Missing LightningModule datamodule reference

[carmocca]

🐛 Bug

This docs snippet does not work:

https://pytorch-lightning.readthedocs.io/en/latest/common/lightning_module.html#datamodule

To Reproduce

def test_bug(tmpdir):
    class TestModel(BoringModel):
        def configure_optimizers(self):
            # works
            len(self.trainer.datamodule.train_dataloader())
            # does not
            len(self.datamodule.train_dataloader())
            
            return super().configure_optimizers()

    dm = BoringDataModule()
    trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=True)
    trainer.fit(TestModel(), datamodule=dm)

line 128, in configure_optimizers
    num_training_samples = len(self.datamodule.train_dataloader())
AttributeError: 'NoneType' object has no attribute 'train_dataloader'

Expected behavior

self.datamodule works

Alternatives

Update docs

Environment

master

Additional context

Reported by Brian Staber in Slack

[mesejo]

Can I work on this?

[ananthsub]

@carmocca is the model expected to have a reference to the data module during training?

[carmocca]

Can I work on this?

Sure!

@carmocca is the model expected to have a reference to the data module during training?

I guess so, given the docs link above and the fact that it has a property for it

https://github.com/PyTorchLightning/pytorch-lightning/blob/f29ecbfd909ff431ef837fcc8ebff451e897cb0b/pytorch_lightning/core/lightning.py#L164-L170

I personally wouldn't mind removing this and forcing access through the trainer.

[tchaton]

Dear @mesejo,

Could you make a PR for this ?

Best,
T.C

[mesejo]

Hi @carmocca, when you said:

I personally wouldn't mind removing this and forcing access through the trainer.

Were you referring to the datamodule property?

[carmocca]

Were you referring to the datamodule property?

I was referring to deprecating model.datamodule in favor of model.trainer.datamodule

cc: @PyTorchLightning/core-contributors Any opinions about this? Should we keep both ways?

[awaelchli]

I would prefer if we don't attach the datamodule to the LightningModule and instead reference it through trainer.

[carmocca]

Okay, let's deprecate it then and update the docs/ocurrences