Interview question

How does the contextlib module simplify writing context managers? contextlib module context managers लिखना कैसे आसान बनाता है?

Answer

The contextlib.contextmanager decorator lets you write a context manager using a generator function instead of a full class with __enter__/__exit__, using yield to separate setup from teardown.

from contextlib import contextmanager

@contextmanager
def managed_file(filename, mode):
    print('Setup: opening file')
    f = open(filename, mode)
    try:
        yield f  # everything before yield = __enter__, after = __exit__
    finally:
        print('Teardown: closing file')
        f.close()  # runs even if an exception occurs in the with block

with managed_file('data.txt', 'w') as f:
    f.write('Hello')
# Setup: opening file
# Teardown: closing file

# Comparing class-based vs generator-based approaches
# Class-based (more code, more control)
class Timer:
    def __enter__(self):
        import time
        self.start = time.time()
        return self
    def __exit__(self, *args):
        print(f'{time.time() - self.start:.4f}s')

# Generator-based equivalent (less code, same result)
import time
@contextmanager
def timer():
    start = time.time()
    yield
    print(f'{time.time() - start:.4f}s')

with timer():
    sum(range(1000000))

# contextlib.suppress - shorthand for ignoring specific exceptions
from contextlib import suppress

with suppress(FileNotFoundError):
    open('might_not_exist.txt')  # no error even if file doesn't exist

# Equivalent to:
try:
    open('might_not_exist.txt')
except FileNotFoundError:
    pass

contextlib.contextmanager decorator generator function से context manager लिखने देता है, पूरी class की बजाय, yield से setup को teardown से अलग करता है।

from contextlib import contextmanager

@contextmanager
def managed_file(filename, mode):
    print('Setup: file खोलना')
    f = open(filename, mode)
    try:
        yield f  # yield से पहले = __enter__, बाद = __exit__
    finally:
        print('Teardown: file बंद करना')
        f.close()

with managed_file('data.txt', 'w') as f:
    f.write('Hello')

# Generator-based timer
import time
@contextmanager
def timer():
    start = time.time()
    yield
    print(f'{time.time() - start:.4f}s')

with timer():
    sum(range(1000000))

# contextlib.suppress - specific exceptions ignore करने का shorthand
from contextlib import suppress

with suppress(FileNotFoundError):
    open('might_not_exist.txt')

# बराबर है:
try:
    open('might_not_exist.txt')
except FileNotFoundError:
    pass

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