How To Create Customized Context Managers in Python

Context managers in Python allow you to work extra effectively with sources—facilitating setup and teardown of sources even when there are errors when working with the sources. Within the tutorial on writing environment friendly Python code, I lined what context managers are and why they’re useful. And in 3 Fascinating Makes use of of Python’s Context Managers, I went over the usage of context managers in managing subprocesses, database connections, and extra.

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

What Are Context Managers in Python?

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

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

How Do Context Managers Work?

When working with sources in Python, it’s a must to take into account establishing the useful resource, anticipate errors, implement exception dealing with, and at last release the useful resource. To do that, you’ll most likely use a try-except-finally block like so:

strive: 
    # Organising the useful resource
    # Working with the useful resource
besides ErrorType:
    # Deal with exceptions
lastly:
    # Liberate the useful resource

Primarily, we strive provisioning and dealing with the useful resource, aside from any errors that will come up throughout the course of, and at last release the useful resource. The lastly block is all the time executed no matter whether or not the operation succeeds or not. However with context managers and the with assertion, you may have reusable try-except-finally blocks.

Now let’s go over how context managers work.

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

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

Exit Part (__exit__() technique):
After the code block completes execution—both usually or as a consequence of an exception—the __exit__() technique of the context supervisor is known as. The __exit__() technique handles cleanup duties, corresponding to closing the sources. If an exception happens throughout the code block, details about the exception (kind, worth, traceback) is handed to __exit__() for error dealing with.

To sum up:

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

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

Creating Customized Context Managers in Python

You’ll be able to 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 jot down a context supervisor utilizing generator features.

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

You’ll be able to 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 Optionally available

# Writing a context supervisor class
class ConnectionManager:
    def __init__(self, db_name: str):
        self.db_name = db_name
        self.conn: Optionally available[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__() technique is known as when the execution enters the context of the with assertion. It’s chargeable for establishing 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 Optionally available kind from the typing module for the connection object conn. We use Optionally available when the worth will be one in every of two sorts: right here a sound connection object or None.
• The __exit__() technique: It is referred to 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 throughout 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:

• Attempt 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 does not exist
	cursor.execute("""
    	CREATE TABLE IF NOT EXISTS books (
        	id INTEGER PRIMARY KEY,
        	title TEXT,
        	creator TEXT,
        	publication_year INTEGER
    	)
	""")

	# Insert pattern ebook 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, author, publication_year) VALUES (?, ?, ?)", books_data)
	conn.commit()

	# Retrieve and print all ebook 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"Book ID: {book_id}, Title: {title}, Author: {author}, Year: {publication_year}")
            cursor.shut()

Operating the above code ought to provide the following output:

Output >>>

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

2. Utilizing the @contextmanager Decorator From contextlib

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

# Writing a generator operate 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 operate works:

• The database_connection operate 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 isn’t resistant to exceptions, we wrap it in a strive block.
• The lastly block ensures that the connection is all the time closed, whether or not an exception was raised or not, making certain there are not 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 ebook 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, author, publication_year) VALUES (?, ?, ?)", more_books_data)
	conn.commit()

	# Retrieve and print all ebook data
	cursor.execute("SELECT * FROM books")
	data = cursor.fetchall()
	print("Updated Library Catalog:")
	for document in data:
    	    book_id, title, creator, publication_year = document
    	    print(f"Book ID: {book_id}, Title: {title}, Author: {author}, Year: {publication_year}")
        cursor.shut()

We connect 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, Writer: F. Scott Fitzgerald, Yr: 1925
E book ID: 2, Title: To Kill a Mockingbird, Writer: Harper Lee, Yr: 1960
E book ID: 3, Title: 1984, Writer: George Orwell, Yr: 1949
E book ID: 4, Title: Satisfaction and Prejudice, Writer: Jane Austen, Yr: 1813
E book ID: 5, Title: The Catcher within the Rye, Writer: J.D. Salinger, Yr: 1951
E book ID: 6, Title: To the Lighthouse, Writer: Virginia Woolf, Yr: 1927
E book ID: 7, Title: Dune, Writer: Frank Herbert, Yr: 1965
E book ID: 8, Title: Slaughterhouse-5, Writer: Kurt Vonnegut, Yr: 1969

Notice that we open and shut the cursor object. So it’s also possible to 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 realized how one can create your personal customized context managers. We checked out two approaches: utilizing a category with __enter__() and __exit()__ strategies and utilizing a generator operate embellished with the @contextmanager decorator.

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

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

As all the time, you may discover the code on GitHub. Maintain coding!

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