diff --git a/12_functions_advanced.ipynb b/12_functions_advanced.ipynb
index 3950c49b..42d4a39d 100644
--- a/12_functions_advanced.ipynb
+++ b/12_functions_advanced.ipynb
@@ -56,9 +56,7 @@
   {
    "cell_type": "markdown",
    "id": "2",
-   "metadata": {
-    "jp-MarkdownHeadingCollapsed": true
-   },
+   "metadata": {},
    "source": [
     "## Recap"
    ]
@@ -295,7 +293,9 @@
   {
    "cell_type": "markdown",
    "id": "19",
-   "metadata": {},
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "## Mutable objects as default values of function's parameters"
    ]
@@ -745,7 +745,9 @@
   {
    "cell_type": "markdown",
    "id": "57",
-   "metadata": {},
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "## Lambdas"
    ]
@@ -2005,7 +2007,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "f = incrementer(2)\n",
+    "f = incrementer(n=2)\n",
     "f.__code__.co_freevars"
    ]
   },
@@ -2043,14 +2045,16 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "inc_10 = incrementer(10)(100)\n",
+    "inc_10 = incrementer(n=10)(start=100)\n",
     "inc_10()"
    ]
   },
   {
    "cell_type": "markdown",
    "id": "177",
-   "metadata": {},
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "### Quiz: Closures"
    ]
@@ -2072,7 +2076,9 @@
   {
    "cell_type": "markdown",
    "id": "179",
-   "metadata": {},
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "### Closures: examples"
    ]
@@ -2511,7 +2517,7 @@
    },
    "outputs": [],
    "source": [
-    "def fcounter(function):\n",
+    "def function_counter(function):\n",
     "    count = 0\n",
     "\n",
     "    def inner(*args, **kwargs):\n",
@@ -2556,7 +2562,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "counter_fact = fcounter(factorial)"
+    "counter_fact = function_counter(factorial)"
    ]
   },
   {
@@ -2627,7 +2633,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "@fcounter\n",
+    "@function_counter\n",
     "def mult(a: float, b: float) -> float:\n",
     "    \"\"\"Multiplies two floats\"\"\"\n",
     "    return a * b"
@@ -2895,13 +2901,21 @@
    "id": "250",
    "metadata": {},
    "source": [
-    "#### Example 1: timer"
+    "Check out also the notebook in the `extra/` folder: [Decorators example: retry & validate](./extra/12a_decorators_retry_validate.ipynb) for two examples of usage patterns built from scratch."
    ]
   },
   {
    "cell_type": "markdown",
    "id": "251",
    "metadata": {},
+   "source": [
+    "#### Example 1: timer"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "252",
+   "metadata": {},
    "source": [
     "This is classic example of using decorators: creating a timer for a generic function."
    ]
@@ -2909,7 +2923,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "252",
+   "id": "253",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -2943,7 +2957,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "253",
+   "id": "254",
    "metadata": {},
    "source": [
     "Let's test it with a function to calculate the n-th Fibonacci number: `1, 1, 2, 3, 5, 8, 11, ...`\n",
@@ -2959,7 +2973,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "254",
+   "id": "255",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -2970,7 +2984,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "255",
+   "id": "256",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -2983,7 +2997,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "256",
+   "id": "257",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -2993,7 +3007,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "257",
+   "id": "258",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3003,7 +3017,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "258",
+   "id": "259",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3015,7 +3029,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "259",
+   "id": "260",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3025,7 +3039,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "260",
+   "id": "261",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3035,7 +3049,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "261",
+   "id": "262",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3046,7 +3060,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "262",
+   "id": "263",
    "metadata": {},
    "source": [
     "Sounds a bit long, doesn't it?\n",
@@ -3056,7 +3070,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "263",
+   "id": "264",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3067,7 +3081,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "264",
+   "id": "265",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3085,7 +3099,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "265",
+   "id": "266",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3095,7 +3109,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "266",
+   "id": "267",
    "metadata": {},
    "source": [
     "Incredibly more efficient!\n",
@@ -3104,7 +3118,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "267",
+   "id": "268",
    "metadata": {},
    "source": [
     "##### Fibonacci using `reduce`"
@@ -3112,7 +3126,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "268",
+   "id": "269",
    "metadata": {},
    "source": [
     "First, a quick refresher:"
@@ -3121,7 +3135,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "269",
+   "id": "270",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3131,7 +3145,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "270",
+   "id": "271",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3142,7 +3156,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "271",
+   "id": "272",
    "metadata": {},
    "source": [
     "It's just the progressive sum of pairs of numbers.\n",
@@ -3155,7 +3169,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "272",
+   "id": "273",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3190,7 +3204,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "273",
+   "id": "274",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3204,7 +3218,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "274",
+   "id": "275",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3214,7 +3228,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "275",
+   "id": "276",
    "metadata": {},
    "source": [
     "If we compare the three methods:"
@@ -3223,7 +3237,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "276",
+   "id": "277",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3236,7 +3250,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "277",
+   "id": "278",
    "metadata": {},
    "source": [
     "Although the recursive method is the __easiest__ to understand, it's also the slowest because it's written inefficiently.\n",
@@ -3245,7 +3259,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "278",
+   "id": "279",
    "metadata": {},
    "source": [
     "#### Example 2: memoization"
@@ -3253,7 +3267,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "279",
+   "id": "280",
    "metadata": {},
    "source": [
     "The previous example showed one task that a decorator can accomplish pretty well: adding some feature to a predefined function.\n",
@@ -3262,7 +3276,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "280",
+   "id": "281",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3274,7 +3288,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "281",
+   "id": "282",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3286,7 +3300,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "282",
+   "id": "283",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3297,7 +3311,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "283",
