python - 防止可以使用 "with"调用的类中的方法被调用两次

Question

我正在编写一个可以与 with 语句一起使用的类，如下所示:

with Context() as context:
    if context:
        ...

该类有一个 enter 函数，该函数只能调用一次并返回一个 bool 值。我想防止它被调用两次(例如 with Context().enter() as context):

class Context(object):
    def __init__(self):
        self.ENTER_HAS_BEEN_CALLED = False

    def __enter__(self):
        return self.enter()

    def __exit__(self):
        self.exit()

    def enter(self):
        """Do things once and only once
        Returns boolean, not self
        """
        if self.ENTER_HAS_BEEN_CALLED: 
            # not sure what to do here, return?
            # call self.exit()?

        self.ENTER_HAS_BEEN_CALLED = True

        # do things that should only be done once
        value = True # or False
        return value

    def exit(self):
        pass

这是防止函数被调用两次的正确方法吗？我想要返回值，并且还允许此代码工作:

context_manager = Context()
context = context_manager.enter()
if context: ...
context_manager.exit()

Answer 1

您已经很接近了，但是，如果在使用装饰器设置标志后已经调用了 contextmanager，您可以简单地引发错误。下面的代码编写为 enter和__enter__可以按照最初的预期返回单独的变量( True 和 self )，并符合 contextmanager 的一般思想:

def control_manager(f):
   def wrapper(cls):
      if getattr(cls, 'flag'):
        raise Exception("Already expended the context manager")
      setattr(cls, 'flag', True)
      return f(cls)
   return wrapper

class Context:
   def __init__(self):
     self.flag = False
   @control_manager
   def __enter__(self):
     return self
   def __exit__(self, *args):
     pass
   @control_manager
   def enter(self):
     return True
   def exit(self):
     #do something
     pass

with Context() as f:
 v = f.enter()

Traceback (most recent call last): File "", line 2, in File "", line 4, in wrapper Exception: Already expended the context manager

但是，它将在第二次测试中起作用:

c = Context()
v = c.enter()
c.exit()

但是，如果您想使用enter作为类中 contextmanager 的主要 block ，您可以将其视为 classmethod与 contextlib.contextmanager :

import contextlib

class Control:
   flag = False
   def __init__(self):
     pass
   def __enter__(self):
     with Control.enter() as f:
       v = f
     return self
   def __exit__(self, *args):
     pass
   @classmethod
   @control_manager
   @contextlib.contextmanager
   def enter(cls):
     yield True


with Control() as f:
  pass
#runs without exception

with Control.enter() as t:
  pass

Traceback (most recent call last): File "", line 1, in File "", line 4, in wrapper Exception: Already expended the context manager