Context Managers

# Using the with statement to open and write a file
with open('sample.txt', 'w') as file:
  file.write('Hello world')

# Using the with statement to open and read a file
with open('sample.txt', 'r') as file:
    content = file.read()
    print(content)

from contextlib import contextmanager

@contextmanager
def managed_file(filename):
    file = open(filename, 'w')
    try:
        yield file
    finally:
        file.close()

# Using the custom context manager
with managed_file('sample2.txt') as file:
    file.write('Hello, World!')

class ManagedFile:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        self.file = open(self.filename, 'w')
        return self.file

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()
        if exc_type is not None:
            print(f"An exception occurred: {exc_type}, {exc_val}")

# Using the class-based context manager
with ManagedFile('sample3.txt') as file:
    file.write('Hello, again!')

import sqlite3
from contextlib import contextmanager

@contextmanager
def open_database(db_name):
    connection = sqlite3.connect(db_name)
    cursor = connection.cursor()
    try:
        yield cursor
    finally:
        connection.commit()
        connection.close()

# Using the database context manager
with open_database('example.db') as cursor:
    cursor.execute('CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, name TEXT)')
    cursor.execute('INSERT INTO users (name) VALUES (?)', ('Alice',))

import time
from contextlib import contextmanager

@contextmanager
def timer():
    start = time.time()
    try:
        yield
    finally:
        end = time.time()
        print(f"Elapsed time: {end - start} seconds")

# Using the timer context manager
with timer():
    time.sleep(2)  # Simulate a time-consuming task