mirror of
https://github.com/rasbt/python_reference.git
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3 more addition
This commit is contained in:
rasbt
parent
b065840e59
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@ -1,7 +1,7 @@
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{
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{
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"metadata": {
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"metadata": {
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||||||
"name": "",
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"name": "",
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||||||
"signature": "sha256:a9475a6f1134f87066829ecff7dab70b1fab2b2cc8ae3ee110b3e719b9626c23"
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"signature": "sha256:2e4322ee7bffc6746194af4b3ac6bb9cd8794c14bea2575e3097821a7d5f9b52"
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"nbformat": 3,
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"nbformat": 3,
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"nbformat_minor": 0,
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"nbformat_minor": 0,
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@ -63,7 +63,11 @@
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||||||
"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)\n",
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||||||
"- [When mutable contents of immutable tuples aren't so mutable](#immutable_tuple)\n",
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"- [When mutable contents of immutable tuples aren't so mutable](#immutable_tuple)\n",
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"- [List comprehensions are fast, but generators are faster!?](#list_generator)\n",
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"- [List comprehensions are fast, but generators are faster!?](#list_generator)\n",
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"- [Public vs. private class methods and name mangling](#private_class)"
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"- [Public vs. private class methods and name mangling](#private_class)\n",
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"- [The consequences of modifying a list when looping through it](#looping_pitfall)\n",
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"- [Dynamic binding and typos in variable names](#dynamic_binding)\n",
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"- [List slicing using indexes that are \"out of range](#out_of_range_slicing)\n",
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"- [Reusing global variable names and UnboundLocalErrors](#unboundlocalerror)"
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]
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]
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},
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},
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{
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{
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||||||
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@ -515,7 +519,7 @@
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||||||
"cell_type": "markdown",
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"cell_type": "markdown",
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"metadata": {},
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"metadata": {},
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||||||
"source": [
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"source": [
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||||||
"Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: Python will create the mutable object (default parameter) the first time the function is defined - not when it is called, see the following code:\n",
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"Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: **Python will create the mutable object (default parameter) the first time the function is defined - not when it is called**, see the following code:\n",
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"\n",
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"\n",
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"(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
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"(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
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]
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]
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@ -548,6 +552,48 @@
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],
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],
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"prompt_number": 1
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"prompt_number": 1
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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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"Another good example showing that demonstrates that default arguments are created when the function is created (**and not when it is called!**):"
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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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"import time\n",
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"def report_arg(my_default=time.time()):\n",
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" print(my_default)\n",
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"\n",
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"report_arg()\n",
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"\n",
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"time.sleep(5)\n",
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"\n",
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"report_arg()"
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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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"1397764090.456688\n",
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"1397764090.456688"
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]
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},
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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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"\n"
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]
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}
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],
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"prompt_number": 10
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},
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{
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{
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"cell_type": "markdown",
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"cell_type": "markdown",
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"metadata": {},
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"metadata": {},
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@ -1299,6 +1345,358 @@
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],
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],
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"prompt_number": 28
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"prompt_number": 28
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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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"<br>\n",
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"<br>\n",
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"<a name='looping_pitfall'></a>\n",
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"## The consequences of modifying a list when looping through it"
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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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"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavior! \n",
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"Look at the following examples, and for a fun exercise: try to figure out what is going on before you skip to the solution!"
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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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"a = [1, 2, 3, 4, 5]\n",
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"for i in a:\n",
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" if not i % 2:\n",
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" a.remove(i)\n",
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"print(a)"
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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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"[1, 3, 5]\n"
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]
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}
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],
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"prompt_number": 3
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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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"b = [2, 4, 5, 6]\n",
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"for i in b:\n",
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" if not i % 2:\n",
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" b.remove(i)\n",
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"print(b)"
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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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"[4, 5]\n"
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]
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}
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],
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"prompt_number": 4
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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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"**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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]
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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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"b = [2, 4, 5, 6]\n",
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"for index, item in enumerate(b):\n",
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" print(index, item)\n",
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" if not item % 2:\n",
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" b.remove(item)\n",
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"print(b)"
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],
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"language": "python",
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"metadata": {},
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||||||
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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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"0 2\n",
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"1 5\n",
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"2 6\n",
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"[4, 5]\n"
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]
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}
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],
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"prompt_number": 7
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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='dynamic_binding'></a>\n",
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"## Dynamic binding and typos in variable names\n",
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"Be careful, dynamic binding is convenient, but can also quickly become dangerous!"
