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Comprehensive Guide to Python's with Statement

1. What is the with Statement?

Python's with statement simplifies resource management and exception handling. It ensures that resources like files, network connections, and databases are properly acquired and released.

2. Basic Usage

with open('example.txt', 'r') as file:
    data = file.read()
    print(data)

In this example, the file example.txt is opened in read mode, its contents are read, and the file is automatically closed after the block is executed.

3. What is a Context Manager?

The with statement works with context managers, which implement the __enter__() and __exit__() methods to manage resource setup and teardown.

4. Custom Context Manager

class CustomContextManager:
    def __enter__(self):
        print("Setting up resource")
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        print("Releasing resource")

with CustomContextManager() as manager:
    print("Performing task")

This example demonstrates a custom context manager that manages resource setup and release.

5. Using contextlib Module

from contextlib import contextmanager

@contextmanager
def managed_resource():
    print("Setting up resource")
    yield
    print("Releasing resource")

with managed_resource():
    print("Performing task")

The contextlib module's @contextmanager decorator allows for easy creation of function-based context managers.

6. Nested with Statements

with open('file1.txt', 'r') as f1, open('file2.txt', 'r') as f2:
    data1 = f1.read()
    data2 = f2.read()

When managing multiple resources simultaneously, nested with statements can be used as shown above.

7. Advanced Usage Examples

7.1. Database Connection

import sqlite3

with sqlite3.connect('example.db') as conn:
    cursor = conn.cursor()
    cursor.execute('SELECT * FROM users')
    results = cursor.fetchall()

7.2. Network Socket

import socket

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect(('localhost', 8080))
    s.sendall(b'Hello, world')
    data = s.recv(1024)

7.3. Measuring Execution Time

import time

class Timer:
    def __enter__(self):
        self.start = time.time()
        return self

    def __exit__(self, *args):
        self.end = time.time()
        print(f"Execution time: {self.end - self.start} seconds")

with Timer():
    time.sleep(2)

8. References

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