mutable objects as default arguments

rasbt · rasbt · commit 38092ac58331 · 2014-04-15T17:10:19.000-04:00
diff --git a/.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb b/.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb
@@ -1,7 +1,7 @@
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@@ -45,7 +45,8 @@
       "- [Picking True values from and and or expressions](#false_true_expressions)\n",
       "- [Don't use mutable objects as default arguments for functions!](#def_mutable_func)\n",
       "- [Be aware of the consuming generator](#consuming_generator)\n",
-      "- [`bool` is a subclass of `int`](#bool_int)"
+      "- [`bool` is a subclass of `int`](#bool_int)\n",
+      "- [About lambda and closures-in-a-loop pitfall](#lambda_closure)"
      ]
     },
     {
@@ -583,6 +584,76 @@
      ],
      "prompt_number": 16
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name='lambda_closure'></a>\n",
+      "\n",
+      "## About lambda and closures-in-a-loop pitfall\n",
+      "\n",
+      "The following example illustrates how the (last) `lambda` is being reused:"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "my_list = [lambda: i for i in range(5)]\n",
+      "for l in my_list:\n",
+      "    print(l())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "4\n",
+        "4\n",
+        "4\n",
+        "4\n",
+        "4\n"
+       ]
+      }
+     ],
+     "prompt_number": 24
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "**Here, a generator can save you some pain:**"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "my_gen = (lambda: n for n in range(5))\n",
+      "for l in my_gen:\n",
+      "    print(l())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "0\n",
+        "1\n",
+        "2\n",
+        "3\n",
+        "4\n"
+       ]
+      }
+     ],
+     "prompt_number": 25
+    },
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      "collapsed": false,
diff --git a/not_so_obvious_python_stuff.ipynb b/not_so_obvious_python_stuff.ipynb
@@ -1,7 +1,7 @@
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+  "signature": "sha256:faa74a34746bf250ef2d72e308074083ee5e60789203d70f630f8c67a709e6fe"
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@@ -45,7 +45,8 @@
       "- [Picking True values from and and or expressions](#false_true_expressions)\n",
       "- [Don't use mutable objects as default arguments for functions!](#def_mutable_func)\n",
       "- [Be aware of the consuming generator](#consuming_generator)\n",
-      "- [`bool` is a subclass of `int`](#bool_int)"
+      "- [`bool` is a subclass of `int`](#bool_int)\n",
+      "- [About lambda and closures-in-a-loop pitfall](#lambda_closure)"
      ]
     },
     {
@@ -583,6 +584,76 @@
      ],
      "prompt_number": 16
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name='lambda_closure'></a>\n",
+      "\n",
+      "## About lambda and closures-in-a-loop pitfall\n",
+      "\n",
+      "The following example illustrates how the (last) `lambda` is being reused:"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "my_list = [lambda: i for i in range(5)]\n",
+      "for l in my_list:\n",
+      "    print(l())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "4\n",
+        "4\n",
+        "4\n",
+        "4\n",
+        "4\n"
+       ]
+      }
+     ],
+     "prompt_number": 24
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "**Here, a generator can save you some pain:**"
+     ]
+    },
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "my_gen = (lambda: n for n in range(5))\n",
+      "for l in my_gen:\n",
+      "    print(l())"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "0\n",
+        "1\n",
+        "2\n",
+        "3\n",
+        "4\n"
+       ]
+      }
+     ],
+     "prompt_number": 25
+    },
     {
      "cell_type": "code",
      "collapsed": false,