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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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"print('first list:')\n",
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"for i in range(3):\n",
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" print(i)\n",
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" \n",
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"print('\\nsecond list:')\n",
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"for j in range(3):\n",
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" print(i) # I (intentionally) made typo here!"
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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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||||||
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"first list:\n",
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"0\n",
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"1\n",
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"2\n",
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"\n",
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"second list:\n",
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"2\n",
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"2\n",
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"2\n"
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]
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}
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],
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"prompt_number": 14
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||||||
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},
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||||||
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{
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"cell_type": "markdown",
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||||||
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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='out_of_range_slicing'></a>\n",
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"## List slicing using indexes that are \"out of range\""
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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 have all encountered it 1 (x10000) time(s) in our live, the infamous `IndexError`:"
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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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"my_list = [1, 2, 3, 4, 5]\n",
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"print(my_list[5])"
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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": "IndexError",
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"evalue": "list index out of range",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mIndexError\u001b[0m Traceback (most recent call last)",
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"\u001b[0;32m<ipython-input-15-eb273dc36fdc>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mmy_list\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m2\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m3\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m4\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mmy_list\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[0;31mIndexError\u001b[0m: list index out of range"
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]
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}
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],
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"prompt_number": 15
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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 suprisingly, it is not raised when we are doing list slicing, which can be a really pain for debugging:"
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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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"my_list = [1, 2, 3, 4, 5]\n",
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"print(my_list[5:])"
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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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"[]\n"
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|
]
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}
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],
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"prompt_number": 16
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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='unboundlocalerror'></a>\n",
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|
"## Reusing global variable names and `UnboundLocalErrors`"
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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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"Usually, it is no problem to access global variables in the local scope of a function:"
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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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"def my_func():\n",
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" print(var)\n",
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"\n",
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"var = 'global'\n",
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"my_func()"
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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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"global\n"
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]
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}
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|
],
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|
"prompt_number": 37
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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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|
"And is also no problem to use the same variable name in the local scope without affecting the local counterpart: "
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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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"def my_func():\n",
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|
" var = 'locally changed'\n",
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"\n",
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"var = 'global'\n",
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"my_func()\n",
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"print(var)"
