How To Create Customized Context Managers in Python

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On this tutorial, you’ll discover ways to create your personal customized context managers. We’ll evaluate how context manages work after which take a look at the other ways you’ll be able to write your personal. Let’s get began.

What Are Context Managers in Python?

Context managers in Python are objects that allow the administration of assets equivalent to file operations, database connections, or community sockets inside a managed block of code. They make sure that assets are correctly initialized earlier than the block of code executes and robotically cleaned up afterward, no matter whether or not the code block completes usually or raises an exception.

Generally, context managers in Python have the next two particular strategies: __enter__() and __exit__(). These strategies outline the conduct of the context supervisor when getting into and exiting a context.

How Do Context Managers Work?

When working with assets in Python, you must take into account organising the useful resource, anticipate errors, implement exception dealing with, and eventually unencumber the useful resource. To do that, you’ll in all probability use a try-except-finally block like so:

strive: 
    # Establishing the useful resource
    # Working with the useful resource
besides ErrorType:
    # Deal with exceptions
lastly:
    # Unencumber the useful resource

Basically, we strive provisioning and dealing with the useful resource, apart from any errors that will come up through the course of, and eventually unencumber the useful resource. The lastly block is at all times executed no matter whether or not the operation succeeds or not. However with context managers and the with assertion, you’ll be able to have reusable try-except-finally blocks.

Now let’s go over how context managers work.

Enter Section (__enter__() methodology):
When a with assertion is encountered, the __enter__() methodology of the context supervisor is invoked. This methodology is chargeable for initializing and organising the useful resource equivalent to opening a file, establishing a database connection, and the like. The worth returned by __enter__() (if any) is made out there to the context block after the `as` key phrase.

Execute the Block of Code:
As soon as the useful resource is ready up (after __enter__() is executed), the code block related to the with assertion is executed. That is the operation you wish to carry out on the useful resource.

Exit Section (__exit__() methodology):
After the code block completes execution—both usually or as a consequence of an exception—the __exit__() methodology of the context supervisor is named. The __exit__() methodology handles cleanup duties, equivalent to closing the assets. If an exception happens inside the code block, details about the exception (sort, worth, traceback) is handed to __exit__() for error dealing with.

To sum up:

• Context managers present a method to handle assets effectively by guaranteeing that assets are correctly initialized and cleaned up.
• We use the with assertion to outline a context the place assets are managed.
• The __enter__() methodology initializes the useful resource, and the __exit__() methodology cleans up the useful resource after the context block completes.

Now that we all know how context managers work, let’s proceed to put in writing a customized context supervisor for dealing with database connections.

Creating Customized Context Managers in Python

You may write your personal context managers in Python utilizing one of many following two strategies:

1. Writing a category with __enter__() and __exit__() strategies.
2. Utilizing the contextlib module which supplies the contextmanager decorator to put in writing a context supervisor utilizing generator features.

1. Writing a Class with __enter__() and __exit__() Strategies

You may outline a category that implements the 2 particular strategies: __enter__() and __exit__() that management useful resource setup and teardown respectively. Right here we write a ConnectionManager class that establishes a connection to an SQLite database and closes the database connection:

import sqlite3
from typing import Non-compulsory

# Writing a context supervisor class
class ConnectionManager:
    def __init__(self, db_name: str):
        self.db_name = db_name
        self.conn: Non-compulsory[sqlite3.Connection] = None

    def __enter__(self):
        self.conn = sqlite3.join(self.db_name)
        return self.conn

    def __exit__(self, exc_type, exc_value, traceback):
        if self.conn:
        self.conn.shut()

Let’s break down how the ConnectionManager works:

• The __enter__() methodology is named when the execution enters the context of the with assertion. It’s chargeable for organising the context, on this case, connecting to a database. It returns the useful resource that must be managed: the database connection. Notice that we’ve used the Non-compulsory sort from the typing module for the connection object conn. We use Non-compulsory when the worth may be certainly one of two varieties: right here a sound connection object or None.
• The __exit__() methodology: It is known as when the execution leaves the context of the with assertion. It handles the cleanup motion of closing the connection. The parameters exc_type, exc_value, and traceback are for dealing with exceptions inside the `with` block. These can be utilized to find out whether or not the context was exited as a consequence of an exception.

Now let’s use the ConnectionManager within the with assertion. We do the next:

• Strive to connect with the database
• Create a cursor to run queries
• Create a desk and insert data
• Question the database desk and retrieve the outcomes of the question

db_name = "library.db"

# Utilizing ConnectionManager context supervisor immediately
with ConnectionManager(db_name) as conn:
	cursor = conn.cursor()

	# Create a books desk if it would not exist
	cursor.execute("""
    	CREATE TABLE IF NOT EXISTS books (
        	id INTEGER PRIMARY KEY,
        	title TEXT,
        	creator TEXT,
        	publication_year INTEGER
    	)
	""")

	# Insert pattern e-book data
	books_data = [
    	("The Great Gatsby", "F. Scott Fitzgerald", 1925),
    	("To Kill a Mockingbird", "Harper Lee", 1960),
    	("1984", "George Orwell", 1949),
    	("Pride and Prejudice", "Jane Austen", 1813)
	]
	cursor.executemany("INSERT INTO books (title, creator, publication_year) VALUES (?, ?, ?)", books_data)
	conn.commit()

