sorry for the late reply,
I try to add it twice but it did not solve the problem (I use python 3.6.6 maybe that’s related). Anyway you were right, I make a circular reference (between the buffers and the object), so using weakref.ref in self.Conv_1_act = weakref.ref(out) solves the problem. I think it would be more natural to put the weak link in the lambda though, do you know if it’s possible?
Thank you very much for your help!