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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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"global\n"
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]
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}
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],
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"prompt_number": 38
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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 we have to be careful if we use a variable name that occurs in the global scope, and we want to access it in the local function scope if we want to reuse this name:"
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|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" print(var) # want to access global variable\n",
|
||||||
|
" var = 'locally changed' # but Python thinks we forgot to define the local variable!\n",
|
||||||
|
" \n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"my_func()"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"ename": "UnboundLocalError",
|
||||||
|
"evalue": "local variable 'var' referenced before assignment",
|
||||||
|
"output_type": "pyerr",
|
||||||
|
"traceback": [
|
||||||
|
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mUnboundLocalError\u001b[0m Traceback (most recent call last)",
|
||||||
|
"\u001b[0;32m<ipython-input-40-3afd870b7c35>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 4\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 5\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'global'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 6\u001b[0;31m \u001b[0mmy_func\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
|
||||||
|
"\u001b[0;32m<ipython-input-40-3afd870b7c35>\u001b[0m in \u001b[0;36mmy_func\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0mmy_func\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[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mvar\u001b[0m\u001b[0;34m)\u001b[0m \u001b[0;31m# want to access global variable\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 3\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'locally changed'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 4\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 5\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'global'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
|
||||||
|
"\u001b[0;31mUnboundLocalError\u001b[0m: local variable 'var' referenced before assignment"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 40
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"In this case, we have to use the `global` keyword!"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" global var\n",
|
||||||
|
" print(var) # want to access global variable\n",
|
||||||
|
" var = 'locally changed' # changes the gobal variable\n",
|
||||||
|
"\n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"\n",
|
||||||
|
"my_func()\n",
|
||||||
|
"print(var)"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"global\n",
|
||||||
|
"locally changed\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 43
|
||||||
|
},
|
||||||
{
|
{
|
||||||
"cell_type": "code",
|
"cell_type": "code",
|
||||||
"collapsed": false,
|
"collapsed": false,
|
||||||
|
|
|
||||||
|
|
@ -1,7 +1,7 @@
|
||||||
{
|
{
|
||||||
"metadata": {
|
"metadata": {
|
||||||
"name": "",
|
"name": "",
|
||||||
"signature": "sha256:02c6c63beb1de9373d69615a4ba37640a7b01c8f2d088dbfaa84bdaf3452f1c5"
|
"signature": "sha256:2e4322ee7bffc6746194af4b3ac6bb9cd8794c14bea2575e3097821a7d5f9b52"
|
||||||
},
|
},
|
||||||
"nbformat": 3,
|
"nbformat": 3,
|
||||||
"nbformat_minor": 0,
|
"nbformat_minor": 0,
|
||||||
|
|
@ -64,7 +64,10 @@
|
||||||
"- [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)\n",
|
"- [Public vs. private class methods and name mangling](#private_class)\n",
|
||||||
"- [The consequences of modifying a list when looping through it](#looping_pitfall)"
|
"- [The consequences of modifying a list when looping through it](#looping_pitfall)\n",
|
||||||
|
"- [Dynamic binding and typos in variable names](#dynamic_binding)\n",
|
||||||
|
"- [List slicing using indexes that are \"out of range](#out_of_range_slicing)\n",
|
||||||
|
"- [Reusing global variable names and UnboundLocalErrors](#unboundlocalerror)"
|
||||||
]
|
]
|
||||||
},
|
},
|
||||||
{
|
{
|
||||||
|
|
@ -516,7 +519,7 @@
|
||||||
"cell_type": "markdown",
|
"cell_type": "markdown",
|
||||||
"metadata": {},
|
"metadata": {},
|
||||||
"source": [
|
"source": [
|
||||||
"Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: Python will create the mutable object (default parameter) the first time the function is defined - not when it is called, see the following code:\n",
|
"Don't use mutable objects (e.g., dictionaries, lists, sets, etc.) as default arguments for functions! You might expect that a new list is created every time when we call the function without providing an argument for the default parameter, but this is not the case: **Python will create the mutable object (default parameter) the first time the function is defined - not when it is called**, see the following code:\n",
|
||||||
"\n",
|
"\n",
|
||||||
"(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
|
"(Original source: [http://docs.python-guide.org/en/latest/writing/gotchas/](http://docs.python-guide.org/en/latest/writing/gotchas/)"
|
||||||
]
|
]
|
||||||
|
|
@ -549,6 +552,48 @@
|
||||||
],
|
],
|
||||||
"prompt_number": 1
|
"prompt_number": 1
|
||||||
},
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"Another good example showing that demonstrates that default arguments are created when the function is created (**and not when it is called!**):"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"import time\n",
|
||||||
|
"def report_arg(my_default=time.time()):\n",
|
||||||
|
" print(my_default)\n",
|
||||||
|
"\n",
|
||||||
|
"report_arg()\n",
|
||||||
|
"\n",
|
||||||
|
"time.sleep(5)\n",
|
||||||
|
"\n",
|
||||||
|
"report_arg()"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"1397764090.456688\n",
|
||||||
|
"1397764090.456688"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 10
|
||||||
|
},
|
||||||
{
|
{
|
||||||
"cell_type": "markdown",
|
"cell_type": "markdown",
|
||||||
"metadata": {},
|
"metadata": {},
|
||||||
|
|
@ -1314,7 +1359,8 @@
|
||||||
"cell_type": "markdown",
|
"cell_type": "markdown",
|
||||||
"metadata": {},
|
"metadata": {},
|
||||||
"source": [
|
"source": [
|
||||||
"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavour!"
|
"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavior! \n",
|
||||||
|
"Look at the following examples, and for a fun exercise: try to figure out what is going on before you skip to the solution!"