+   "id": "284",
    "metadata": {},
    "source": [
     "You can see that `fib(2)` is calculated **three times**.\n",
@@ -3307,7 +3321,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "284",
+   "id": "285",
    "metadata": {},
    "source": [
     "We'll see how we can improve this approach using a decorator and a caching mechanism for previously calculated numbers.\n",
@@ -3316,7 +3330,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "285",
+   "id": "286",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3327,7 +3341,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "286",
+   "id": "287",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3345,7 +3359,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "287",
+   "id": "288",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3356,7 +3370,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "288",
+   "id": "289",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3367,7 +3381,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "289",
+   "id": "290",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3380,7 +3394,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "290",
+   "id": "291",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3400,7 +3414,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "291",
+   "id": "292",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3411,7 +3425,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "292",
+   "id": "293",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3420,7 +3434,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "293",
+   "id": "294",
    "metadata": {},
    "source": [
     "Once again, cached valued are just returned and not recalculated."
@@ -3428,7 +3442,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "294",
+   "id": "295",
    "metadata": {},
    "source": [
     "How can we implement this as a decorator?"
@@ -3437,7 +3451,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "295",
+   "id": "296",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3459,7 +3473,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "296",
+   "id": "297",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3472,7 +3486,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "297",
+   "id": "298",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3482,7 +3496,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "298",
+   "id": "299",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3492,7 +3506,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "299",
+   "id": "300",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3503,7 +3517,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "300",
+   "id": "301",
    "metadata": {},
    "source": [
     "`fib(6)` was literally instantaneous because we already had it in the cache."
@@ -3511,7 +3525,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "301",
+   "id": "302",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3523,7 +3537,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "302",
+   "id": "303",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3543,7 +3557,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "303",
+   "id": "304",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3554,7 +3568,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "304",
+   "id": "305",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3567,7 +3581,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "305",
+   "id": "306",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3577,7 +3591,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "306",
+   "id": "307",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3587,7 +3601,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "307",
+   "id": "308",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3596,7 +3610,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "308",
+   "id": "309",
    "metadata": {},
    "source": [
     "Caching and decorators play a crucial role in optimizing function performance.\n",
@@ -3609,7 +3623,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "309",
+   "id": "310",
    "metadata": {},
    "source": [
     "Additionally, our current implementation does not handle keyword arguments (`**kwargs`), which can be a significant limitation in more complex scenarios.\n",
@@ -3623,7 +3637,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "310",
+   "id": "311",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3633,7 +3647,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "311",
+   "id": "312",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3646,7 +3660,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "312",
+   "id": "313",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3656,7 +3670,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "313",
+   "id": "314",
    "metadata": {},
    "source": [
     "Once again, the last value `fact(8)` was simply fetched from the cache."
@@ -3664,7 +3678,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "314",
+   "id": "315",
    "metadata": {},
    "source": [
     "Now let's see if we have improved on our recursive approach of calculating Fibonacci numbers.\n",
@@ -3674,7 +3688,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "315",
+   "id": "316",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3684,7 +3698,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "316",
+   "id": "317",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3695,7 +3709,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "317",
+   "id": "318",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3709,7 +3723,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "318",
+   "id": "319",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3721,7 +3735,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "319",
+   "id": "320",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3734,7 +3748,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "320",
+   "id": "321",
    "metadata": {},
    "source": [
     "It's about **4 orders of magnitude** faster than the naive approach! 🔥\n",
@@ -3744,7 +3758,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "321",
+   "id": "322",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3758,7 +3772,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "322",
+   "id": "323",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3773,7 +3787,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "323",
+   "id": "324",
    "metadata": {},
    "source": [
     "Not the same time, but about the same order of magnitude.\n",
@@ -3782,7 +3796,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "324",
+   "id": "325",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3795,7 +3809,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "325",
+   "id": "326",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3808,7 +3822,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "326",
+   "id": "327",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3818,7 +3832,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "327",
+   "id": "328",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3828,7 +3842,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "328",
+   "id": "329",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3837,7 +3851,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "329",
+   "id": "330",
    "metadata": {},
    "source": [
     "We had to recalculate `fib(1)` because when we called `fib(9)` the least recent item in the cache (the result of `fib(1)`) was evicted from the cache."
