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David Buchaca
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update: context manager example
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‎python_basics/context_manager/01_context_manager_timer.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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"id": "c053fde3",
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"metadata": {},
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"source": [
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"# How to create a context manager\n",
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"\n",
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"A context manager is a class that needs to have at least two methods:\n",
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"\n",
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"- **`__enter__`**: executed when the interpreter enters the context block.\n",
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"\n",
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"\n",
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"- **`__exit__`**: executed allways either after the last line in the context block OR if an error happens.\n",
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"\n",
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"\n",
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"### Example: Context manager to print execution time \n",
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"\n",
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"Let us create a context manager that tells us the execution time of the code within it.\n",
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"\n",
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"To make it more useful let us print also a user defined string to faciliate knowing what we are timing, in the event we want to time more than one line in a piece of code. "
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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": 14,
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"id": "35fb1490",
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"metadata": {},
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"outputs": [],
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"source": [
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"import time\n",
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"from time import sleep\n",
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"\n",
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"class timer():\n",
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"\n",
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" def __init__(self, name = ''):\n",
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" self.start = time.time()\n",
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" self.name = name\n",
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"\n",
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" def __enter__(self):\n",
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" pass\n",
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"\n",
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" def __exit__(self, *args):\n",
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" print(f'time {self.name} {round(time.time() - self.start,6)} sec')"
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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": 24,
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"id": "c04fe165",
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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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"time par for 0.205038 sec\n"
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]
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}
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],
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"source": [
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"with timer('par for'):\n",
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" sleep(0.2)"
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]
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},
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{
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"cell_type": "markdown",
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"id": "7f0e1f69",
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"metadata": {},
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"source": [
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"Note that the execution time is printed even if there is an error"
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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": 26,
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"id": "77a696f1",
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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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"time non existing file 0.000449 sec\n"
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]
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},
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{
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"ename": "FileNotFoundError",
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"evalue": "[Errno 2] No such file or directory: 'does_not_exists.md'",
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"output_type": "error",
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"traceback": [
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"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
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"\u001b[0;31mFileNotFoundError\u001b[0m Traceback (most recent call last)",
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"Cell \u001b[0;32mIn[26], line 2\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[38;5;28;01mwith\u001b[39;00m timer(\u001b[38;5;124m'\u001b[39m\u001b[38;5;124mnon existing file\u001b[39m\u001b[38;5;124m'\u001b[39m):\n\u001b[0;32m----> 2\u001b[0m \u001b[38;5;28;43mopen\u001b[39;49m\u001b[43m(\u001b[49m\u001b[38;5;124;43m\"\u001b[39;49m\u001b[38;5;124;43mdoes_not_exists.md\u001b[39;49m\u001b[38;5;124;43m\"\u001b[39;49m\u001b[43m,\u001b[49m\u001b[43m \u001b[49m\u001b[38;5;124;43m'\u001b[39;49m\u001b[38;5;124;43mr\u001b[39;49m\u001b[38;5;124;43m'\u001b[39;49m\u001b[43m)\u001b[49m\n",
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"File \u001b[0;32m~/opt/anaconda3/lib/python3.9/site-packages/IPython/core/interactiveshell.py:284\u001b[0m, in \u001b[0;36m_modified_open\u001b[0;34m(file, *args, **kwargs)\u001b[0m\n\u001b[1;32m 277\u001b[0m \u001b[38;5;28;01mif\u001b[39;00m file \u001b[38;5;129;01min\u001b[39;00m {\u001b[38;5;241m0\u001b[39m, \u001b[38;5;241m1\u001b[39m, \u001b[38;5;241m2\u001b[39m}:\n\u001b[1;32m 278\u001b[0m \u001b[38;5;28;01mraise\u001b[39;00m \u001b[38;5;167;01mValueError\u001b[39;00m(\n\u001b[1;32m 279\u001b[0m \u001b[38;5;124mf\u001b[39m\u001b[38;5;124m\"\u001b[39m\u001b[38;5;124mIPython won\u001b[39m\u001b[38;5;124m'\u001b[39m\u001b[38;5;124mt let you open fd=\u001b[39m\u001b[38;5;132;01m{\u001b[39;00mfile\u001b[38;5;132;01m}\u001b[39;00m\u001b[38;5;124m by default \u001b[39m\u001b[38;5;124m\"\u001b[39m\n\u001b[1;32m 280\u001b[0m \u001b[38;5;124m\"\u001b[39m\u001b[38;5;124mas it is likely to crash IPython. If you know what you are doing, \u001b[39m\u001b[38;5;124m\"\u001b[39m\n\u001b[1;32m 281\u001b[0m \u001b[38;5;124m\"\u001b[39m\u001b[38;5;124myou can use builtins\u001b[39m\u001b[38;5;124m'\u001b[39m\u001b[38;5;124m open.\u001b[39m\u001b[38;5;124m\"\u001b[39m\n\u001b[1;32m 282\u001b[0m )\n\u001b[0;32m--> 284\u001b[0m \u001b[38;5;28;01mreturn\u001b[39;00m \u001b[43mio_open\u001b[49m\u001b[43m(\u001b[49m\u001b[43mfile\u001b[49m\u001b[43m,\u001b[49m\u001b[43m \u001b[49m\u001b[38;5;241;43m*\u001b[39;49m\u001b[43margs\u001b[49m\u001b[43m,\u001b[49m\u001b[43m \u001b[49m\u001b[38;5;241;43m*\u001b[39;49m\u001b[38;5;241;43m*\u001b[39;49m\u001b[43mkwargs\u001b[49m\u001b[43m)\u001b[49m\n",
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"\u001b[0;31mFileNotFoundError\u001b[0m: [Errno 2] No such file or directory: 'does_not_exists.md'"
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]
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}
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],
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"source": [
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"with timer('non existing file'):\n",
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" open(\"does_not_exists.md\", 'r')"