|
||||||
]
|
]
|
||||||
},
|
},
|
||||||
{
|
{
|
||||||
|
|
@ -1398,6 +1444,266 @@
|
||||||
}
|
}
|
||||||
],
|
],
|
||||||
"prompt_number": 7
|
"prompt_number": 7
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"<br>\n",
|
||||||
|
"<br>\n",
|
||||||
|
"<a name='dynamic_binding'></a>\n",
|
||||||
|
"## Dynamic binding and typos in variable names\n",
|
||||||
|
"Be careful, dynamic binding is convenient, but can also quickly become dangerous!"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"print('first list:')\n",
|
||||||
|
"for i in range(3):\n",
|
||||||
|
" print(i)\n",
|
||||||
|
" \n",
|
||||||
|
"print('\\nsecond list:')\n",
|
||||||
|
"for j in range(3):\n",
|
||||||
|
" print(i) # I (intentionally) made typo here!"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"first list:\n",
|
||||||
|
"0\n",
|
||||||
|
"1\n",
|
||||||
|
"2\n",
|
||||||
|
"\n",
|
||||||
|
"second list:\n",
|
||||||
|
"2\n",
|
||||||
|
"2\n",
|
||||||
|
"2\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 14
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"<br>\n",
|
||||||
|
"<br>\n",
|
||||||
|
"<a name='out_of_range_slicing'></a>\n",
|
||||||
|
"## List slicing using indexes that are \"out of range\""
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"As we have all encountered it 1 (x10000) time(s) in our live, the infamous `IndexError`:"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"my_list = [1, 2, 3, 4, 5]\n",
|
||||||
|
"print(my_list[5])"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"ename": "IndexError",
|
||||||
|
"evalue": "list index out of range",
|
||||||
|
"output_type": "pyerr",
|
||||||
|
"traceback": [
|
||||||
|
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mIndexError\u001b[0m Traceback (most recent call last)",
|
||||||
|
"\u001b[0;32m<ipython-input-15-eb273dc36fdc>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mmy_list\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m[\u001b[0m\u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m2\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m3\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m4\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mmy_list\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
|
||||||
|
"\u001b[0;31mIndexError\u001b[0m: list index out of range"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 15
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"But suprisingly, it is not raised when we are doing list slicing, which can be a really pain for debugging:"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"my_list = [1, 2, 3, 4, 5]\n",
|
||||||
|
"print(my_list[5:])"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"[]\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 16
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"<br>\n",
|
||||||
|
"<br>\n",
|
||||||
|
"<a name='unboundlocalerror'></a>\n",
|
||||||
|
"## Reusing global variable names and `UnboundLocalErrors`"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"Usually, it is no problem to access global variables in the local scope of a function:"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" print(var)\n",
|
||||||
|
"\n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"my_func()"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"global\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 37
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"And is also no problem to use the same variable name in the local scope without affecting the local counterpart: "
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" var = 'locally changed'\n",
|
||||||
|
"\n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"my_func()\n",
|
||||||
|
"print(var)"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"global\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 38
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"But we have to be careful if we use a variable name that occurs in the global scope, and we want to access it in the local function scope if we want to reuse this name:"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" print(var) # want to access global variable\n",
|
||||||
|
" var = 'locally changed' # but Python thinks we forgot to define the local variable!\n",
|
||||||
|
" \n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"my_func()"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"ename": "UnboundLocalError",
|
||||||
|
"evalue": "local variable 'var' referenced before assignment",
|
||||||
|
"output_type": "pyerr",
|
||||||
|
"traceback": [
|
||||||
|
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mUnboundLocalError\u001b[0m Traceback (most recent call last)",
|
||||||
|
"\u001b[0;32m<ipython-input-40-3afd870b7c35>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 4\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 5\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'global'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 6\u001b[0;31m \u001b[0mmy_func\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
|
||||||
|
"\u001b[0;32m<ipython-input-40-3afd870b7c35>\u001b[0m in \u001b[0;36mmy_func\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0mmy_func\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[0;32m----> 2\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mvar\u001b[0m\u001b[0;34m)\u001b[0m \u001b[0;31m# want to access global variable\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 3\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'locally changed'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 4\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 5\u001b[0m \u001b[0mvar\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m'global'\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
|
||||||
|
"\u001b[0;31mUnboundLocalError\u001b[0m: local variable 'var' referenced before assignment"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 40
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "markdown",
|
||||||
|
"metadata": {},
|
||||||
|
"source": [
|
||||||
|
"In this case, we have to use the `global` keyword!"
|
||||||
|
]
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [
|
||||||
|
"def my_func():\n",
|
||||||
|
" global var\n",
|
||||||
|
" print(var) # want to access global variable\n",
|
||||||
|
" var = 'locally changed' # changes the gobal variable\n",
|
||||||
|
"\n",
|
||||||
|
"var = 'global'\n",
|
||||||
|
"\n",
|
||||||
|
"my_func()\n",
|
||||||
|
"print(var)"
|
||||||
|
],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": [
|
||||||
|
{
|
||||||
|
"output_type": "stream",
|
||||||
|
"stream": "stdout",
|
||||||
|
"text": [
|
||||||
|
"global\n",
|
||||||
|
"locally changed\n"
|
||||||
|
]
|
||||||
|
}
|
||||||
|
],
|
||||||
|
"prompt_number": 43
|
||||||
|
},
|
||||||
|
{
|
||||||
|
"cell_type": "code",
|
||||||
|
"collapsed": false,
|
||||||
|
"input": [],
|
||||||
|
"language": "python",
|
||||||
|
"metadata": {},
|
||||||
|
"outputs": []
|
||||||
}
|
}
|
||||||
],
|
],
|
||||||
"metadata": {}
|
"metadata": {}
|
||||||
|
|
|
||||||
Loading…
Reference in New Issue
Block a user