有没有一种有意义的方法可以在生成器中使用上下文管理器？

发布于 2024-11-10 14:40:26 字数 566 浏览 3 评论 0原文

from contextlib import contextmanager

@contextmanager
def context():
    print "entering"
    yield
    print "exiting"

def test():
    with context():
        for x in range(10):
            yield x

for x in test():
    if x == 5:
        break  # or raise

输出：

entering

有没有办法让Python在for循环中断时自动调用context()的__exit__方法？或者通过其他方式实现相同的目标？我对生成器和上下文管理器的了解让我怀疑这是不可能的，但这使得上下文管理器在生成器中毫无用处，不是吗？在我看来，with块内的yield语句应该引发一个红旗，上下文管理器__exit__可能无法运行。

原文

from contextlib import contextmanager

@contextmanager
def context():
    print "entering"
    yield
    print "exiting"

def test():
    with context():
        for x in range(10):
            yield x

for x in test():
    if x == 5:
        break  # or raise

output:

entering

Is there a way to make python automatically invoke the __exit__ method of context() when the for-loop is interrupted? Or some other way of achieving the same aim? What I know about generators and context managers makes me suspect it's not possible, but this makes context managers rather useless inside generators, doesn't it? It seems to me, a yield statement inside a with block should raise a red flag, context manager __exit__ may not run.

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风筝有风，海豚有海 2024-11-17 14:40:27

好吧，您可以使用 try/finally 子句将yield 包装在 context() 函数中：

from contextlib import contextmanager

@contextmanager
def context():
    print "entering"
    try:
        yield
    finally:
        print "exiting"

def test():
    with context():
        for x in range(10):
            yield x

for x in test():
    if x == 5:
        break  # or raise

输出：

entering
exiting

编辑： 如果您尝试：help(contextmanager)，它将显示这是“典型”的用法示例，其中他们用 try/finally 子句包装了收益。

Well, you could wrap the yield in context() function with a try/finally clause:

from contextlib import contextmanager

@contextmanager
def context():
    print "entering"
    try:
        yield
    finally:
        print "exiting"

def test():
    with context():
        for x in range(10):
            yield x

for x in test():
    if x == 5:
        break  # or raise

output: