With class as contextmanager

Wed Jan 25 07:36:34 EST 2017

Thank you for your answers. Very appreciated.
I ended up doing it as follows:
class MetaLock(type):
 def __init__(cls, *args):
 super().__init__(*args)
 cls.lock = Lock()
 cls.instances = []
 def register(cls, instance):
 cls.instances.append(instance)
 def __enter__(cls):
 cls.lock.acquire()
 def __exit__(cls, exc_type, exc_val, exc_tb):
 for instance in cls.instances:
 instance.cleanup()
 cls.instances = []
 cls.lock.release()
 return False
class Klass(metaclass=MetaLock):
 def __init__(self):
 if not self.__class__.lock.locked():
 raise Exception("You have to use the context manager on
the Class Object!")
 self.__class__.register(self)
 def cleaup(self):
 pass
with Klass:
 inst1 = Klass()
 inst2 = Klass()
 # on leaving the context the cleanup of each instance of Klass is called.
Regards
This
2017年01月25日 3:21 GMT+01:00 Terry Reedy <tjreedy at udel.edu>:
> On 1/24/2017 4:31 PM, This Wiederkehr wrote:
>> having a class definition:
>>>> class Test():
>>>> @classmethod
>> def __enter__(cls):
>> pass
>>>> @classmethod
>> def __exit__(cls, exception_type, execption_value, callback):
>> pass
>>>> now using this as a contextmanager does not work, even though Test is an
>> object and has the two required methods __enter__ and __exit__.
>>>> it fails with:
>> #Attribute Error: __enter__
>>>>>> This is not working because behind the scene it does something like:
>> type(Test).__enter__(Test)
>>>> But isn't this supposed to be working?
>>>> No. Unqualified 'method' means instance method, not class method.
>> One can simulate instance methods like so:
>> >>> import types
> >>> def f(self): print('f called')
>> >>> class C: pass
>> >>> c = C()
> >>> c.m = types.MethodType(f, c)
> >>> c.m()
> f called
>> However, this will not work for dunder methods where the lookup for the
> method starts with the class and not the object itself.
>> >>> def h(self): return 1
>> >>> c.__hash__ = types.MethodType(h, c)
> >>> hash(c) # calls type(c).__hash__, not c.__hash__
> -9223371924496369383
>> --
> Terry Jan Reedy
>> --
> https://mail.python.org/mailman/listinfo/python-list
>
-- 
This Wiederkehr
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