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mutable objects as default arguments

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rasbt 2014-04-15 17:58:34 -04:00
parent c177a57002
commit 7b07896b8e
2 changed files with 286 additions and 4 deletions
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145
.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb
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@ -1,7 +1,7 @@
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@ -47,7 +47,8 @@
"- [Be aware of the consuming generator](#consuming_generator)\n", "- [Be aware of the consuming generator](#consuming_generator)\n",
"- [`bool` is a subclass of `int`](#bool_int)\n", "- [`bool` is a subclass of `int`](#bool_int)\n",
"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n", "- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)" "- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
"- [When immutable Tuples aren't so immutable](#immutable_tuple)"
] ]
}, },
{ {
@ -818,6 +819,146 @@
} }
], ],
"prompt_number": 35 "prompt_number": 35
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"<br>\n",
"<br>\n",
"<a name='immutable_tuple'></a>\n",
"## When immutable Tuples aren't so immutable"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"As we all know, tuples are immutable objects in Python, right!?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = (1,)\n",
"tup[0] += 1"
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "TypeError",
"evalue": "'tuple' object does not support item assignment",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-41-c3bec6c3fe6f>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mtup\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mtup\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m+=\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
]
}
],
"prompt_number": 41
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### But what if we put a mutable object into the immutable tuple? Well, modification works, but we **also** get a type error, funny, isn't it?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = ([],)\n",
"print('tup before: ', tup)\n",
"tup[0] += [1]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup before: ([],)\n"
]
},
{
"ename": "TypeError",
"evalue": "'tuple' object does not support item assignment",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-42-aebe9a31dbeb>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mtup\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m,\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 2\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'tup before: '\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mtup\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 3\u001b[0;31m \u001b[0mtup\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m+=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
]
}
],
"prompt_number": 42
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('tup after: ', tup)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup after: ([1],)\n"
]
}
],
"prompt_number": 43
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"<br>\n",
"<br>\n",
"However, there **IS** a way (where there should be none!) to modify our immutable tuple without raising the `TypeError`, the solution is the `.extend()`:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = ([],)\n",
"print('tup before: ', tup)\n",
"tup[0].extend([1])\n",
"print('tup after: ', tup)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup before: ([],)\n",
"tup after: ([1],)\n"
]
}
],
"prompt_number": 44
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### Explanation\n",
"\n",
"**A. Jesse Jiryu Davis** has a nice explanation for this phenomenon (Original source: [http://emptysqua.re/blog/python-increment-is-weird-part-ii/](http://emptysqua.re/blog/python-increment-is-weird-part-ii/))\n",
"\n",
"If we try to extend the list via `+=` *\"then the statement executes STORE_SUBSCR, which calls the C function PyObject_SetItem, which checks if the object supports item assignment. In our case the object is a tuple, so PyObject_SetItem throws the TypeError. Mystery solved.\"*"
]
} }
], ],
"metadata": {} "metadata": {}

145
not_so_obvious_python_stuff.ipynb
View File

@ -1,7 +1,7 @@
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@ -47,7 +47,8 @@
"- [Be aware of the consuming generator](#consuming_generator)\n", "- [Be aware of the consuming generator](#consuming_generator)\n",
"- [`bool` is a subclass of `int`](#bool_int)\n", "- [`bool` is a subclass of `int`](#bool_int)\n",
"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n", "- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)" "- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
"- [When immutable Tuples aren't so immutable](#immutable_tuple)"
] ]
}, },
{ {
@ -818,6 +819,146 @@
} }
], ],
"prompt_number": 35 "prompt_number": 35
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"<br>\n",
"<br>\n",
"<a name='immutable_tuple'></a>\n",
"## When immutable Tuples aren't so immutable"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"As we all know, tuples are immutable objects in Python, right!?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = (1,)\n",
"tup[0] += 1"
],
"language": "python",
"metadata": {},
"outputs": [
{
"ename": "TypeError",
"evalue": "'tuple' object does not support item assignment",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-41-c3bec6c3fe6f>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mtup\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mtup\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m+=\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
]
}
],
"prompt_number": 41
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"#### But what if we put a mutable object into the immutable tuple? Well, modification works, but we **also** get a type error, funny, isn't it?"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = ([],)\n",
"print('tup before: ', tup)\n",
"tup[0] += [1]"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup before: ([],)\n"
]
},
{
"ename": "TypeError",
"evalue": "'tuple' object does not support item assignment",
"output_type": "pyerr",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-42-aebe9a31dbeb>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mtup\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m,\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 2\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'tup before: '\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mtup\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 3\u001b[0;31m \u001b[0mtup\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m]\u001b[0m \u001b[0;34m+=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
]
}
],
"prompt_number": 42
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"print('tup after: ', tup)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup after: ([1],)\n"
]
}
],
"prompt_number": 43
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"<br>\n",
"<br>\n",
"However, there **IS** a way (where there should be none!) to modify our immutable tuple without raising the `TypeError`, the solution is the `.extend()`:"
]
},
{
"cell_type": "code",
"collapsed": false,
"input": [
"tup = ([],)\n",
"print('tup before: ', tup)\n",
"tup[0].extend([1])\n",
"print('tup after: ', tup)"
],
"language": "python",
"metadata": {},
"outputs": [
{
"output_type": "stream",
"stream": "stdout",
"text": [
"tup before: ([],)\n",
"tup after: ([1],)\n"
]
}
],
"prompt_number": 44
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### Explanation\n",
"\n",
"**A. Jesse Jiryu Davis** has a nice explanation for this phenomenon (Original source: [http://emptysqua.re/blog/python-increment-is-weird-part-ii/](http://emptysqua.re/blog/python-increment-is-weird-part-ii/))\n",
"\n",
"If we try to extend the list via `+=` *\"then the statement executes STORE_SUBSCR, which calls the C function PyObject_SetItem, which checks if the object supports item assignment. In our case the object is a tuple, so PyObject_SetItem throws the TypeError. Mystery solved.\"*"
]
} }
], ],
"metadata": {} "metadata": {}