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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", "- [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", "- [When mutable contents of immutable tuples aren't so mutable](#immutable_tuple)\n",
"- [List comprehensions are fast, but generators are faster!?](#list_generator)\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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@ -1299,6 +1300,23 @@
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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"
]
},
{
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"source": [
"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavour!"
]
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@ -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:"
]
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"c = [2, 4, 5, 6]\n", "b = [2, 4, 5, 6]\n",
"for i in c:\n", "for index, item in enumerate(b):\n",
" if i % 2 != 0:\n", " print(index, item)\n",
" c.remove(i)\n", " if not item % 2:\n",
"print(c)" " b.remove(item)\n",
"print(b)"
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@ -1362,39 +1390,14 @@
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"text": [ "text": [
"[2, 4, 6]\n" "0 2\n",
"1 5\n",
"2 6\n",
"[4, 5]\n"
] ]
} }
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"not 4 % 2"
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