mix the order\n",
    "print(my_fancy_calculation(third=1, first=3, second=2))\n",
    "\n",
    "# You can mix arguments and keyword arguments but you have to start with arguments\n",
    "print(my_fancy_calculation(3, third=1, second=2))  "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Default arguments"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "scrolled": true
   },
   "outputs": [],
   "source": [
    "def create_person_info(name, age, job=None, salary=300):\n",
    "    info = {'name': name, 'age': age, 'salary': salary}\n",
    "    \n",
    "    # Add 'job' key only if it's provided as parameter\n",
    "    if job:  \n",
    "        info.update(dict(job=job))\n",
    "        \n",
    "    return info\n",
    "\n",
    "person1 = create_person_info('John Doe', 82)  # use default values for job and salary\n",
    "person2 = create_person_info('Lisa Doe', 22, 'hacker', 10000)\n",
    "print(person1)\n",
    "print(person2)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "**Don't use mutable objects as default arguments!**"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def append_if_multiple_of_five(number, magical_list=[]):\n",
    "    if number % 5 == 0:\n",
    "        magical_list.append(number)\n",
    "    return magical_list\n",
    "\n",
    "print(append_if_multiple_of_five(100))\n",
    "print(append_if_multiple_of_five(105))\n",
    "print(append_if_multiple_of_five(123))\n",
    "print(append_if_multiple_of_five(123, []))\n",
    "print(append_if_multiple_of_five(123))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Here's how you can achieve desired behavior:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def append_if_multiple_of_five(number, magical_list=None):\n",
    "    if not magical_list:\n",
    "        magical_list = []\n",
    "    if number % 5 == 0:\n",
    "        magical_list.append(number)\n",
    "    return magical_list\n",
    "\n",
    "print(append_if_multiple_of_five(100))\n",
    "print(append_if_multiple_of_five(105))\n",
    "print(append_if_multiple_of_five(123))\n",
    "print(append_if_multiple_of_five(123, []))\n",
    "print(append_if_multiple_of_five(123))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Docstrings\n",
    "Strings for documenting your functions, methods, modules and variables."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def print_sum(val1, val2):\n",
    "    \"\"\"Function which prints the sum of given arguments.\"\"\"\n",
    "    print('sum: {}'.format(val1 + val2))\n",
    "\n",
    "print(help(print_sum))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "def calculate_sum(val1, val2):\n",
    "    \"\"\"This is a longer docstring defining also the args and the return value. \n",
    "\n",
    "    Args:\n",
    "        val1: The first parameter.\n",
    "        val2: The second parameter.\n",
    "\n",
    "    Returns:\n",
    "        The sum of val1 and val2.\n",
    "        \n",
    "    \"\"\"\n",
    "    return val1 + val2\n",
    "\n",
    "print(help(calculate_sum))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### [`pass`](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement) statement\n",
    "`pass` is a statement which does nothing when it's executed. It can be used e.g. a as placeholder to make the code syntatically correct while sketching the functions and/or classes of your application. For example, the following is valid Python. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def my_function(some_argument):\n",
    "    pass\n",
    "\n",
    "def my_other_function():\n",
    "    pass"
   ]
  }
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