@@ -3845,7 +3859,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "330",
+   "id": "331",
    "metadata": {},
    "source": [
     "### Parametrized decorators"
@@ -3853,7 +3867,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "331",
+   "id": "332",
    "metadata": {},
    "source": [
     "Here comes a natural question: what if I need to pass some argument to my decorator?\n",
@@ -3862,7 +3876,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "332",
+   "id": "333",
    "metadata": {},
    "source": [
     "Let's bring back our `timed` decorator and make a small change.\n",
@@ -3872,7 +3886,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "333",
+   "id": "334",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3901,7 +3915,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "334",
+   "id": "335",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3917,7 +3931,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "335",
+   "id": "336",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3928,7 +3942,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "336",
+   "id": "337",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -3943,7 +3957,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "337",
+   "id": "338",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3968,7 +3982,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "338",
+   "id": "339",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3982,7 +3996,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "339",
+   "id": "340",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -3991,7 +4005,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "340",
+   "id": "341",
    "metadata": {},
    "source": [
     "But wait: why did we use the fancy `@-` syntax?\n",
@@ -4000,7 +4014,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "341",
+   "id": "342",
    "metadata": {},
    "source": [
     "To fix this behavior we need to rethink of what `@` is doing.\n",
@@ -4024,7 +4038,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "342",
+   "id": "343",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4034,7 +4048,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "343",
+   "id": "344",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4043,7 +4057,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "344",
+   "id": "345",
    "metadata": {},
    "source": [
     "So, for the syntax `@timed(10)` to work, where `10` is the number of repetition, `timed` should return **a decorator itself**, and not our closure.\n",
@@ -4053,7 +4067,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "345",
+   "id": "346",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4086,7 +4100,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "346",
+   "id": "347",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4102,7 +4116,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "347",
+   "id": "348",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4112,7 +4126,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "348",
+   "id": "349",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4132,7 +4146,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "349",
+   "id": "350",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4141,7 +4155,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "350",
+   "id": "351",
    "metadata": {},
    "source": [
     "And yes, you can **stack multiple decorators**! 😎"
@@ -4149,7 +4163,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "351",
+   "id": "352",
    "metadata": {},
    "source": [
     "### Quiz: Decorators"
@@ -4158,7 +4172,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "352",
+   "id": "353",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4171,15 +4185,17 @@
   },
   {
    "cell_type": "markdown",
-   "id": "353",
-   "metadata": {},
+   "id": "354",
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "## Generators"
    ]
   },
   {
    "cell_type": "markdown",
-   "id": "354",
+   "id": "355",
    "metadata": {},
    "source": [
     "The concept of generators is very much tied to that of \"looping over some kind of container\".\n",
@@ -4196,7 +4212,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "355",
+   "id": "356",
    "metadata": {},
    "source": [
     "To understand generators, we first need to review what it means to be **iterable** and, more importantly, what is an **iterator**.\n",
@@ -4207,7 +4223,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "356",
+   "id": "357",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4220,7 +4236,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "357",
+   "id": "358",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4243,7 +4259,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "358",
+   "id": "359",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4255,7 +4271,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "359",
+   "id": "360",
    "metadata": {},
    "source": [
     "We see that we can indeed loop over our custom `Squares` class.\n",
@@ -4268,7 +4284,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "360",
+   "id": "361",
    "metadata": {},
    "source": [
     "As you might have learned by now, we can implement some built-in behavior in our classes by using the so-called \"special methods\" or **dunder methods**: `__method__`.\n",
@@ -4282,7 +4298,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "361",
+   "id": "362",
    "metadata": {},
    "source": [
     "Python also has the built-in `next()` which does what you think: it takes an **iterator** object and returns the **next element** in the stream of data by calling the `__next__` method implemented by that object.\n",
@@ -4295,7 +4311,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "362",
+   "id": "363",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4306,7 +4322,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "363",
+   "id": "364",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4327,7 +4343,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "364",
+   "id": "365",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4339,7 +4355,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "365",
+   "id": "366",
    "metadata": {},
    "source": [
     "If the value returned by `square()` is 25 (our sentinel), then a `StopIteration` is raised."
@@ -4347,7 +4363,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "366",
+   "id": "367",
    "metadata": {},
    "source": [
     "These two ways are identical: in the first case (the class), we built the iterator ourselves. In the second case, Python built it for us.\n",
@@ -4358,7 +4374,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "367",
+   "id": "368",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4377,7 +4393,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "368",
+   "id": "369",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4393,7 +4409,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "369",
+   "id": "370",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4403,7 +4419,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "370",
+   "id": "371",
    "metadata": {},
    "source": [
     "Here it is: our function returned _something_ different than the usual \"function\" object.\n",
@@ -4413,7 +4429,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "371",
+   "id": "372",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4423,7 +4439,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "372",
+   "id": "373",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4433,7 +4449,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "373",
+   "id": "374",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4443,7 +4459,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "374",
+   "id": "375",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4452,7 +4468,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "375",
+   "id": "376",
    "metadata": {},
    "source": [
     "A `StopIteration` is raised if we are trying to go past the last `yield` statement.\n",
@@ -4465,7 +4481,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "376",
+   "id": "377",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4474,7 +4490,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "377",
+   "id": "378",
    "metadata": {},
    "source": [
     "And also the `__next__` method"
@@ -4483,7 +4499,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "378",
+   "id": "379",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4492,7 +4508,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "379",
+   "id": "380",
    "metadata": {},
    "source": [
     "We can also check that `iter()` applied on our object returns indeed the same thing.\n",
@@ -4502,7 +4518,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "380",
+   "id": "381",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4512,7 +4528,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "381",
+   "id": "382",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4521,7 +4537,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "382",
+   "id": "383",
    "metadata": {},
    "source": [
     "Precisely the same object."
