将上下文管理器的动态可迭代链接到单个 with 语句

Question

我有一堆要链接的上下文管理器。乍一看，contextlib.nested 看起来是一个合适的解决方案。但是，此方法在文档中被标记为已弃用，该文档还指出最新的 with 语句直接允许这样做：

Deprecated since version 2.7: The with-statement now supports this functionality directly (without the confusing error prone quirks).

但是我无法让 Python 3.4.3 使用上下文管理器的动态迭代：

class Foo():
    def __enter__(self):
        print('entering:', self.name)
        return self
    def __exit__(self, *_):
        pass
    def __init__(self, name):
        self.name = name

foo = Foo('foo')
bar = Foo('bar')

是否链接：

from itertools import chain
m = chain([foo], [bar])
with m:
     pass

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __exit__
m = [foo, bar]

直接提供列表：

with m:
     pass

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __exit__

或解包：

with (*m):
    pass

  File "<stdin>", line 1
SyntaxError: can use starred expression only as assignment target

那么，如何在 with 语句中正确链接动态数量的上下文管理器？

Answer 1

"multiple manager form of the with statement"，如 the statement's documentation 所示，将是：

with foo, bar:

即它不支持动态数量的经理。正如 the documentation for contextlib.nested 所说：

Developers that need to support nesting of a variable number of context managers can either use the warnings module to suppress the DeprecationWarning raised by this function or else use this function as a model for an application specific implementation.

Answer 2

你误解了那句话。 with 语句采用多个上下文管理器，以逗号分隔，但不是可迭代的：

with foo, bar:

有效。

如果需要支持动态上下文管理器集，请使用 contextlib.ExitStack() object：

from contextlib import ExitStack

with ExitStack() as stack:
    for cm in (foo, bar):
        stack.enter_context(cm)

将上下文管理器的动态可迭代链接到单个 with 语句

Chain dynamic iterable of context managers to a single with statement

python

python-3.x

with-statement

iterable

chain