How To Create Customized Context Managers in Python

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

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

What Are Context Managers in Python?

Context managers in Python are objects that allow the administration of assets similar to file operations, database connections, or community sockets inside a managed block of code. They be sure that assets are correctly initialized earlier than the block of code executes and routinely 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, it’s a must to think about 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: 
    # Organising the useful resource
    # Working with the useful resource
besides ErrorType:
    # Deal with exceptions
lastly:
    # Liberate the useful resource

Basically, we strive provisioning and dealing with the useful resource, apart from any errors that will come up in the course of 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 Part (__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 similar 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__() methodology):
After the code block completes execution—both usually or attributable to an exception—the __exit__() methodology of the context supervisor is known as. The __exit__() methodology handles cleanup duties, similar to closing the assets. If an exception happens throughout 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 strategy 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 possibly can write your individual context managers in Python utilizing one of many following two strategies:

Writing a category with __enter__() and __exit__() strategies.

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 possibly can 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-obligatory

# Writing a context supervisor class
class ConnectionManager:
    def __init__(self, db_name: str):
        self.db_name = db_name
        self.conn: Non-obligatory[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 known as 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. Word that we’ve used the Non-obligatory sort from the typing module for the connection object conn. We use Non-obligatory when the worth will be one in all two sorts: right here a sound connection object or None.

The __exit__() methodology: 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 attributable to an exception.

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

Attempt to hook up with the database

Create a cursor to run queries
Create a desk and insert information
Question the database desk and retrieve the outcomes of the question

db_name = "library.db"

# Utilizing ConnectionManager context supervisor straight
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 information
	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 ebook information
	cursor.execute("SELECT * FROM books")
	information = cursor.fetchall()
	print("Library Catalog:")
	for file in information:
    	    book_id, title, creator, publication_year = file
    	    print(f"Guide ID: {book_id}, Title: {title}, Creator: {creator}, Yr: {publication_year}")
            cursor.shut()

Working the above code ought to provide the following output:

Output >>>

Library Catalog:
Guide ID: 1, Title: The Nice Gatsby, Creator: F. Scott Fitzgerald, Yr: 1925
Guide ID: 2, Title: To Kill a Mockingbird, Creator: Harper Lee, Yr: 1960
Guide ID: 3, Title: 1984, Creator: George Orwell, Yr: 1949
Guide ID: 4, Title: Delight and Prejudice, Creator: 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. Word that as a result of yield itself just isn’t resistant to 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 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 straight
with database_connection(db_name) as conn:
	cursor = conn.cursor()

	# Insert a set of ebook information
	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 ebook information
	cursor.execute("SELECT * FROM books")
	information = cursor.fetchall()
	print("Up to date Library Catalog:")
	for file in information:
    	    book_id, title, creator, publication_year = file
    	    print(f"Guide ID: {book_id}, Title: {title}, Creator: {creator}, Yr: {publication_year}")
        cursor.shut()

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

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

Word 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 tips on how to create your individual 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 straightforward to see that you just get the next benefits when utilizing a context supervisor:

Setup and teardown of assets are routinely 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. Hold 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 occasional! 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 participating useful resource overviews and coding tutorials.