@@ -4529,7 +4545,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "383",
+   "id": "384",
    "metadata": {},
    "source": [
     "How does Python know when to stop the iteration?\n",
@@ -4546,7 +4562,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "384",
+   "id": "385",
    "metadata": {},
    "source": [
     "Here's a generator that yields numbers and returns a message at the end:"
@@ -4555,7 +4571,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "385",
+   "id": "386",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -4570,7 +4586,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "386",
+   "id": "387",
    "metadata": {},
    "source": [
     "When we iterate through the generator in a for loop, we only see the yielded values:"
@@ -4579,7 +4595,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "387",
+   "id": "388",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4589,7 +4605,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "388",
+   "id": "389",
    "metadata": {},
    "source": [
     "But we can capture the return value by catching the StopIteration exception:"
@@ -4598,7 +4614,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "389",
+   "id": "390",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -4615,7 +4631,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "390",
+   "id": "391",
    "metadata": {},
    "source": [
     "This ability to attach return values to generators is used internally by Python features like `yield from`.\n",
@@ -4626,7 +4642,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "391",
+   "id": "392",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -4642,7 +4658,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "392",
+   "id": "393",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -4658,7 +4674,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "393",
+   "id": "394",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4669,7 +4685,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "394",
+   "id": "395",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4683,7 +4699,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "395",
+   "id": "396",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4700,7 +4716,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "396",
+   "id": "397",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4711,7 +4727,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "397",
+   "id": "398",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4721,7 +4737,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "398",
+   "id": "399",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4731,7 +4747,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "399",
+   "id": "400",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4742,7 +4758,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "400",
+   "id": "401",
    "metadata": {},
    "source": [
     "Note how in the generator function above we incremented the number `i` **after** the `yield` statement.\n",
@@ -4751,7 +4767,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "401",
+   "id": "402",
    "metadata": {},
    "source": [
     "### Create an interable from a generator"
@@ -4759,7 +4775,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "402",
+   "id": "403",
    "metadata": {},
    "source": [
     "As we know, generators are iterators.\n",
@@ -4771,7 +4787,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "403",
+   "id": "404",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4782,7 +4798,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "404",
+   "id": "405",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4794,7 +4810,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "405",
+   "id": "406",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4804,7 +4820,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "406",
+   "id": "407",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4814,7 +4830,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "407",
+   "id": "408",
    "metadata": {},
    "source": [
     "But our generator is now exhausted and it has nothing left to return:"
@@ -4823,7 +4839,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "408",
+   "id": "409",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4834,7 +4850,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "409",
+   "id": "410",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4846,7 +4862,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "410",
+   "id": "411",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4861,7 +4877,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "411",
+   "id": "412",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4871,7 +4887,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "412",
+   "id": "413",
    "metadata": {},
    "source": [
     "And we can do it again:"
@@ -4880,7 +4896,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "413",
+   "id": "414",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4891,7 +4907,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "414",
+   "id": "415",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4902,7 +4918,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "415",
+   "id": "416",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4922,7 +4938,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "416",
+   "id": "417",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4932,7 +4948,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "417",
+   "id": "418",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4942,7 +4958,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "418",
+   "id": "419",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4953,7 +4969,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "419",
+   "id": "420",
    "metadata": {},
    "source": [
     "### Combining generators"
@@ -4961,7 +4977,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "420",
+   "id": "421",
    "metadata": {
     "lines_to_next_cell": 2
    },
@@ -4973,7 +4989,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "421",
