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‎14-Advanced Python Objects and Data Structures/08-BONUS - With Statement Context Managers.ipynb

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{
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"cells": [
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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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"# With Statement Context Managers\n",
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"\n",
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"When you open a file using `f = open('test.txt')`, the file stays open until you specifically call `f.close()`. Should an exception be raised while working with the file, it remains open. This can lead to vulnerabilities in your code, and inefficient use of resources.\n",
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"\n",
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"A context manager handles the opening and closing of resources, and provides a built-in `try/finally` block should any exceptions occur.\n",
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"\n",
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"The best way to demonstrate this is with an example.\n",
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"\n",
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"### Standard `open()` procedure, with a raised exception:\n"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 1,
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"metadata": {},
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"outputs": [
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{
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"ename": "UnsupportedOperation",
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"evalue": "not readable",
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"output_type": "error",
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"traceback": [
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"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
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"\u001b[1;31mUnsupportedOperation\u001b[0m Traceback (most recent call last)",
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"\u001b[1;32m<ipython-input-1-ad7a2000735b>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m 1\u001b[0m \u001b[0mp\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mopen\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'oops.txt'\u001b[0m\u001b[1;33m,\u001b[0m\u001b[1;34m'a'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[0mp\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mreadlines\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m 3\u001b[0m \u001b[0mp\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mclose\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
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"\u001b[1;31mUnsupportedOperation\u001b[0m: not readable"
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]
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}
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],
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"source": [
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"p = open('oops.txt','a')\n",
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"p.readlines()\n",
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"p.close()"
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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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"Let's see if we can modify our file:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 2,
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"13"
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]
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},
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"execution_count": 2,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"p.write('add more text')"
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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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"Ouch! I may not have wanted to do that until I traced the exception! Unfortunately, the exception prevented the last line, `p.close()` from running. Let's close the file manually:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 3,
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"metadata": {},
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"outputs": [],
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"source": [
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"p.close()"
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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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"### Protect the file with `try/except/finally`\n",
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"\n",
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"A common workaround is to insert a `try/except/finally` clause to close the file whenever an exception is raised:\n"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 4,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"An exception was raised!\n"
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]
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}
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],
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"source": [
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"p = open('oops.txt','a')\n",
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"try:\n",
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" p.readlines()\n",
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"except:\n",
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" print('An exception was raised!')\n",
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"finally:\n",
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" p.close()"
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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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"Let's see if we can modify our file this time:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 5,
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"metadata": {},
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"outputs": [
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{
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"ename": "ValueError",
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"evalue": "I/O operation on closed file.",
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"output_type": "error",
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"traceback": [
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"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
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"\u001b[1;31mValueError\u001b[0m Traceback (most recent call last)",
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"\u001b[1;32m<ipython-input-5-1209a18e617d>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0mp\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mwrite\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'add more text'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[1;31mValueError\u001b[0m: I/O operation on closed file."
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]
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}
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],
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"source": [
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"p.write('add more text')"
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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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"Excellent! Our file is safe.\n",
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"\n",
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"### Save steps with `with`\n",
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"\n",
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"Now we'll employ our context manager. The syntax follows `with [resource] as [target]: do something`"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 6,
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"metadata": {},
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"outputs": [
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{
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"ename": "UnsupportedOperation",
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"evalue": "not readable",
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"output_type": "error",
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"traceback": [
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"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
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"\u001b[1;31mUnsupportedOperation\u001b[0m Traceback (most recent call last)",
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"\u001b[1;32m<ipython-input-6-7ccc44e332f9>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m 1\u001b[0m \u001b[1;32mwith\u001b[0m \u001b[0mopen\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'oops.txt'\u001b[0m\u001b[1;33m,\u001b[0m\u001b[1;34m'a'\u001b[0m\u001b[1;33m)\u001b[0m \u001b[1;32mas\u001b[0m \u001b[0mp\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[0mp\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mreadlines\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[1;31mUnsupportedOperation\u001b[0m: not readable"
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]
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}
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],
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"source": [
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"with open('oops.txt','a') as p:\n",
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" p.readlines()"
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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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"Can we modify the file?"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 7,
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"metadata": {},
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"outputs": [
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{
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"ename": "ValueError",
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"evalue": "I/O operation on closed file.",
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"output_type": "error",
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"traceback": [
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"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
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"\u001b[1;31mValueError\u001b[0m Traceback (most recent call last)",
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"\u001b[1;32m<ipython-input-7-1209a18e617d>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[0mp\u001b[0m\u001b[1;33m.\u001b[0m\u001b[0mwrite\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m'add more text'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
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"\u001b[1;31mValueError\u001b[0m: I/O operation on closed file."
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]
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}
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],
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"source": [
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"p.write('add more text')"
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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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"Great! With just one line of code we've handled opening the file, enclosing our code in a `try/finally` block, and closing our file all at the same time.\n",
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"\n",
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"Now you should have a basic understanding of context managers."
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 3",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.6.2"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 2
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}

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