	# Retrieve and print all e-book data
	cursor.execute("SELECT * FROM books")
	data = cursor.fetchall()
	print("Library Catalog:")
	for document in data:
    	    book_id, title, creator, publication_year = document
    	    print(f"E book ID: {book_id}, Title: {title}, Creator: {creator}, 12 months: {publication_year}")
            cursor.shut()

Working the above code ought to provide the following output:

Output >>>

Library Catalog:
E book ID: 1, Title: The Nice Gatsby, Creator: F. Scott Fitzgerald, 12 months: 1925
E book ID: 2, Title: To Kill a Mockingbird, Creator: Harper Lee, 12 months: 1960
E book ID: 3, Title: 1984, Creator: George Orwell, 12 months: 1949
E book ID: 4, Title: Delight and Prejudice, Creator: Jane Austen, 12 months: 1813

2. Utilizing the @contextmanager Decorator From contextlib

The contextlib module supplies the @contextmanager decorator which can be utilized to outline a generator perform as a context supervisor. Here is how we do it for the database connection instance:

# Writing a generator perform with the `@contextmanager` decorator
import sqlite3
from contextlib import contextmanager

@contextmanager
def database_connection(db_name: str):
    conn = sqlite3.join(db_name)
    strive:
        yield conn  # Present the connection to the 'with' block
    lastly:
        conn.shut()  # Shut the connection upon exiting the 'with' block

Right here’s how the database_connection perform works:

• The database_connection perform first establishes a connection which the yield assertion then provisions the connection to the block of code within the with assertion block. Notice that as a result of yield itself just isn’t proof against exceptions, we wrap it in a strive block.
• The lastly block ensures that the connection is at all times closed, whether or not an exception was raised or not, guaranteeing there aren’t any useful resource leaks.

Like we did beforehand, let’s use this in a with assertion:

db_name = "library.db"

# Utilizing database_connection context supervisor immediately
with database_connection(db_name) as conn:
	cursor = conn.cursor()

	# Insert a set of e-book data
	more_books_data = [
    	("The Catcher in the Rye", "J.D. Salinger", 1951),
    	("To the Lighthouse", "Virginia Woolf", 1927),
    	("Dune", "Frank Herbert", 1965),
    	("Slaughterhouse-Five", "Kurt Vonnegut", 1969)
	]
	cursor.executemany("INSERT INTO books (title, creator, publication_year) VALUES (?, ?, ?)", more_books_data)
	conn.commit()

	# Retrieve and print all e-book data
	cursor.execute("SELECT * FROM books")
	data = cursor.fetchall()
	print("Up to date Library Catalog:")
	for document in data:
    	    book_id, title, creator, publication_year = document
    	    print(f"E book ID: {book_id}, Title: {title}, Creator: {creator}, 12 months: {publication_year}")
        cursor.shut()

We hook up with the database, insert some extra data, question the db, and fetch the outcomes of the question. Right here’s the output:

Output >>>

Up to date Library Catalog:
E book ID: 1, Title: The Nice Gatsby, Creator: F. Scott Fitzgerald, 12 months: 1925
E book ID: 2, Title: To Kill a Mockingbird, Creator: Harper Lee, 12 months: 1960
E book ID: 3, Title: 1984, Creator: George Orwell, 12 months: 1949
E book ID: 4, Title: Delight and Prejudice, Creator: Jane Austen, 12 months: 1813
E book ID: 5, Title: The Catcher within the Rye, Creator: J.D. Salinger, 12 months: 1951
E book ID: 6, Title: To the Lighthouse, Creator: Virginia Woolf, 12 months: 1927
E book ID: 7, Title: Dune, Creator: Frank Herbert, 12 months: 1965
E book ID: 8, Title: Slaughterhouse-5, Creator: Kurt Vonnegut, 12 months: 1969

Notice that we open and shut the cursor object. So you can too use the cursor object in a with assertion. I recommend attempting that as a fast train!

Wrapping Up

And that’s a wrap. I hope you discovered methods to create your personal customized context managers. We checked out two approaches: utilizing a category with __enter__() and __exit()__ strategies and utilizing a generator perform adorned with the @contextmanager decorator.

It’s fairly straightforward to see that you simply get the next benefits when utilizing a context supervisor:

• Setup and teardown of assets are robotically managed, minimizing useful resource leaks.
• The code is cleaner and simpler to keep up.
• Cleaner exception dealing with when working with assets.

As at all times, you’ll be able to discover the code on GitHub. Preserve coding!

Bala Priya C is a developer and technical author from India. She likes working on the intersection of math, programming, knowledge science, and content material creation. Her areas of curiosity and experience embody DevOps, knowledge science, and pure language processing. She enjoys studying, writing, coding, and low! At present, she’s engaged on studying and sharing her information with the developer group by authoring tutorials, how-to guides, opinion items, and extra. Bala additionally creates participating useful resource overviews and coding tutorials.