+   "id": "422",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4985,7 +5001,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "422",
+   "id": "423",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -4995,7 +5011,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "423",
+   "id": "424",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5004,7 +5020,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "424",
+   "id": "425",
    "metadata": {},
    "source": [
     "Now, `enumerate` builds a generator itself, so `sq` had not been consumed yet at this point:"
@@ -5013,7 +5029,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "425",
+   "id": "426",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5023,7 +5039,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "426",
+   "id": "427",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5032,7 +5048,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "427",
+   "id": "428",
    "metadata": {},
    "source": [
     "But since we now have consumed **2 elements** from `sq`, when we use `enumerate` it will also have two less items from `sq`:"
@@ -5041,7 +5057,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "428",
+   "id": "429",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -5052,7 +5068,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "429",
+   "id": "430",
    "metadata": {},
    "source": [
     "And this might not be what you expected: the value is the **third** element of `sq` ($2^2$), while the index is `0`, as if we were starting from the beginning.\n",
@@ -5063,7 +5079,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "430",
+   "id": "431",
    "metadata": {},
    "source": [
     "### Beware: Generators can only be consumed once!\n",
@@ -5074,7 +5090,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "431",
+   "id": "432",
    "metadata": {
     "lines_to_next_cell": 1
    },
@@ -5088,7 +5104,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "432",
+   "id": "433",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5100,7 +5116,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "433",
+   "id": "434",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5113,7 +5129,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "434",
+   "id": "435",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5125,7 +5141,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "435",
+   "id": "436",
    "metadata": {},
    "source": [
     "Compare this with a list, which can be iterated multiple times:"
@@ -5134,7 +5150,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "436",
+   "id": "437",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5146,7 +5162,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "437",
+   "id": "438",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5159,7 +5175,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "438",
+   "id": "439",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5171,7 +5187,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "439",
+   "id": "440",
    "metadata": {},
    "source": [
     "If you need to iterate through generator values multiple times, convert it to a list first:\n",
@@ -5184,7 +5200,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "440",
+   "id": "441",
    "metadata": {},
    "source": [
     "### Quiz: Generators"
@@ -5193,7 +5209,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "441",
+   "id": "442",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5204,7 +5220,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "442",
+   "id": "443",
    "metadata": {},
    "source": [
     "## Exercises"
@@ -5213,7 +5229,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "443",
+   "id": "444",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5222,7 +5238,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "444",
+   "id": "445",
    "metadata": {},
    "source": [
     "### Exercise 1: Password checker factory"
@@ -5230,7 +5246,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "445",
+   "id": "446",
    "metadata": {
     "jp-MarkdownHeadingCollapsed": true
    },
@@ -5254,7 +5270,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "446",
+   "id": "447",
    "metadata": {},
    "source": [
     "For example, to create a password checker that requires a password to have at least 2 uppercase letters, at least 3 lowercase letters, at least 1 punctuation mark, and at least 4 digits, we can write\n",
@@ -5284,7 +5300,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "447",
+   "id": "448",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5310,20 +5326,24 @@
     "        min_dig: Minimum number of digits\n",
     "    Returns:\n",
     "        - A function that checks if a password meets the criteria\n",
-    "    \"\"\""
+    "    \"\"\"\n",
+    "    def noop(*args, **kwargs): ...\n",
+    "    return noop"
    ]
   },
   {
    "cell_type": "markdown",
-   "id": "448",
-   "metadata": {},
+   "id": "449",
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "### Exercise 2: String range"
    ]
   },
   {
    "cell_type": "markdown",
-   "id": "449",
+   "id": "450",
    "metadata": {},
    "source": [
     "Complete the function called `solution_str_range` so that it emulates the built-in `range`, but for characters.\n",
@@ -5337,7 +5357,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "450",
+   "id": "451",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-info\">\n",
@@ -5347,7 +5367,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "451",
+   "id": "452",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-warning\">\n",
@@ -5358,7 +5378,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "452",
+   "id": "453",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5381,15 +5401,17 @@
   },
   {
    "cell_type": "markdown",
-   "id": "453",
-   "metadata": {},
+   "id": "454",
+   "metadata": {
+    "jp-MarkdownHeadingCollapsed": true
+   },
    "source": [
     "### Exercise 3: Read `n` lines"
    ]
   },
   {
    "cell_type": "markdown",
-   "id": "454",
+   "id": "455",
    "metadata": {},
