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mutable objects as default arguments

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rasbt 2014-04-15 15:50:39 -04:00
parent 19e5613f57
commit 0bab1bbca1
2 changed files with 15 additions and 10 deletions
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15
.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb
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@ -1,7 +1,7 @@
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@ -40,9 +40,10 @@
"- [The C3 class resolution algorithm for multiple class inheritance](#c3_class_res)\n", "- [The C3 class resolution algorithm for multiple class inheritance](#c3_class_res)\n",
"- [The behavior of += for lists](#pm_in_lists)\n", "- [The behavior of += for lists](#pm_in_lists)\n",
"- [`True` and `False` in the datetime module](#datetime_module)\n", "- [`True` and `False` in the datetime module](#datetime_module)\n",
"- [Python reuses objects for small integers](#python_small_int)\n", "- [Python reuses objects for small integers - always use \"==\" for equality, \"is\" for identity](#python_small_int)\n",
"- [Shallow vs. deep copies if list contains other structures and objects](#shallow_vs_deep)\n", "- [Shallow vs. deep copies if list contains other structures and objects](#shallow_vs_deep)\n",
"- [Picking True values from and and or expressions](#false_true_expressions)" "- [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)"
] ]
}, },
{ {
@ -189,10 +190,12 @@
"<br>\n", "<br>\n",
"<br>\n", "<br>\n",
"<a name='python_small_int'></a>\n", "<a name='python_small_int'></a>\n",
"## Python reuses objects for small integers \n", "## Python reuses objects for small integers - always use \"==\" for equality, \"is\" for identity\n",
"\n", "\n",
"This oddity occurs, because Python tends to stores small integers as the same object, but not so for larger ones! \n", "This oddity occurs, because Python tends to stores small integers as the same object, but not so for larger ones! \n",
"(*I received a comment that this is in fact a CPython artefact and must not necessarily be true in all implementations of Python!*)" "(*I received a comment that this is in fact a CPython artefact and must not necessarily be true in all implementations of Python!*)\n",
"\n",
"So the take home message is: always use \"==\" for equality, \"is\" for identity!"
] ]
}, },
{ {
@ -335,7 +338,7 @@
"source": [ "source": [
"<br>\n", "<br>\n",
"<br>\n", "<br>\n",
"<a name='def_mutable_func></a>\n", "<a name='def_mutable_func'></a>\n",
"## Don't use mutable objects as default arguments for functions!" "## Don't use mutable objects as default arguments for functions!"
] ]
}, },

10
not_so_obvious_python_stuff.ipynb
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@ -1,7 +1,7 @@
{ {
"metadata": { "metadata": {
"name": "", "name": "",
"signature": "sha256:158ad35cd56abe8905983dc63421b3b7b54d28a947df54f67a3b7f0a53e31481" "signature": "sha256:0c860c383edcd7622b0c660eb989513e90496c7350916bae1ddb6c10f29fab2c"
}, },
"nbformat": 3, "nbformat": 3,
"nbformat_minor": 0, "nbformat_minor": 0,
@ -40,7 +40,7 @@
"- [The C3 class resolution algorithm for multiple class inheritance](#c3_class_res)\n", "- [The C3 class resolution algorithm for multiple class inheritance](#c3_class_res)\n",
"- [The behavior of += for lists](#pm_in_lists)\n", "- [The behavior of += for lists](#pm_in_lists)\n",
"- [`True` and `False` in the datetime module](#datetime_module)\n", "- [`True` and `False` in the datetime module](#datetime_module)\n",
"- [Python reuses objects for small integers](#python_small_int)\n", "- [Python reuses objects for small integers - always use \"==\" for equality, \"is\" for identity](#python_small_int)\n",
"- [Shallow vs. deep copies if list contains other structures and objects](#shallow_vs_deep)\n", "- [Shallow vs. deep copies if list contains other structures and objects](#shallow_vs_deep)\n",
"- [Picking True values from and and or expressions](#false_true_expressions)\n", "- [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)" "- [Don't use mutable objects as default arguments for functions!](#def_mutable_func)"
@ -190,10 +190,12 @@
"<br>\n", "<br>\n",
"<br>\n", "<br>\n",
"<a name='python_small_int'></a>\n", "<a name='python_small_int'></a>\n",
"## Python reuses objects for small integers \n", "## Python reuses objects for small integers - always use \"==\" for equality, \"is\" for identity\n",
"\n", "\n",
"This oddity occurs, because Python tends to stores small integers as the same object, but not so for larger ones! \n", "This oddity occurs, because Python tends to stores small integers as the same object, but not so for larger ones! \n",
"(*I received a comment that this is in fact a CPython artefact and must not necessarily be true in all implementations of Python!*)" "(*I received a comment that this is in fact a CPython artefact and must not necessarily be true in all implementations of Python!*)\n",
"\n",
"So the take home message is: always use \"==\" for equality, \"is\" for identity!"
] ]
}, },
{ {