Tracking warnings and deprecations

[ananthsub]

What's the recommendation for how the project should track these?

In some parts of the codebase, we use rank_zero_warn/rank_zero_deprecation
In other parts of the codebase we use the warning cache
https://github.com/PyTorchLightning/pytorch-lightning/blob/master/pytorch_lightning/utilities/warnings.py

Why do we have both? When should we use one over another?

And for deprecated functions or classes, should we use a decorator for them? https://pypi.org/project/Deprecated/

cc @Borda @awaelchli @carmocca @tchaton @edward-io

[awaelchli]

Not sure myself, I will let @carmocca and @Borda deliver the official answer :)
My understanding:

rank_zero_warn: simple to use, when we know for sure it will print only once
warning_cache: when we are in a loop and prevent repeated warnings. needs to be assigned as an attribute or global variable within the module however.

I believe we used to have this extension https://pypi.org/project/pyDeprecate/

[carmocca]

rank_zero_warn. Calls warnings.warn with the rank zero wrapper. warnings.warn is supposed to print the same warning only once in the process.

WarningCache. Calls rank_zero_warn internally. I believe it was added because the "once in the process" requirement is unfulfilled in some distributed setting (i'm not sure if it's dp or what, but it was added with #3618). Maybe worth testing when it's necessary exactly and writing a test for that.

I believe we used to have this extension https://pypi.org/project/pyDeprecate/

We still install it:

https://github.com/PyTorchLightning/pytorch-lightning/blob/master/requirements.txt#L11

Although we pretty much don't use it now that torchmetrics has been removed from the project.

$ grep -iIrn "from deprecate" pytorch_lightning
pytorch_lightning/loops/epoch/evaluation_epoch_loop.py:18:from deprecate import void
pytorch_lightning/loops/epoch/prediction_epoch_loop.py:4:from deprecate import void
pytorch_lightning/loops/batch/training_batch_loop.py:23:from deprecate import void
pytorch_lightning/loops/base.py:18:from deprecate import void
pytorch_lightning/loops/dataloader/evaluation_loop.py:17:from deprecate.utils import void
pytorch_lightning/loops/dataloader/prediction_loop.py:3:from deprecate.utils import void