How do you write a custom context manager using __enter__ and __exit__? __enter__ और __exit__ से custom context manager कैसे लिखें?

A class becomes usable with 'with' by implementing __enter__ (setup, returns the value bound to 'as') and __exit__ (cleanup, receives exception details if one occurred).

class FileManager:
    def __init__(self, filename, mode):
        self.filename = filename
        self.mode = mode
        self.file = None

    def __enter__(self):
        print(f'Opening {self.filename}')
        self.file = open(self.filename, self.mode)
        return self.file  # this becomes the 'as' variable

    def __exit__(self, exc_type, exc_value, traceback):
        print(f'Closing {self.filename}')
        if self.file:
            self.file.close()
        if exc_type is not None:
            print(f'An exception occurred: {exc_value}')
        return False  # False/None means exceptions propagate normally

with FileManager('test.txt', 'w') as f:
    f.write('Hello World')
# Opening test.txt
# Closing test.txt

# Timing context manager - practical example
import time

class Timer:
    def __enter__(self):
        self.start = time.time()
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        elapsed = time.time() - self.start
        print(f'Elapsed: {elapsed:.4f}s')

with Timer():
    total = sum(range(1000000))
# Elapsed: 0.0234s (example)

# Database connection pattern - very common real-world use
class DatabaseConnection:
    def __enter__(self):
        print('Connecting to database')
        self.conn = 'connection_object'
        return self.conn

    def __exit__(self, exc_type, exc_value, traceback):
        print('Closing database connection')
        # cleanup happens regardless of success or failure