mirror of
https://github.com/rasbt/python_reference.git
synced 2024-11-24 04:21:15 +00:00
mutable objects as default arguments
This commit is contained in:
rasbt
parent
c177a57002
commit
7b07896b8e
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@ -1,7 +1,7 @@
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{
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"metadata": {
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"name": "",
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"signature": "sha256:06e485535e22c756262f09d4477108b57d0fa1029f9831ee77a177267f5afc7f"
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"signature": "sha256:1b145c7412627e0fa1c08e3b663174712154077c8d8b7740f66d3021a06216a4"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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@ -47,7 +47,8 @@
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"- [Be aware of the consuming generator](#consuming_generator)\n",
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"- [`bool` is a subclass of `int`](#bool_int)\n",
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)"
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
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"- [When immutable Tuples aren't so immutable](#immutable_tuple)"
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]
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},
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{
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@ -818,6 +819,146 @@
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}
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],
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"prompt_number": 35
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='immutable_tuple'></a>\n",
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"## When immutable Tuples aren't so immutable"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"As we all know, tuples are immutable objects in Python, right!?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = (1,)\n",
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"tup[0] += 1"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"ename": "TypeError",
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"evalue": "'tuple' object does not support item assignment",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
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"\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",
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"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
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]
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}
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],
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"prompt_number": 41
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### 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?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = ([],)\n",
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"print('tup before: ', tup)\n",
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"tup[0] += [1]"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup before: ([],)\n"
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]
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},
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{
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"ename": "TypeError",
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"evalue": "'tuple' object does not support item assignment",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
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"\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",
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"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
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]
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}
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],
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"prompt_number": 42
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"print('tup after: ', tup)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup after: ([1],)\n"
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]
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}
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],
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"prompt_number": 43
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"However, there **IS** a way (where there should be none!) to modify our immutable tuple without raising the `TypeError`, the solution is the `.extend()`:"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = ([],)\n",
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"print('tup before: ', tup)\n",
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"tup[0].extend([1])\n",
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"print('tup after: ', tup)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup before: ([],)\n",
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"tup after: ([1],)\n"
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]
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}
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],
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"prompt_number": 44
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Explanation\n",
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"\n",
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"**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",
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"\n",
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"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.\"*"
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]
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}
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],
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"metadata": {}
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@ -1,7 +1,7 @@
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{
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"metadata": {
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"name": "",
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||||
"signature": "sha256:06e485535e22c756262f09d4477108b57d0fa1029f9831ee77a177267f5afc7f"
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"signature": "sha256:1b145c7412627e0fa1c08e3b663174712154077c8d8b7740f66d3021a06216a4"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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|
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@ -47,7 +47,8 @@
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"- [Be aware of the consuming generator](#consuming_generator)\n",
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"- [`bool` is a subclass of `int`](#bool_int)\n",
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)"
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
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"- [When immutable Tuples aren't so immutable](#immutable_tuple)"
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]
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},
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{
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@ -818,6 +819,146 @@
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}
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],
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"prompt_number": 35
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='immutable_tuple'></a>\n",
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"## When immutable Tuples aren't so immutable"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"As we all know, tuples are immutable objects in Python, right!?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = (1,)\n",
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"tup[0] += 1"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"ename": "TypeError",
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"evalue": "'tuple' object does not support item assignment",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
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"\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",
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"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
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]
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}
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],
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"prompt_number": 41
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"#### 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?"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = ([],)\n",
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"print('tup before: ', tup)\n",
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"tup[0] += [1]"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup before: ([],)\n"
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]
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},
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{
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"ename": "TypeError",
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"evalue": "'tuple' object does not support item assignment",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)",
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"\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",
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"\u001b[0;31mTypeError\u001b[0m: 'tuple' object does not support item assignment"
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]
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}
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],
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"prompt_number": 42
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"print('tup after: ', tup)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup after: ([1],)\n"
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]
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}
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],
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"prompt_number": 43
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"However, there **IS** a way (where there should be none!) to modify our immutable tuple without raising the `TypeError`, the solution is the `.extend()`:"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"tup = ([],)\n",
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"print('tup before: ', tup)\n",
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"tup[0].extend([1])\n",
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"print('tup after: ', tup)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"tup before: ([],)\n",
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"tup after: ([1],)\n"
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]
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}
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],
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"prompt_number": 44
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### Explanation\n",
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"\n",
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"**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",
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"\n",
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"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.\"*"
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]
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}
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],
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"metadata": {}
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|
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