Python 具有可变数量上下文管理器的contextlib.nested的替代方案

我们的代码根据运行时参数调用不同数量的上下文管理器：

from contextlib import nested, contextmanager

@contextmanager
def my_context(arg):
    print("entering", arg)
    try:
        yield arg
    finally:
        print("exiting", arg)

def my_fn(items): 
    with nested(*(my_context(arg) for arg in items)) as managers:
        print("processing under", managers)

my_fn(range(3))

然而：

回答表明

contextlib.nested

有一些“容易混淆的错误怪癖”，但建议的使用多个管理器

with

语句的替代方案对数量可变的上下文管理器不起作用（也破坏了向后兼容性）

除了

contextlib.nested

之外，还有其他没有被弃用并且（最好）没有相同bug的方法吗

或者我应该继续使用

contextlib.nested

并忽略警告吗？如果是这样，我是否应该计划在将来某个时候删除

contextlib.nested

？

新添加的内容是作为

contextlib.nested（）

的替换内容（请参阅）

它的编码方式可以直接在Python 2中使用：

import sys
from collections import deque


class ExitStack(object):
    """Context manager for dynamic management of a stack of exit callbacks

    For example:

        with ExitStack() as stack:
            files = [stack.enter_context(open(fname)) for fname in filenames]
            # All opened files will automatically be closed at the end of
            # the with statement, even if attempts to open files later
            # in the list raise an exception

    """
    def __init__(self):
        self._exit_callbacks = deque()

    def pop_all(self):
        """Preserve the context stack by transferring it to a new instance"""
        new_stack = type(self)()
        new_stack._exit_callbacks = self._exit_callbacks
        self._exit_callbacks = deque()
        return new_stack

    def _push_cm_exit(self, cm, cm_exit):
        """Helper to correctly register callbacks to __exit__ methods"""
        def _exit_wrapper(*exc_details):
            return cm_exit(cm, *exc_details)
        _exit_wrapper.__self__ = cm
        self.push(_exit_wrapper)

    def push(self, exit):
        """Registers a callback with the standard __exit__ method signature

        Can suppress exceptions the same way __exit__ methods can.

        Also accepts any object with an __exit__ method (registering a call
        to the method instead of the object itself)
        """
        # We use an unbound method rather than a bound method to follow
        # the standard lookup behaviour for special methods
        _cb_type = type(exit)
        try:
            exit_method = _cb_type.__exit__
        except AttributeError:
            # Not a context manager, so assume its a callable
            self._exit_callbacks.append(exit)
        else:
            self._push_cm_exit(exit, exit_method)
        return exit # Allow use as a decorator

    def callback(self, callback, *args, **kwds):
        """Registers an arbitrary callback and arguments.

        Cannot suppress exceptions.
        """
        def _exit_wrapper(exc_type, exc, tb):
            callback(*args, **kwds)
        # We changed the signature, so using @wraps is not appropriate, but
        # setting __wrapped__ may still help with introspection
        _exit_wrapper.__wrapped__ = callback
        self.push(_exit_wrapper)
        return callback # Allow use as a decorator

    def enter_context(self, cm):
        """Enters the supplied context manager

        If successful, also pushes its __exit__ method as a callback and
        returns the result of the __enter__ method.
        """
        # We look up the special methods on the type to match the with statement
        _cm_type = type(cm)
        _exit = _cm_type.__exit__
        result = _cm_type.__enter__(cm)
        self._push_cm_exit(cm, _exit)
        return result

    def close(self):
        """Immediately unwind the context stack"""
        self.__exit__(None, None, None)

    def __enter__(self):
        return self

    def __exit__(self, *exc_details):
        # We manipulate the exception state so it behaves as though
        # we were actually nesting multiple with statements
        frame_exc = sys.exc_info()[1]
        def _fix_exception_context(new_exc, old_exc):
            while 1:
                exc_context = new_exc.__context__
                if exc_context in (None, frame_exc):
                    break
                new_exc = exc_context
            new_exc.__context__ = old_exc

        # Callbacks are invoked in LIFO order to match the behaviour of
        # nested context managers
        suppressed_exc = False
        while self._exit_callbacks:
            cb = self._exit_callbacks.pop()
            try:
                if cb(*exc_details):
                    suppressed_exc = True
                    exc_details = (None, None, None)
            except:
                new_exc_details = sys.exc_info()
                # simulate the stack of exceptions by setting the context
                _fix_exception_context(new_exc_details[1], exc_details[1])
                if not self._exit_callbacks:
                    raise
                exc_details = new_exc_details
        return suppressed_exc

