With class as contextmanager

Tue Jan 24 21:21:02 EST 2017

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