mutable objects as default arguments

.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb

@@ -1,7 +1,7 @@
 {
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   "name": "",
-  "signature": "sha256:ed543f19540359765672e007e235b9a4f6e76963cacf74e34dac5612ea7d8b94"
+  "signature": "sha256:6bc56360203df31596f6332e4b5c73effb2572d18b60bdf6a792042e2fde8c12"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -328,6 +328,61 @@
       }
      ],
      "prompt_number": 9
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name='def_mutable_func></a>\n",
+      "## Don't use mutable objects as default arguments for functions!"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: Python will create the mutable object (default parameter) only the first time the function is called, see the following code:\n",
+      "\n",
+      "(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def append_to_list(value, def_list=[]):\n",
+      "    def_list.append(value)\n",
+      "    return def_list\n",
+      "\n",
+      "my_list = append_to_list(1)\n",
+      "print(my_list)\n",
+      "\n",
+      "my_other_list = append_to_list(2)\n",
+      "print(my_other_list)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "[1]\n",
+        "[1, 2]\n"
+       ]
+      }
+     ],
+     "prompt_number": 1
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [],
+     "language": "python",
+     "metadata": {},
+     "outputs": []
     }
    ],
    "metadata": {}

not_so_obvious_python_stuff.ipynb

@@ -1,7 +1,7 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:ed543f19540359765672e007e235b9a4f6e76963cacf74e34dac5612ea7d8b94"
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  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -328,6 +328,61 @@
       }
      ],
      "prompt_number": 9
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name='def_mutable_func></a>\n",
+      "## Don't use mutable objects as default arguments for functions!"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: Python will create the mutable object (default parameter) only the first time the function is called, see the following code:\n",
+      "\n",
+      "(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "def append_to_list(value, def_list=[]):\n",
+      "    def_list.append(value)\n",
+      "    return def_list\n",
+      "\n",
+      "my_list = append_to_list(1)\n",
+      "print(my_list)\n",
+      "\n",
+      "my_other_list = append_to_list(2)\n",
+      "print(my_other_list)"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "[1]\n",
+        "[1, 2]\n"
+       ]
+      }
+     ],
+     "prompt_number": 1
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [],
+     "language": "python",
+     "metadata": {},
+     "outputs": []
     }
    ],
    "metadata": {}