将其用作上下文管理器，然后随意添加嵌套的上下文管理器：

with ExitStack() as stack:
    managers = [stack.enter_context(my_context(arg)) for arg in items]
    print("processing under", managers)

对于您的示例上下文管理器，这将打印：

>>> my_fn(range(3))
('entering', 0)
('entering', 1)
('entering', 2)
('processing under', [0, 1, 2])
('exiting', 2)
('exiting', 1)
('exiting', 0)

---

您也可以安装；它包括

ExitStack

作为一个后端口。

python3维护者选择破坏向后兼容性有点令人烦恼，因为在

ExitStack

方面实现

nested

非常简单：

try:
    from contextlib import nested  # Python 2
except ImportError:
    from contextlib import ExitStack, contextmanager

    @contextmanager
    def nested(*contexts):
        """
        Reimplementation of nested in python 3.
        """
        with ExitStack() as stack:
            for ctx in contexts:
                stack.enter_context(ctx)
            yield contexts

---

关于任意数量的经理，你提出了一个很好的观点。。。在我看来，这一点应该在邮件列表的某个地方提出……这就是为什么引入Python 3的原因。请看，此解决方案仅适用于将要使用的上下文管理器“输入”在其组装的相同上下文中的情况。例如，在中，

nested

的结果将返回给调用方，以便调用方稍后输入。我看不到在Python 3中实现这一点的方法。您的意思是希望在函数或类似函数中构造一个堆栈CM，然后在

with

语句中使用该堆栈？您也可以使用

ExitStack（）

来实现这一点；只需在变量中创建一个实例，使用

。输入_context（）

在其上推送上下文管理器，然后在

with

语句中使用该实例。您可以像其他Python对象一样传递该对象。如果您想推迟输入，也可以使用一个子类，该子类接受队列并调用

。在
上输入上下文（）
（并在发生异常时正确处理退出）你需要注意的一点是，创建CM也会引发异常；因此，将可调用项传递到该队列可能是最好的方法，将CMs的创建推迟到调用
\uuuu enter\uuuu
为止。我想知道为什么他们没有将此添加到Python 2.7的最新版本中，因为
nested
已经被弃用，而
ExitStack
的代码在Python 2.7中可以直接使用。你认为只要我想支持Python2.7，就把它复制到我的项目中是最好的选择吗？这个实现与原始的
nested
有相同的问题。调用
nested
函数时，上下文管理器已经创建。如果索引>0的上下文管理器的构造函数引发异常，则不会退出前面的上下文管理器。它确实存在该“问题”，是的。这仅仅是在构建资源时抓住资源的上下文的问题（不幸的是，
open
就是这样）。有许多上下文不是这样工作的，嵌套的
允许您返回单个返回值，该值可以用作所有子上下文的上下文。
try:
    from contextlib import nested  # Python 2
except ImportError:
    from contextlib import ExitStack, contextmanager

    @contextmanager
    def nested(*contexts):
        """
        Reimplementation of nested in python 3.
        """
        with ExitStack() as stack:
            for ctx in contexts:
                stack.enter_context(ctx)
            yield contexts

import sys
import contextlib

class nodeA(object):

    def __init__(self):
        print( '__init__ nodeA')

    def __enter__(self):
        print( '__enter__ nodeA')

    def __exit__(self, a, b, c):
        print( '__exit__ nodeA')

class nodeB(object):

    def __init__(self):
        print( '__init__ nodeB')

    def __enter__(self):
        print( '__enter__ nodeB')

    def __exit__(self, a, b, c):
        print( '__exit__ nodeB')

class nodeC(object):

    def __init__(self):
        print( '__init__ nodeC')

    def __enter__(self):
        print( '__enter__ nodeC')

    def __exit__(self, a, b, c):
        print( '__exit__ nodeC')

print( 'Start...')

a = nodeA()
b = nodeB()
c = nodeC()

print( 'Python version: %s' % (sys.version))

if sys.version.startswith('2'):
    print('Use python 2!')
    with contextlib.nested(a, b, c):
        print('hallo?')

if sys.version.startswith('3'):
    print('Use python 3!')
    with contextlib.ExitStack() as stack:
        [stack.enter_context(arg) for arg in [a,b,c]]

print('...end!')