    "source": [
     "Create a function called `read_n_lines` that takes two arguments: the filename from which to read, and the **maximum number of lines** that should be returned with each iteration.\n",
@@ -5433,7 +5455,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "455",
+   "id": "456",
    "metadata": {},
    "source": [
     "We could also do:\n",
@@ -5459,7 +5481,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "456",
+   "id": "457",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-warning\">\n",
@@ -5470,7 +5492,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "457",
+   "id": "458",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5489,7 +5511,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "458",
+   "id": "459",
    "metadata": {},
    "source": [
     "### Exercise 4: Only run once"
@@ -5497,7 +5519,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "459",
+   "id": "460",
    "metadata": {},
    "source": [
     "Create a decorator called `solution_once` that restricts a function to run at most **once every `allowed_time` seconds**, where `allowed_time` is a parameter with a default value of `15`.\n",
@@ -5508,7 +5530,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "460",
+   "id": "461",
    "metadata": {},
    "source": [
     "For example, the following code:\n",
@@ -5532,7 +5554,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "461",
+   "id": "462",
    "metadata": {},
    "source": [
     "Should print something like:\n",
@@ -5557,7 +5579,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "462",
+   "id": "463",
    "metadata": {},
    "source": [
     "<div class=\"alert alert-block alert-warning\">\n",
@@ -5576,7 +5598,7 @@
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "463",
+   "id": "464",
    "metadata": {},
    "outputs": [],
    "source": [
@@ -5597,8 +5619,17 @@
     "        allowed_time: The time in seconds to wait before allowing the function to run again. Default is 15 seconds.\n",
     "    Returns:\n",
     "        - A decorator that runs the function at most once per given seconds\n",
-    "    \"\"\""
+    "    \"\"\"\n",
+    "    return"
    ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "465",
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
@@ -5620,7 +5651,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.12.10"
+   "version": "3.12.11"
   }
  },
  "nbformat": 4,
diff --git a/extra/12a_decorators_retry_validate.ipynb b/extra/12a_decorators_retry_validate.ipynb
new file mode 100644
index 00000000..576c5de5
--- /dev/null
+++ b/extra/12a_decorators_retry_validate.ipynb
@@ -0,0 +1,591 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "0",
+   "metadata": {},
+   "source": [
+    "# Decorators example\n",
+    "## Retry & Validate\n",
+    "\n",
+    "In this notebook we build two practical, parametrized decorators and stack them together.\n",
+    "The goal is to see how decorators let us compose **different concerns** (like retrying on failure and validating inputs) without touching the function body.\n",
+    "\n",
+    "**Prerequisites:** closures, `functools.wraps`, decorator factories, stacking\n",
+    "(see [Functions (advanced)](../12_functions_advanced.ipynb))."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "1",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from functools import wraps\n",
+    "import time\n",
+    "import random"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "2",
+   "metadata": {},
+   "source": [
+    "## 1. The `@retry` decorator"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "3",
+   "metadata": {},
+   "source": [
+    "Network calls fail, file I/O can be flaky, external services go down.\n",
+    "Instead of writing a try/except loop every time we call an unreliable function, we can wrap it once with a decorator.\n",
+    "\n",
+    "Let's start with a function that simulates a flaky service:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "4",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import random\n",
+    "\n",
+    "random.seed(None)\n",
+    "\n",
+    "\n",
+    "def flaky_function():\n",
+    "    \"\"\"Simulate an unreliable network call.\"\"\"\n",
+    "    if random.random() < 0.6:\n",
+    "        raise ConnectionError(\"Server not responding\")\n",
+    "    return {\"status\": \"ok\", \"data\": 42}"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "5",
+   "metadata": {},
+   "source": [
+    "Try calling it a few times — sometimes it works, sometimes it doesn't:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "6",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "for i in range(5):\n",
+    "    try:\n",
+    "        result = flaky_function()\n",
+    "        print(f\"Call {i + 1}: {result}\")\n",
+    "    except ConnectionError as e:\n",
+    "        print(f\"Call {i + 1}: FAILED — {e}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "7",
+   "metadata": {},
+   "source": [
+    "We *could* wrap every call site in a retry loop, but that quickly becomes tedious and clutters our code.\n",
+    "Let's build a decorator instead."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "8",
+   "metadata": {},
+   "source": [
+    "### First version: simple retry (hard-coded)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "9",
+   "metadata": {
+    "lines_to_next_cell": 1
+   },
+   "outputs": [],
+   "source": [
+    "def retry_simple(fn):\n",
+    "    \"\"\"Retry a function up to 3 times with a 0.5s delay.\"\"\"\n",
+    "\n",
+    "    @wraps(fn)\n",
+    "    def wrapper(*args, **kwargs):\n",
+    "        last_exception = None\n",
+    "        for attempt in range(1, 4):\n",
+    "            try:\n",
+    "                return fn(*args, **kwargs)\n",
+    "            except Exception as e:\n",
+    "                last_exception = e\n",
+    "                print(f\"  Attempt {attempt} failed: {e}\")\n",
+    "                time.sleep(0.5)\n",
+    "        raise last_exception\n",
+    "\n",
+    "    return wrapper\n",
+    "\n",
+    "\n",
+    "@retry_simple\n",
+    "def flaky_function():\n",
