add beartype

Author: khuyentran1401

Chapter2/dataclasses.ipynb

@@ -398,11 +398,112 @@
     "if not isinstance(dog.age, int):\n",
     "    raise ValueError(\"Dog's age must be an integer.\")"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "ec7e1d48",
+   "metadata": {},
+   "source": [
+    "### Python Best Practices: Using default_factory for Mutable Defaults"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "feb1a7c8-abb3-4972-b3c1-e538ca3cbe22",
+   "metadata": {},
+   "source": [
+    "When defining classes in Python, using mutable default values for instance variables can lead to unexpected behavior."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "765adba5-1145-4cd8-b714-c7c88424e47a",
+   "metadata": {},
+   "source": [
+    "For example, if you use a list as a default value in a class's `__init__` method, all instances of the class will share the same list object:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 8,
+   "id": "2790fc25-56ff-4797-953b-fdbf2608a87e",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "['Author 1']\n"
+     ]
+    }
+   ],
+   "source": [
+    "class Book:\n",
+    "    def __init__(self, title, authors=[]):\n",
+    "        self.title = title\n",
+    "        self.authors = authors\n",
+    "\n",
+    "\n",
+    "book1 = Book(\"Book 1\")\n",
+    "book1.authors.append(\"Author 1\")\n",
+    "\n",
+    "book2 = Book(\"Book 2\")\n",
+    "print(book2.authors)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "9c56cdc2-7033-4edb-b505-8b456e733c64",
+   "metadata": {},
+   "source": [
+    "In this example, `book1` and `book2` share the same list object, which is why modifying the list in `book1` affects `book2`.\n",
+    "\n",
+    "To avoid this issue, you can use the `default_factory` parameter in dataclasses, which creates a new object for each instance:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "id": "aa062527-c688-4200-aa8d-8b65970f4297",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "[]\n"
+     ]
+    }
+   ],
+   "source": [
+    "from dataclasses import dataclass, field\n",
+    "\n",
+    "\n",
+    "@dataclass\n",
+    "class Book:\n",
+    "    title: str\n",
+    "    authors: list = field(default_factory=list)\n",
+    "\n",
+    "\n",
+    "book1 = Book(\"Book 1\")\n",
+    "book1.authors.append(\"Author 1\")\n",
+    "\n",
+    "book2 = Book(\"Book 2\")\n",
+    "print(book2.authors)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "8be32d8d-c083-425e-889d-0548dd4f3183",
+   "metadata": {},
+   "source": [
+    "Now, each instance has its own separate list object, and modifying one instance's list does not affect others."
+   ]
   }
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 3.8.9 ('venv': venv)",
+   "display_name": "Python 3 (ipykernel)",
    "language": "python",
    "name": "python3"
   },
@@ -430,11 +531,6 @@
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