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Python : Building Powerful Context Managers and Decorators

   January 2, 2024

Python’s context managers and decorators are indispensable tools for creating clean, concise, and efficient code. In this article, we’ll explore the process of building context managers in Python, along with real-world examples and their outputs.

Understanding Context Managers:

Context managers in Python facilitate resource management by providing a way to allocate and release resources automatically. They ensure that resources are properly managed, even in the presence of exceptions or other errors. Let’s dive into building custom context managers:

1. Using Classes:

The most common approach to creating context managers is by defining a class with __enter__ and __exit__ methods. These methods handle the setup and teardown of resources. Consider the following example of a file opener context manager:

class FileOpener:
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode

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

    def __exit__(self, exc_type, exc_value, traceback):
        self.file.close()

with FileOpener('example.txt', 'r') as file:
    content = file.read()
    print(content)
Output:
Contents of 'example.txt'

2. Using Contextlib Module:

The contextlib module provides utilities for creating context managers with less boilerplate code. The contextmanager decorator can be used to define a generator function that yields the resource to be managed. Let’s see an example:

from contextlib import contextmanager
@contextmanager
def file_opener(filename, mode):
    file = open(filename, mode)
    try:
        yield file
    finally:
        file.close()
with file_opener('example.txt', 'r') as file:
    content = file.read()
    print(content)

Output:

Contents of 'example.txt'

Database Connection Context Manager

Let’s create a context manager for managing database connections using SQLite:

import sqlite3
from contextlib import contextmanager
@contextmanager
def sqlite_connection(db_name):
    conn = sqlite3.connect(db_name)
    try:
        yield conn
    finally:
        conn.close()
with sqlite_connection('example.db') as conn:
    cursor = conn.cursor()
    cursor.execute("CREATE TABLE IF NOT EXISTS users (id INTEGER PRIMARY KEY, name TEXT)")
    cursor.execute("INSERT INTO users (name) VALUES ('Alice')")
    cursor.execute("INSERT INTO users (name) VALUES ('Bob')")
    conn.commit()
    cursor.execute("SELECT * FROM users")
    print(cursor.fetchall())

Output:

[(1, 'Alice'), (2, 'Bob')]
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