python - 如何重用自定义的@contextlib.contextmanager？

Question

我正在尝试创建一个包装器来使上下文对象成为可选的。当条件为真时，事物应该表现得像包装的上下文对象，否则它应该表现得像无操作上下文对象。此示例适用于使用一次包装对象，但失败了，它被重新使用。

例子:

import contextlib
from threading import Lock

@contextlib.contextmanager
def conditional_context(context, condition):
    if condition and context:
        with context:
            yield
    else:
        yield

use_locking = False
lock = conditional_context(Lock(), use_locking)
with lock:
    print('Foo')
with lock:
    print('Bar')

输出:

Foo
Traceback (most recent call last):
  File "example.py", line 16, in <module>
    with lock:
  File "/usr/lib/python3.5/contextlib.py", line 61, in __enter__
    raise RuntimeError("generator didn't yield") from None
RuntimeError: generator didn't yield

Answer 1

你不能用 contextlib.contextmanager 做到这一点。正如在传递中提到的the docs , 由 contextmanager 创建的上下文管理器是一次性的。

如果您希望同一个对象在多个 with 语句中可重用，则必须使用 __enter__ 和 __exit__ 方法编写自己的类:

from threading import Lock


class ConditionalContext:

    def __init__(self, context, condition):
        self._context = context
        self._condition = condition

    def __enter__(self, *args, **kwargs):
        if self._condition:
            self._context.__enter__(*args, **kwargs)

    def __exit__(self, *args, **kwargs):
        if self._condition:
            self._context.__exit__(*args, **kwargs)


use_locking = False
lock = ConditionalContext(Lock(), use_locking)
with lock:
    print('Foo')
with lock:
    print('Bar')