197. Contextlib's closing

contextlib.closing is a utility in the contextlib module that helps ensure resources such as files or sockets are properly closed when they are no longer needed, by automatically calling their close() method. It works in a context manager, ensuring that the resource is closed even if an exception occurs within the context.

Here are several Python code snippets demonstrating the use of contextlib.closing for safely handling and closing resources:

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1. Basic Example with File Handling

from contextlib import closing
import urllib.request

# Using closing to ensure the file is closed properly after use
with closing(open('example.txt', 'w')) as file:
    file.write("Hello, World!")

In this example, the file is opened and written to within the with block, and the closing context manager ensures that the file is automatically closed after the block is executed.

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2. Using closing with Socket

from contextlib import closing
import socket

# Example of using closing with socket
with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as sock:
    sock.connect(('example.com', 80))
    sock.sendall(b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n")
    response = sock.recv(1024)
    print(response.decode())

Here, closing ensures the socket is properly closed after the communication, even if an exception is raised during the request.

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3. Using closing with Database Connections

from contextlib import closing
import sqlite3

# Using closing with a database connection
with closing(sqlite3.connect('my_database.db')) as conn:
    cursor = conn.cursor()
    cursor.execute('SELECT * FROM my_table')
    result = cursor.fetchall()
    print(result)

This ensures that the database connection is automatically closed after the database operations, preventing potential resource leakage.

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4. Using closing with a Custom Resource

from contextlib import closing

class CustomResource:
    def __enter__(self):
        print("Opening resource...")
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        print("Closing resource...")

# Using closing with a custom resource
with closing(CustomResource()) as resource:
    print("Using resource...")

In this example, the closing context manager wraps the CustomResource, ensuring the __exit__ method is called, which closes the resource.

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5. Handling Multiple Resources Safely

from contextlib import closing
import os

# Using closing with multiple resources
with closing(open('file1.txt', 'w')) as file1, closing(open('file2.txt', 'w')) as file2:
    file1.write("Data for file 1")
    file2.write("Data for file 2")

This ensures both files are closed safely even if an error occurs while writing to one of them.

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6. Using closing for Network Resources

from contextlib import closing
import socket

# Ensuring the network connection is closed
with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as sock:
    sock.connect(('localhost', 8080))
    sock.sendall(b'GET / HTTP/1.1\r\nHost: localhost\r\n\r\n')
    response = sock.recv(1024)
    print(response.decode())

Using closing guarantees that the socket is closed properly after the communication.

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7. Using closing for a Web Request

from contextlib import closing
import urllib.request

# Closing after fetching a URL
with closing(urllib.request.urlopen('http://example.com')) as response:
    html = response.read()
    print(html[:100])  # Print first 100 characters of the response

This makes sure that the connection to the URL is closed after fetching the data, even if an exception occurs.

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8. Custom Context Manager for Closing Resources

from contextlib import closing

class Resource:
    def close(self):
        print("Resource closed.")

with closing(Resource()) as res:
    print("Resource is being used.")

This demonstrates using closing with a custom object that implements a close() method.

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9. Using closing for a Temporary File

from contextlib import closing
import tempfile

# Using closing with temporary file
with closing(tempfile.NamedTemporaryFile(delete=False)) as temp_file:
    temp_file.write(b"Temporary data")
    print(f"Temporary file created: {temp_file.name}")

The closing context manager ensures the temporary file is closed after use.

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10. Ensuring Network Connection Closes with Exceptions

from contextlib import closing
import socket

# Ensuring socket is closed even with exceptions
try:
    with closing(socket.socket(socket.AF_INET, socket.SOCK_STREAM)) as sock:
        sock.connect(('localhost', 9000))
        sock.sendall(b"Hello server!")
        raise Exception("Something went wrong!")  # Simulate an exception
except Exception as e:
    print(f"Exception caught: {e}")

Even if an exception is raised during the communication, the closing context manager ensures that the socket is closed properly.

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Summary:

• closing Context Manager: This is useful for any resource that has a close() method, like files, sockets, and database connections.

• Automatic Cleanup: Ensures that resources are closed after they are no longer needed, preventing resource leakage.

• Handling Exceptions: Even if an exception occurs inside the with block, the close() method will always be called when the block is exited.

Using contextlib.closing is a great way to manage resources, ensuring that they are properly cleaned up even if errors occur during their use.