上下文管理器数目可变的contextlib.nested的替代方法

Question

我们的代码根据运行时参数调用可变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))

但是，^{} is deprecated since Python 2.7：

DeprecationWarning: With-statements now directly support multiple context managers

对Multiple variables in Python 'with' statement的回答表明contextlib.nested有一些“容易引起混淆的错误的怪癖”，但是建议的使用multiple managerwith语句的替代方法对于可变数量的上下文管理器不起作用（而且还会破坏向后兼容性）。

除了contextlib.nested之外，还有没有不被弃用且（最好）没有相同错误的替代品？

还是继续使用contextlib.nested并忽略警告？如果是，我是否应该计划将来某个时候删除contextlib.nested？

Answer 1

似乎应该对多个上下文管理器实例使用with语句。

来自https://pymotw.com/2/contextlib/：

In Python 2.7 and later, nested() is deprecated because the with statement supports nesting directly.

import contextlib

@contextlib.contextmanager
def make_context(name):
    print 'entering:', name
    yield name
    print 'exiting :', name

with make_context('A') as A, make_context('B') as B, make_context('C') as C:
    print 'inside with statement:', A, B, C

Answer 2

python3维护人员选择打破向后兼容性有点恼火，因为用ExitStack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

Answer 3

新的Python 3 ^{} class被添加为contextlib.nested()的替代品（参见issue 13585）。

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，将打印：

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

您还可以安装^{} module；它包含ExitStack作为后台端口。