Decoding Python Magic : __enter__ and __exit__

Let’s understand how context managers work.

Hello Everyone,

Welcome to the next installment of my magic methods series! Today, let’s delve into understanding how the with statement works in Python.

Image Credit Unsplash

Context Manager

Context managers in Python are objects that are used to manage resources and define setup and cleanup actions that need to be performed around a block of code.

Some Use case of Context Managers

• File Handling: Managing file operations like opening and closing files.
• Database Connections: Managing connections to databases.
• Locking: Ensuring that only one thread or process accesses a resource at a time.
• Managing network connections
• Resource Management

Managing Files with Context Manager

One Operation that almost every developer does is to write into a file or read from a file. This operation is very common, Only precaution that we need to keep it, release assignedfd.

Non Pythonic Way

f = None
try:
    f = open("test.txt", "w")
    f.write("Hello World")
finally:
    if f:
        f.close()

Class Based Context Manager

class FileContextManager:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        self.open_file = open(self.filename, "w")
        return self.open_file

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.open_file.close()

with FileContextManager("test.txt") as f:
    f.write("Hello World")

Function based Context Managers

from contextlib import contextmanager


@contextmanager
def file_manager(filename):
    f = open(filename, "w")
    try:
        yield f
    finally:
        f.close()


with file_manager("test.txt") as f:
    f.write("Hello World")

How to close Sockets automatically?

import socket

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect(("localhost", 80))
    s.sendall(b"GET / HTTP/1.1\r\nHost: localhost\r\n\r\n")
    print(s.recv(4096))

Conclusion

Context Managers are very useful and used heavily. These are mostly used to manage resources especially deallocate the occupied ones. Similar methods __aenter__ and __aexit__ are used to support async programming. If you want to learn it deeply, I already have a blog that covers pretty much everything.

Context Manager in Python

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