替代contextlib.nested，具有可变数量的上下文管理器 [英] Alternative to contextlib.nested with variable number of context managers
问题描述
我们有根据运行时参数调用可变数量的上下文管理器的代码：
We have code that invokes a variable number of context managers depending on runtime parameters:
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， contextlib.nested ：
DeprecationWarning: With-statements now directly support multiple context managers
多个变量Python'with'语句中的les指示 contextlib.nested 有一些容易出错的易怪癖，但是建议使用多管理器 with 语句不适用于可变数量的上下文管理器（并且还会破坏向后兼容性）。
The answers to Multiple variables in Python 'with' statement indicate that contextlib.nested has some "confusing error prone quirks", but the suggested alternative of using the multiple-manager with statement won't work for a variable number of context managers (and also breaks backward compatibility).
有没有 contextlib.nested 的替代版本不被弃用，并且（最好）没有相同的错误？
Are there any alternatives to contextlib.nested that aren't deprecated and (preferably) don't have the same bugs?
还是应该继续使用 contextlib.nested 并忽略警告？如果是这样，我是否应该计划在将来的某个时间删除 contextlib.nested ？
Or should I continue to use contextlib.nested and ignore the warning? If so, should I plan for contextlib.nested to be removed at some time in the future?
推荐答案
新的 Python 3 contextlib.nested（）（请参见问题13585 ）。
它的编码方式可用于直接使用Python 2
It is coded in such a way you can use it in Python 2 directly:
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：
Use this as your context manager, then add nested context managers at will:
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：
For your example context manager, this prints:
>>> 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 contextlib2 模块；它包括 ExitStack 作为反向端口。
You can also install the contextlib2 module; it includes ExitStack as a backport.
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