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function annotation
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‎.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb‎

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{
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"metadata": {
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"name": "",
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"signature": "sha256:ef925e4eaf58c6daf50a495a77d38aa0b02a365dfc972bb5c0174332360b5adb"
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"nbformat_minor": 0,
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"- [List slicing using indexes that are \"out of range](#out_of_range_slicing)\n",
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"- [Reusing global variable names and UnboundLocalErrors](#unboundlocalerror)\n",
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"- [Creating copies of mutable objects](#copy_mutable)\n",
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"- [Key differences between Python 2 and 3](#python_differences)"
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"- [Key differences between Python 2 and 3](#python_differences)\n",
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"- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)"
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]
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},
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{
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='python_differences'></a>\n",
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"## Key differences between Python 2 and 3\n",
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"\n",
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"There are some good articles already that are summarizing the differences between Python 2 and 3, e.g., \n",
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"metadata": {},
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### Handling exceptions\n",
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"\n",
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"Also the handling of excecptions has slightly changed in Python 3. Now, we have to use the `as` keyword!"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"... except NameError as err:\n",
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"... print(err, '--> our error msg')\n",
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"... \n",
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"name 'blabla' is not defined --> our error msg\n"
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"name 'blabla' is not defined --> our error msg"
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],
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"language": "python",
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"metadata": {},
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### The `next()` function and `.next()` method\n",
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"\n",
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"Where you can use both function and method in Python 2.7.5, the `next()` function is all that remain in Python 3!"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"# Python 2\n",
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">>> my_generator = (letter for letter in 'abcdefg')\n",
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">>> my_generator.next()\n",
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"'a'\n",
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">>> next(my_generator)\n",
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"'b'\n",
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"\n",
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"# Python 3\n",
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">>> my_generator = (letter for letter in 'abcdefg')\n",
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">>> next(my_generator)\n",
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"'a'\n",
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">>> my_generator.next()\n",
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"Traceback (most recent call last):\n",
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" File \"<stdin>\", line 1, in <module>\n",
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"AttributeError: 'generator' object has no attribute 'next'"
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],
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"language": "python",
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"metadata": {},
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='function_annotation'></a>\n",
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"## Function annotations - What are those `->`'s in my Python code?\n"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Have you ever seen any Python code that used colons inside the parantheses of a function definition?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"def foo1(x: 'insert x here', y: 'insert x^2 here'):\n",
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" print('Hello, World')\n",
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" return"
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],
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"language": "python",
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"metadata": {},
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"outputs": [],
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"prompt_number": 8
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"And what about the fancy arrow here?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"def foo2(x, y) -> 'Hi!':\n",
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" print('Hello, World')\n",
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" return"
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],
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"language": "python",
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"metadata": {},
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"outputs": [],
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"prompt_number": 10
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Q: Is this valid Python syntax? \n",
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"A: Yes!\n",
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" \n",
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" \n",
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"Q: So, what happens if I *just call* the function? \n",
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"A: Nothing!\n",
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" \n",
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"Here is the proof!"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"foo1(1,2)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"Hello, World\n"
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]
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}
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],
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"prompt_number": 9
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"foo2(1,2) "
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"Hello, World\n"
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]
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}
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],
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"prompt_number": 11
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"**So, those are function annotations ... ** \n",
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"- the colon for the function parameters \n",
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"- the arrow for the return value \n",
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"\n",
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"You probably will never make use of them (or at least very rarely). Usually, we write good function documentations below the function as a docstring - or at least this is how I would do it (okay this case is a little bit extreme, I have to admit):"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"def is_palindrome(a):\n",
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" \"\"\"\n",
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" Case-and punctuation insensitive check if a string is a palindrom.\n",
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" \n",
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" Keyword arguments:\n",
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" a (str): The string to be checked if it is a palindrome.\n",
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" \n",
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" Returns `True` if input string is a palindrome, else False.\n",
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" \n",
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" \"\"\"\n",
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" stripped_str = [l for l in my_str.lower() if l.isalpha()]\n",
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" return stripped_str == stripped_str[::-1]\n",
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" "
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],
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"language": "python",
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"metadata": {},
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"outputs": []
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"However, function annotations can be useful to indicate that work is still in progress in some cases. But they are optional and I see them very very rarely.\n",
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"\n",
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"As it is stated in [PEP3107](http://legacy.python.org/dev/peps/pep-3107/#fundamentals-of-function-annotations):\n",
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"\n",
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"1. Function annotations, both for parameters and return values, are completely optional.\n",
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"\n",
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"2. Function annotations are nothing more than a way of associating arbitrary Python expressions with various parts of a function at compile-time.\n"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"The nice thing about function annotations is their `__annotations__` attribute, which is dictionary of all the parameters and/or the `return` value you annotated."
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"foo1.__annotations__"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"metadata": {},
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"output_type": "pyout",
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"prompt_number": 17,
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"text": [
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"{'y': 'insert x^2 here', 'x': 'insert x here'}"
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]
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}
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],
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"prompt_number": 17
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"foo2.__annotations__"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"metadata": {},
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"output_type": "pyout",
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"prompt_number": 18,
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"text": [
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"{'return': 'Hi!'}"
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]
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}
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],
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"prompt_number": 18
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"**When are they useful?**"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Function annotations can be useful for a couple of things \n",
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"- Documentation in general\n",
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"- pre-condition testing\n",
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"- [type checking](http://legacy.python.org/dev/peps/pep-0362/#annotation-checker)\n",
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" \n",
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"..."
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [],
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"language": "python",
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"metadata": {},
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"outputs": []

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