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]
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},
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{
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"cell_type": "markdown",
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"id": "7cfe5478",
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"metadata": {},
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"source": [
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"THere is another way to create such context manager, which is using contextlib.contextmanager"
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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": 13,
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"id": "5bc668ac",
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"metadata": {},
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"outputs": [],
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"source": [
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"from contextlib import contextmanager\n",
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"import time\n",
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"\n",
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"@contextmanager\n",
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"def timer(name=''):\n",
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" start = time.time()\n",
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" yield\n",
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" end = time.time()\n",
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" print(f\"Time needed for {name}: {end-start} sec\")"
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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": 14,
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"id": "22a71fe3",
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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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"Time needed for sum: 3.0994415283203125e-06 sec\n"
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]
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}
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],
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"source": [
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"import requests\n",
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"\n",
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"with timer('sum'):\n",
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" 2*3"
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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 (ipykernel)",
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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": "3.9.12"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 5
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}

‎python_basics/context_manager/README.md‎

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# `01_context_manager_timer`
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## How to create a context manager
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A context manager is a class that needs to have at least two methods:
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- **`__enter__`**: executed when the interpreter enters the context block.
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- **`__exit__`**: executed allways either after the last line in the context block OR if an error happens.
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13+
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### Example: Context manager to print execution time
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Let us create a context manager that tells us the execution time of the code within it.
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To make it more useful let us print also a user defined string to faciliate knowing what we are timing, in the event we want to time more than one line in a piece of code.
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```python
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import time
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from time import sleep
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class timer():
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def __init__(self, name = ''):
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self.start = time.time()
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self.name = name
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def __enter__(self):
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pass
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def __exit__(self, *args):
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print(f'time {self.name} {round(time.time() - self.start,6)} sec')
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```
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Let us use the context manager to time how long does it take to run `sleep(0.2)`
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```python
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with timer('par for'):
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sleep(0.2)
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```
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time par for 0.205038 sec
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Note that the execution time is printed even if there is an error
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```python
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with timer('non existing file'):
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open("does_not_exists.md", 'r')
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```
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time non existing file 0.000449 sec
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---------------------------------------------------------------------------
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FileNotFoundError Traceback (most recent call last)
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Cell In[26], line 2
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1 with timer('non existing file'):
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----> 2 open("does_not_exists.md", 'r')
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File ~/opt/anaconda3/lib/python3.9/site-packages/IPython/core/interactiveshell.py:284, in _modified_open(file, *args, **kwargs)
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277 if file in {0, 1, 2}:
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278 raise ValueError(
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279 f"IPython won't let you open fd={file} by default "
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280 "as it is likely to crash IPython. If you know what you are doing, "
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281 "you can use builtins' open."
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282 )
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--> 284 return io_open(file, *args, **kwargs)
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FileNotFoundError: [Errno 2] No such file or directory: 'does_not_exists.md'
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THere is another way to create such context manager, which is using contextlib.contextmanager
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```python
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from contextlib import contextmanager
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import time
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@contextmanager
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def timer(name=''):
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start = time.time()
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yield
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end = time.time()
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print(f"Time needed for {name}: {end-start} sec")
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```
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```python
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import requests
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with timer('sum'):
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2*3
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```
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Time needed for sum: 3.0994415283203125e-06 sec
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