Add example for post-init hook

Author: gjbex

source-code/object-orientation/attr_intro.ipynb

@@ -10,7 +10,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 81,
+   "execution_count": 2,
    "id": "ef9a778a-06a2-4dc0-941b-23c84b7c6418",
    "metadata": {},
    "outputs": [],
@@ -840,6 +840,87 @@
     "except ValueError as error:\n",
     "    print(error)"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "771ff071-c15e-439a-b976-f7cb783a68ab",
+   "metadata": {},
+   "source": [
+    "## Post-init hook"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "658da1f9-f5a0-46fa-b730-2cfb497e48d8",
+   "metadata": {},
+   "source": [
+    "Sometimes you want to do initialization based on the values of the attributes passed to the constructor.  You would typically do that in the `__init__` method.  To do similar things for `attrs` classes, you can use the `__attrs_post_init__` method."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "id": "7a616115-ddd2-4582-ad3a-47b3ab7f8c64",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "@attr.s\n",
+    "class Guessing:\n",
+    "    nr_values : int = attr.ib(converter=int)\n",
+    "    _values : typing.List[int] = attr.ib(init=False)\n",
+    "\n",
+    "    @nr_values.validator\n",
+    "    def nr_values_validate(self, attribute, value):\n",
+    "        if value < 1 or value > 10:\n",
+    "            raise ValueError('number of values should be between 1 and 10')\n",
+    "\n",
+    "    def __attrs_post_init__(self):\n",
+    "        self._values = set()\n",
+    "        while len(self._values) < self.nr_values:\n",
+    "            self._values.add(random.choice(list(range(10))))\n",
+    "    \n",
+    "    def has_value(self, value):\n",
+    "        return value in self._values"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 10,
+   "id": "0cbea0f1-0f55-49f3-aaa2-54fd0006ca04",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Guessing(nr_values=4, _values={0, 2, 5, 6})\n",
+      "trying 7\n",
+      "no luck with 7\n",
+      "trying 4\n",
+      "no luck with 4\n",
+      "trying 8\n",
+      "no luck with 8\n"
+     ]
+    }
+   ],
+   "source": [
+    "guessing = Guessing(4)\n",
+    "print(guessing)\n",
+    "for guess in random.choices(range(10), k=3):\n",
+    "    print(f'trying {guess}')\n",
+    "    if guessing.has_value(guess):\n",
+    "        print(f'hurray for {guess}!')\n",
+    "    else:\n",
+    "        print(f'no luck with {guess}')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "b84b2436-9a82-4872-a087-71fd4cbe5de4",
+   "metadata": {},
+   "outputs": [],
+   "source": []
   }
  ],
  "metadata": {
@@ -858,7 +939,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.10"
+   "version": "3.9.5"
   }
  },
  "nbformat": 4,