Add default arguments mistake

book/pythontricks/Chapter.ipynb

@@ -829,6 +829,56 @@
     "p = Point(x=3.2, y=1.0)\n",
     "print(p.x, p.y)"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Mutable Default Values for Function Arguments"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "One big mistake in Python:\n",
+    "\n",
+    "Using mutable default values for function arguments.\n",
+    "\n",
+    "When using mutable objects like a list as a default value, you have to be careful.\n",
+    "\n",
+    "See the example below, where the default list is shared among all calls to the function.\n",
+    "\n",
+    "To fix this, set the default value to None and create a new list inside the function if no list is provided."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "# Dangerous behaviour:\n",
+    "def increment_numbers(numbers=[]):\n",
+    "    for i in range(3):\n",
+    "        numbers.append(i)\n",
+    "    print(f\"Numbers: {numbers}\")\n",
+    "\n",
+    "increment_numbers()  # Output: Numbers: [0, 1, 2]\n",
+    "increment_numbers()  # Output: Numbers: [0, 1, 2, 0, 1, 2]\n",
+    "\n",
+    "\n",
+    "# Better:\n",
+    "def increment_numbers(numbers=None):\n",
+    "    if numbers is None:\n",
+    "        numbers = []\n",
+    "    for i in range(3):\n",
+    "        numbers.append(i)\n",
+    "    print(f\"Numbers: {numbers}\")\n",
+    "\n",
+    "increment_numbers()  # Output: Numbers: [0, 1, 2]\n",
+    "increment_numbers()  # Output: Numbers: [0, 1, 2]"
+   ]
   }
  ],
  "metadata": {