This is, perhaps, the intended behavior, but I find it quite confusing, and also inconsistent with the underlying Qt C++ function.

If you call disconnect() to disconnect a signal from a function, but they have never been previously connected, a runtime exception is thrown with the message "Failed to disconnect signal".  My main issue is that this message is confusing and misleading, since it doesn't really seem like a "failure" to me - it's just that there's nothing to do.

I also find it inconsistent that the C++ disconnect function returns false on failure, and you can simply ignore that, if you wish.  Why doesn't the PySide2 function return false rather than throwing an exception?