Alternativa a contextlib.nested con número variable de administradores de contexto

Tenemos un código que invoca un número variable de administradores de contexto en función de los parámetros de tiempo de ejecución:

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)) 

Sin embargo, contextlib.nested está en desuso desde Python 2.7 :

 DeprecationWarning: With-statements now directly support multiple context managers 

Las respuestas a Múltiples variables en Python ‘con’ indican que contextlib.nested tiene algunas “peculiaridades propensas a errores confusos”, pero la alternativa sugerida de usar el administrador múltiple with statement no funcionará para un número variable de administradores de contexto (y también rompe la compatibilidad hacia atrás).

¿Hay alternativas a contextlib.nested que no estén en desuso y (preferiblemente) no tengan los mismos errores?

¿O debería seguir usando contextlib.nested e ignorar la advertencia? Si es así, ¿debería planear que contextlib.nested se elimine en algún momento en el futuro?

La nueva clase contextlib.ExitStack Python 3 se agregó como un reemplazo para contextlib.nested() (ver problema 13585 ).

Está codificado de tal manera que puede usarlo directamente en Python 2:

 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 

Use esto como su administrador de contexto, luego agregue administradores de contexto nesteds a voluntad:

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

Para su administrador de contexto de ejemplo, esto imprime:

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

También puede instalar el módulo contextlib2 ; incluye ExitStack como backport.

Es un poco desconcertante que los mantenedores de python3 decidieron romper la compatibilidad hacia atrás, ya que la implementación nested en términos de ExitStack es bastante sencilla:

 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 

Parece que se supone que debes usar la instrucción with con múltiples instancias de administrador de contexto.

de https://pymotw.com/2/contextlib/ :

En Python 2.7 y versiones posteriores, nested () está en desuso porque la instrucción with admite el anidamiento directamente.

 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 
 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!')