contextlib: @contextmanager

เหตุใดจึงใช้ contextlib

from contextlib import contextmanager

@contextmanager
def managed():
    print("setup")
    yield
    print("teardown")

with managed():
    print("inside")

การส่งคืนค่าด้วย Yield

from contextlib import contextmanager

@contextmanager
def temp_list():
    lst = []
    yield lst
    print(f"Final list: {lst}")

with temp_list() as lst:
    lst.append(1)
    lst.append(2)

การจัดการข้อยกเว้นใน @contextmanager

from contextlib import contextmanager

@contextmanager
def safe_open(path):
    f = open(path)
    try:
        yield f
    finally:
        f.close()

with safe_open("data.txt") as f:
    print(f.read())

การระงับข้อยกเว้น

from contextlib import contextmanager

@contextmanager
def ignore_value_errors():
    try:
        yield
    except ValueError:
        pass

with ignore_value_errors():
    raise ValueError("ignored")

contextlib.suppress

from contextlib import suppress

with suppress(FileNotFoundError):
    open("missing.txt")

print("continues")

contextlib.closing

from contextlib import closing
from urllib.request import urlopen

with closing(urlopen("https://example.com")) as page:
    content = page.read()

contextlib.nullcontext

from contextlib import nullcontext

def process(data, lock=None):
    cm = lock if lock is not None else nullcontext()
    with cm:
        return [x*2 for x in data]

contextlib.ExitStack

from contextlib import ExitStack

files = ["a.txt", "b.txt", "c.txt"]
with ExitStack() as stack:
    handles = [stack.enter_context(open(f)) for f in files]
    data = [h.read() for h in handles]

การนำ @contextmanager กลับมาใช้ใหม่

from contextlib import contextmanager

@contextmanager
def counter():
    count = 0
    yield lambda: count.__add__  # each call is independent
    print("done")

with counter():
    pass
with counter():
    pass

asynccontextmanager

from contextlib import asynccontextmanager

@asynccontextmanager
async def async_session():
    session = await create_session()
    try:
        yield session
    finally:
        await session.close()

contextlib.redirect_stdout

import io
from contextlib import redirect_stdout

buf = io.StringIO()
with redirect_stdout(buf):
    print("captured")

print(buf.getvalue())  # captured

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