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looping pitfall

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rasbt 2014-04-17 15:39:55 -04:00
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not_so_obvious_python_stuff.ipynb
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@ -1,7 +1,7 @@
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"- [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)"
]
},
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@ -1299,14 +1300,31 @@
],
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{
"cell_type": "markdown",
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"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 @@
]
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@ -1343,17 +1361,27 @@
]
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{
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"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": [
"c = [2, 4, 5, 6]\n",
"for i in c:\n",
" if i % 2 != 0:\n",
" c.remove(i)\n",
"print(c)"
"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",
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@ -1362,39 +1390,14 @@
"output_type": "stream",
"stream": "stdout",
"text": [
"[2, 4, 6]\n"
"0 2\n",
"1 5\n",
"2 6\n",
"[4, 5]\n"
]
}
],
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},
{
"cell_type": "code",
"collapsed": false,
"input": [
"not 4 % 2"
],
"language": "python",
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"outputs": [
{
"metadata": {},
"output_type": "pyout",
"prompt_number": 9,
"text": [
"True"
]
}
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