self.log raised error when number of dataloader is not consistent

[ding3820]

Hi all,

Recently I've been modifying our codes to adopt the latest lightning logging approach that is self.log. We have two validation dataloaders, A and B. For A we use it every epoch. But for B, we use it every 5 epoch. We implement this mechanism in val_dataloader() and reload dataloader every epoch. See below for implementation.

def val_dataloader():
    if self.should_run_B():
        return [loader_A, loader_B]
    else:
        return [loader_A]

However, when we use self.log in validation_step(), an error shows up

You called self.log({name}, ...) twice in {fx} with different arguments. This is not allowed

(see here)

After debug in the source code, I notice that when we are at the epoch that only use one val dataloader, dataloader_idx is always None. On the other hand, when we have two val dataloaders, dataloader_idx would be sequentially presented as 0 or 1 in validation_step. If I understand correctly, this is the main reason causing the error.

The questions are how can I avoid this error. Is it a lightning implementation issue? Is there any better way to implement it? I'm open for any suggestions. Thanks in advance!