looping pitfall

rasbt · rasbt · commit b065840e5952 · 2014-04-17T15:39:55.000-04:00
diff --git a/not_so_obvious_python_stuff.ipynb b/not_so_obvious_python_stuff.ipynb
@@ -1,7 +1,7 @@
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  "metadata": {
   "name": "",
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+  "signature": "sha256:02c6c63beb1de9373d69615a4ba37640a7b01c8f2d088dbfaa84bdaf3452f1c5"
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@@ -63,7 +63,8 @@
       "- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
       "- [When mutable contents of immutable tuples aren't so mutable](#immutable_tuple)\n",
       "- [List comprehensions are fast, but generators are faster!?](#list_generator)\n",
-      "- [Public vs. private class methods and name mangling](#private_class)"
+      "- [Public vs. private class methods and name mangling](#private_class)\n",
+      "- [The consequences of modifying a list when looping through it](#looping_pitfall)"
      ]
     },
     {
@@ -1299,14 +1300,31 @@
      ],
      "prompt_number": 28
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "<a name='looping_pitfall'></a>\n",
+      "## The consequences of modifying a list when looping through it"
+     ]
+    },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavour!"
+     ]
+    },
     {
      "cell_type": "code",
      "collapsed": false,
      "input": [
       "a = [1, 2, 3, 4, 5]\n",
       "for i in a:\n",
-      "     if not i % 2:\n",
-      "         a.remove(i)\n",
+      "    if not i % 2:\n",
+      "        a.remove(i)\n",
       "print(a)"
      ],
      "language": "python",
@@ -1320,7 +1338,7 @@
        ]
       }
      ],
-     "prompt_number": 1
+     "prompt_number": 3
     },
     {
      "cell_type": "code",
@@ -1343,58 +1361,43 @@
        ]
       }
      ],
-     "prompt_number": 7
+     "prompt_number": 4
     },
     {
-     "cell_type": "code",
-     "collapsed": false,
-     "input": [
-      "c = [2, 4, 5, 6]\n",
-      "for i in c:\n",
-      "     if i % 2 != 0:\n",
-      "         c.remove(i)\n",
-      "print(c)"
-     ],
-     "language": "python",
+     "cell_type": "markdown",
      "metadata": {},
-     "outputs": [
-      {
-       "output_type": "stream",
-       "stream": "stdout",
-       "text": [
-        "[2, 4, 6]\n"
-       ]
-      }
-     ],
-     "prompt_number": 6
+     "source": [
+      "<br>\n",
+      "<br>\n",
+      "**The solution** is that we are iterating through the list index by index, and if we remove one of the items in-between, we inevitably mess around with the indexing, look at the following example, and it will become clear:"
+     ]
     },
     {
      "cell_type": "code",
      "collapsed": false,
      "input": [
-      "not 4 % 2"
+      "b = [2, 4, 5, 6]\n",
+      "for index, item in enumerate(b):\n",
+      "    print(index, item)\n",
+      "    if not item % 2:\n",
+      "        b.remove(item)\n",
+      "print(b)"
      ],
      "language": "python",
      "metadata": {},
      "outputs": [
       {
-       "metadata": {},
-       "output_type": "pyout",
-       "prompt_number": 9,
+       "output_type": "stream",
+       "stream": "stdout",
        "text": [
-        "True"
+        "0 2\n",
+        "1 5\n",
+        "2 6\n",
+        "[4, 5]\n"
        ]
       }
      ],
-     "prompt_number": 9
-    },
-    {
-     "cell_type": "code",
-     "collapsed": false,
-     "input": [],
-     "language": "python",
-     "metadata": {},
-     "outputs": []
+     "prompt_number": 7
     }
    ],
    "metadata": {}
