I've run into this problem too. I think it has more to do with object lifetimes. Suppose you define your lambda in a method of a QWidget that's later destroye

I've run into this problem too. I think it has more to do with object lifetimes. Suppose you define your lambda in a method of a QWidget that's later destroyed. The underlying C++ object has been cleaned up already, but the signal's reference to the lambda is keeping the Python part of the QWidget alive. Then when the slot is triggered you get weird crashy behavior.