class Timer:
    def __enter__(self):
        print('starting')
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('stopping')
        return False
with Timer():
    print('doing work')

from contextlib import contextmanager
@contextmanager
def timer():
    print('starting')
    yield 'timer-object'
    print('stopping')
with timer() as t:
    print(f'doing work with {t}')

@contextmanager
def resource_guard(name):
    print(f'acquiring {name}')
    try:
        yield name
    finally:
        print(f'releasing {name}')
try:
    with resource_guard('db-connection'):
        print('using resource')
        raise ValueError('something broke')
except ValueError as e:
    print(f'caught: {e}')

from contextlib import closing
class Connection:
    def __init__(self, name):
        self.name = name
        self.open = True
    def close(self):
        self.open = False
        print(f'{self.name} closed')
with closing(Connection('conn-a')) as conn:
    print(conn.open)
print(conn.open)

from contextlib import suppress
import os
with suppress(FileNotFoundError):
    os.remove('file_that_does_not_exist.txt')
print('continued past the missing file')

try:
    with suppress(FileNotFoundError):
        result = 10 / 0
except ZeroDivisionError:
    print('ZeroDivisionError was NOT suppressed, exactly as expected')

import io
from contextlib import redirect_stdout
buffer = io.StringIO()
with redirect_stdout(buffer):
    print('this is captured, not printed')
    print('so is this')
captured = buffer.getvalue()
print(f'captured text: {captured!r}')

from contextlib import ExitStack
names = ['a', 'b', 'c']
with ExitStack() as stack:
    resources = [stack.enter_context(resource_guard(name)) for name in names]
    print(resources)

with ExitStack() as stack:
    stack.callback(print, 'cleanup one')
    stack.callback(print, 'cleanup two')
    print('doing work inside the stack')

from contextlib import nullcontext
def process(data, lock=None):
    ctx = lock if lock is not None else nullcontext()
    with ctx:
        return sum(data)
print(process([1, 2, 3]))
import threading
real_lock = threading.Lock()
print(process([4, 5, 6], lock=real_lock))

@contextmanager
def logged(label):
    print(f'[{label}] entering')
    yield
    print(f'[{label}] exiting')
@logged('demo-call')
def do_something():
    print('inside do_something')
do_something()

import os
@contextmanager
def temporary_directory_change(path):
    original = os.getcwd()
    os.chdir(path)
    try:
        yield
    finally:
        os.chdir(original)
print(os.getcwd())
with temporary_directory_change('/tmp'):
    print(os.getcwd())
print(os.getcwd())