+    "    if random.random() < 0.6:\n",
+    "        raise ConnectionError(\"Server not responding\")\n",
+    "    return {\"status\": \"ok\", \"data\": 42}\n",
+    "\n",
+    "\n",
+    "flaky_function()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "10",
+   "metadata": {},
+   "source": [
+    "This works, but the retry count, delay, and which exceptions to catch are all hard-coded.\n",
+    "Let's make it configurable — we need a **decorator factory**."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "11",
+   "metadata": {},
+   "source": [
+    "### Parametrized `@retry`"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "12",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def retry(max_attempts=3, delay=0.5, exceptions=(Exception,)):\n",
+    "    \"\"\"\n",
+    "    Decorator factory: retry a function up to `max_attempts` times,\n",
+    "    waiting `delay` seconds between attempts.\n",
+    "    Only catches exceptions listed in `exceptions`.\n",
+    "    \"\"\"\n",
+    "\n",
+    "    def decorator(fn):\n",
+    "        @wraps(fn)\n",
+    "        def wrapper(*args, **kwargs):\n",
+    "            last_exception = None\n",
+    "            for attempt in range(1, max_attempts + 1):\n",
+    "                try:\n",
+    "                    return fn(*args, **kwargs)\n",
+    "                except exceptions as e:\n",
+    "                    last_exception = e\n",
+    "                    print(f\"  [{fn.__name__}] Attempt {attempt}/{max_attempts} failed: {e}\")\n",
+    "                    if attempt < max_attempts:\n",
+    "                        time.sleep(delay)\n",
+    "            raise last_exception\n",
+    "\n",
+    "        return wrapper\n",
+    "\n",
+    "    return decorator"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "13",
+   "metadata": {},
+   "source": [
+    "Let's try it out:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "14",
+   "metadata": {
+    "lines_to_next_cell": 1
+   },
+   "outputs": [],
+   "source": [
+    "@retry(max_attempts=5, delay=0.3, exceptions=(ConnectionError,))\n",
+    "def flaky_function():\n",
+    "    if random.random() < 0.6:\n",
+    "        raise ConnectionError(\"Server not responding\")\n",
+    "    return {\"status\": \"ok\", \"data\": 42}\n",
+    "\n",
+    "\n",
+    "flaky_function()"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "15",
+   "metadata": {},
+   "source": [
+    "Notice how only `ConnectionError` is caught.\n",
+    "If the function raises something else, the decorator lets it propagate immediately — no retries wasted:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "16",
+   "metadata": {
+    "lines_to_next_cell": 1
+   },
+   "outputs": [],
+   "source": [
+    "@retry(max_attempts=5, delay=0.3, exceptions=(ConnectionError,))\n",
+    "def broken_function():\n",
+    "    raise ValueError(\"This is a bug, not a transient failure\")\n",
+    "\n",
+    "\n",
+    "try:\n",
+    "    broken_function()\n",
+    "except ValueError as e:\n",
+    "    print(f\"ValueError propagated immediately: {e}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "17",
+   "metadata": {},
+   "source": [
+    "## 2. The `@validate_types` decorator"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "18",
+   "metadata": {},
+   "source": [
+    "Python is dynamically typed, which is flexible but can lead to confusing errors deep inside a function.\n",
+    "A type-checking decorator catches bad inputs early, at the \"front door.\"\n",
+    "\n",
+    "Consider a simple calculation:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "19",
+   "metadata": {
+    "lines_to_next_cell": 1
+   },
+   "outputs": [],
+   "source": [
+    "def compute(x, y):\n",
+    "    return x * y + 1\n",
+    "\n",
+    "\n",
+    "# Works fine with numbers:\n",
+    "print(compute(3, 4.5))\n",
+    "\n",
+    "# But with wrong types, the error is confusing:\n",
+    "try:\n",
+    "    compute(\"hello\", 4.5)\n",
+    "except TypeError as e:\n",
+    "    print(f\"Confusing error: {e}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "20",
+   "metadata": {},
+   "source": [
+    "Let's build a decorator that checks types before the function runs.\n",
+    "\n",
+    "To keep things simple, our decorator will require the function to be called with **keyword arguments**.\n",
+    "This way we can match argument names directly — no introspection needed:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "21",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def validate_types(**expected_types):\n",
+    "    \"\"\"\n",
+    "    Decorator factory: check that keyword arguments match the specified types.\n",
+    "\n",
+    "    Usage: @validate_types(x=int, y=float)\n",
+    "    \"\"\"\n",
+    "\n",
+    "    def decorator(fn):\n",
+    "        @wraps(fn)\n",
+    "        def wrapper(**kwargs):\n",
+    "            for param_name, expected_type in expected_types.items():\n",
+    "                if param_name in kwargs:\n",
+    "                    value = kwargs[param_name]\n",
+    "                    if not isinstance(value, expected_type):\n",
+    "                        raise TypeError(\n",
+    "                            f\"Argument '{param_name}' must be {expected_type.__name__}, \"\n",
+    "                            f\"got {type(value).__name__}: {value!r}\"\n",
+    "                        )\n",
+    "            return fn(**kwargs)\n",
+    "\n",
+    "        return wrapper\n",
+    "\n",
+    "    return decorator"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "22",
+   "metadata": {},
+   "source": [
+    "Let's try it:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "23",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "@validate_types(x=int, y=(int, float))\n",
+    "def compute(x, y):\n",
+    "    return x * y + 1\n",
+    "\n",
+    "\n",
+    "# Correct types — works fine:\n",
+    "print(compute(x=3, y=4.5))\n",
+    "\n",
+    "# Wrong types — clear error message:\n",
+    "try:\n",
+    "    compute(x=\"hello\", y=4.5)\n",
+    "except TypeError as e:\n",
