With class as contextmanager

Tue Jan 24 17:31:31 EST 2017

On Tue, Jan 24, 2017 at 2:31 PM, This Wiederkehr
<this.wiederkehr at gmail.com> wrote:
> Hellou
>> 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?
>> I am asking because I'd like to implement the clean up behaviour for
> multiple instances directly into the class:
>> with Test:
> testinstance1 = Test()
> testinstance2 = Test()
> # on context exit Test.__exit__ should take care on cleaning up
> testinstance1 and testinstance2.

Special dunder methods are generally only looked up on the object's
class, not on the object itself. If you wish to use a class object as a
context manager then you need to define __enter__ and __exit__ on the
class's class, i.e. its metaclass.
Alternatively you could separate the context manager from the testing
class and make Test be just an object that defines __enter__, __exit__
and __call__, with __call__ creating and returning a new instance of
the actual testing class.