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https://github.com/rasbt/python_reference.git
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fixes and new section
This commit is contained in:
Sebastian Raschka
parent
a0edd6b293
commit
9f22042812
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@ -1,6 +1,6 @@
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{
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"metadata": {
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"name": "",
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"name": "not_so_obvious_python_stuff",
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"signature": "sha256:9654ab25e9d989ee8f196ec7cd5c2b7222157dbe195ddf70e6ec34329c9db545"
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},
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"nbformat": 3,
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@ -80,7 +80,8 @@
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"- [Key differences between Python 2 and 3](#python_differences)\n",
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"- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)\n",
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"- [Abortive statements in `finally` blocks](#finally_blocks)\n",
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"- [Assigning types to variables as values](#variable_types)"
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"- [Assigning types to variables as values](#variable_types)\n",
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"- [Only the first clause of generators is evaluated immediately](#generator_rhs)"
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]
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},
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{
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@ -274,13 +275,13 @@
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"collapsed": false,
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"input": [
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"a_list = []\n",
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"print('ID:',id(a_list))\n",
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"print(a_list, '\\nID (initial):',id(a_list), '\\n')\n",
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"\n",
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"a_list.append(1)\n",
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"print('ID (append):',id(a_list))\n",
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"print(a_list, '\\nID (append):',id(a_list), '\\n')\n",
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"\n",
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"a_list.append(2)\n",
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"print('ID (extend):',id(a_list))"
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"a_list.extend([2])\n",
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"print(a_list, '\\nID (extend):',id(a_list))"
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],
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"language": "python",
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"metadata": {},
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@ -289,13 +290,18 @@
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"ID: 4366495544\n",
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"ID (append): 4366495544\n",
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"ID (extend): 4366495544\n"
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"[] \n",
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"ID (initial): 140704077653128 \n",
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"\n",
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"[1] \n",
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"ID (append): 140704077653128 \n",
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"\n",
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"[1, 2] \n",
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"ID (extend): 140704077653128\n"
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]
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}
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],
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"prompt_number": 7
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"prompt_number": 6
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},
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{
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"cell_type": "code",
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@ -2979,6 +2985,102 @@
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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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"<a name='generator_rhs'>"
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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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"# Only the first clause of generators is evaluated immediately"
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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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"The main reason why we love to use generators in certain cases (i.e., when we are dealing with large numbers of computations) is that it only computes the next value when it is needed, which is also known as \"lazy\" evaluation.\n",
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"However, the first clause of an generator is already checked upon it's creation, as the following example demonstrates:"
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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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"gen_fails = (i for i in 1/0)"
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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": "ZeroDivisionError",
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"evalue": "division by zero",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mZeroDivisionError\u001b[0m Traceback (most recent call last)",
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"\u001b[0;32m<ipython-input-18-29312e1ece8d>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mgen_fails\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0mi\u001b[0m \u001b[0;32mfor\u001b[0m \u001b[0mi\u001b[0m \u001b[0;32min\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m/\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[0;31mZeroDivisionError\u001b[0m: division by zero"
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]
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}
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],
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"prompt_number": 18
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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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"Certainly, this is a nice feature, since it notifies us about syntax erros immediately. However, this is (unfortunately) not the case if we have multiple cases in our generator."
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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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"gen_succeeds = (i for i in range(5) for j in 1/0)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [],
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"prompt_number": 19
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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('But obviously fails when we iterate ...')\n",
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"for i in gen_succeeds:\n",
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" print(i)"
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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": "ZeroDivisionError",
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"evalue": "division by zero",
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"output_type": "pyerr",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m\n\u001b[0;31mZeroDivisionError\u001b[0m Traceback (most recent call last)",
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"\u001b[0;32m<ipython-input-20-8a83a1022971>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m'But obviously fails when we iterate ...'\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 2\u001b[0;31m \u001b[0;32mfor\u001b[0m \u001b[0mi\u001b[0m \u001b[0;32min\u001b[0m \u001b[0mgen_succeeds\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m 3\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mi\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n",
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"\u001b[0;32m<ipython-input-19-c54c53f2218a>\u001b[0m in \u001b[0;36m<genexpr>\u001b[0;34m(.0)\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mgen_succeeds\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;34m(\u001b[0m\u001b[0mi\u001b[0m \u001b[0;32mfor\u001b[0m \u001b[0mi\u001b[0m \u001b[0;32min\u001b[0m \u001b[0mrange\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;36m5\u001b[0m\u001b[0;34m)\u001b[0m \u001b[0;32mfor\u001b[0m \u001b[0mj\u001b[0m \u001b[0;32min\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m/\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[0;31mZeroDivisionError\u001b[0m: division by zero"
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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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"But obviously fails when we iterate ...\n"
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]
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}
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],
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"prompt_number": 20
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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@ -3010,18 +3112,9 @@
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"source": [
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"#### 04/27/2014\n",
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"- minor fixes of typos \n",
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"- single- vs. double-underscore clarification in the private class section."
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"- new section: \"Only the first clause of generators is evaluated immediately\""
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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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"language": "python",
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"metadata": {},
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"outputs": [],
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"prompt_number": 1
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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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