+    "    print(f\"Clear error: {e}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "24",
+   "metadata": {},
+   "source": [
+    "The function body never changed.\n",
+    "The validation concern is completely **separate** from the business logic.\n",
+    "\n",
+    "> **Note:** requiring keyword arguments is a simplification. A production-grade version would use\n",
+    "> `inspect.signature` to handle positional arguments too — but that's beyond our scope here.\n",
+    "> In practice, you'd use a library like [`pydantic`](https://docs.pydantic.dev/) for this."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "25",
+   "metadata": {},
+   "source": [
+    "### Advanced alternative: handling positional arguments"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "26",
+   "metadata": {},
+   "source": [
+    "If you want `validate_types` to work with **both** positional and keyword arguments,\n",
+    "you can use `inspect.signature` to bind them to parameter names:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "27",
+   "metadata": {
+    "jupyter": {
+     "source_hidden": true
+    },
+    "lines_to_next_cell": 1
+   },
+   "outputs": [],
+   "source": [
+    "import inspect\n",
+    "\n",
+    "\n",
+    "def validate_types_advanced(**expected_types):\n",
+    "    \"\"\"Version that handles both positional and keyword arguments.\"\"\"\n",
+    "\n",
+    "    def decorator(fn):\n",
+    "        sig = inspect.signature(fn)\n",
+    "\n",
+    "        @wraps(fn)\n",
+    "        def wrapper(*args, **kwargs):\n",
+    "            bound = sig.bind(*args, **kwargs)\n",
+    "            bound.apply_defaults()\n",
+    "\n",
+    "            for param_name, expected_type in expected_types.items():\n",
+    "                if param_name in bound.arguments:\n",
+    "                    value = bound.arguments[param_name]\n",
+    "                    if not isinstance(value, expected_type):\n",
+    "                        raise TypeError(\n",
+    "                            f\"Argument '{param_name}' must be {expected_type.__name__}, \"\n",
+    "                            f\"got {type(value).__name__}: {value!r}\"\n",
+    "                        )\n",
+    "            return fn(*args, **kwargs)\n",
+    "\n",
+    "        return wrapper\n",
+    "\n",
+    "    return decorator\n",
+    "\n",
+    "\n",
+    "# Now positional calls work too:\n",
+    "@validate_types_advanced(x=int, y=(int, float))\n",
+    "def compute(x, y):\n",
+    "    return x * y + 1\n",
+    "\n",
+    "\n",
+    "print(compute(3, 4.5))  # positional argumnets work!\n",
+    "\n",
+    "compute(\"hello\", y=4.5)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "28",
+   "metadata": {},
+   "source": [
+    "## 3. Stacking them together"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "29",
+   "metadata": {},
+   "source": [
+    "Now the real power: combining both decorators on a single function.\n",
+    "\n",
+    "Imagine we have a flaky sensor API. We want to:\n",
+    "1. **Validate** that we're passing the right types\n",
+    "2. **Retry** if the sensor doesn't respond\n",
+    "\n",
+    "The stacking order matters! The decorator **closest to `def`** wraps first (its wrapper is the **innermost** layer):\n",
+    "\n",
+    "```python\n",
+    "@retry(...)          # outer: retries the validated call\n",
+    "@validate_types(...) # inner: validates first, then calls the function\n",
+    "def my_function(...):\n",
+    "    ...\n",
+    "```"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "30",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "@retry(max_attempts=4, delay=0.3, exceptions=(ConnectionError,))\n",
+    "@validate_types(sensor_id=int, threshold=(int, float))\n",
+    "def fetch_sensor_reading(sensor_id, threshold):\n",
+    "    \"\"\"Fetch a reading from a flaky sensor API.\"\"\"\n",
+    "    if random.random() < 0.5:\n",
+    "        raise ConnectionError(f\"Sensor {sensor_id} not responding\")\n",
+    "    reading = random.uniform(0, 100)\n",
+    "    return round(reading, 2) if reading > threshold else 0.0"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "31",
+   "metadata": {},
+   "source": [
+    "Call with correct types — retries happen on `ConnectionError`:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "32",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "result = fetch_sensor_reading(sensor_id=42, threshold=10.0)\n",
+    "print(f\"Reading: {result}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "33",
+   "metadata": {},
+   "source": [
+    "Call with wrong types — `TypeError` is raised **immediately**, no retries attempted\n",
+    "(because `TypeError` is not in the retry `exceptions` tuple):"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "34",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "try:\n",
+    "    fetch_sensor_reading(sensor_id=\"abc\", threshold=10.0)\n",
+    "except TypeError as e:\n",
+    "    print(f\"Caught immediately: {e}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "35",
+   "metadata": {},
+   "source": [
+    "This is exactly the point: each decorator handles **one concern**, and stacking them composes behavior cleanly."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "36",
+   "metadata": {},
+   "source": [
+    "## Recap\n",
+    "\n",
+    "- **Decorator factories** use three nested functions: factory → decorator → wrapper.\n",
+    "- Always use `functools.wraps` to preserve the original function's name and docstring.\n",
+    "- **Stacking order matters:** the decorator closest to `def` wraps first (runs first when the function is called).\n",
+    "- These patterns are used in well-known libraries. Use these and do not reivent the well for production code!\n",
+    "  - Retry → [`tenacity`](https://tenacity.readthedocs.io/)\n",
+    "  - Type validation → [`pydantic`](https://docs.pydantic.dev/)\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "37",
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
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