Lighting offers options for logging information about model, gpu usage, etc, via several different logging frameworks. It also offers printing options for training monitoring.


---
### Setting up logging

Initialize your logger, which should inherit from `LightningBaseLogger`, and pass
it to `Trainer`.
```{.python}
my_logger = MyLightningLogger(...)
trainer = Trainer(logger=my_logger)
```

Lightning supports several common experiment tracking frameworks out of the box

---
#### Test tube

Log using [test tube](https://williamfalcon.github.io/test-tube/).

```{.python}
from pytorch_lightning.logging import TestTubeLogger
tt_logger = TestTubeLogger(
    save_dir=".",
    name="default",
    debug=False,
    create_git_tag=False
)
trainer = Trainer(logger=tt_logger)
```

---
#### MLFlow

Log using [mlflow](https://mlflow.org)

```{.python}
from pytorch_lightning.logging import MLFlowLogger
mlf_logger = MLFlowLogger(
    experiment_name="default",
    tracking_uri="file:/."
)
trainer = Trainer(logger=mlf_logger)
```

---
#### Custom logger

You can implement your own logger by writing a class that inherits from
`LightningLoggerBase`. Use the `rank_zero_only` decorator to make sure that
only the first process in DDP training logs data.

```{.python}
from pytorch_lightning.logging import LightningLoggerBase, rank_zero_only

class MyLogger(LightningLoggerBase):

    @rank_zero_only
    def log_hyperparams(self, params):
        # params is an argparse.Namespace
        # your code to record hyperparameters goes here
        pass
    
    @rank_zero_only
    def log_metrics(self, metrics, step_num):
        # metrics is a dictionary of metric names and values
        # your code to record metrics goes here
        pass
    
    def save(self):
        # Optional. Any code necessary to save logger data goes here
        pass
    
    @rank_zero_only
    def finalize(self, status):
        # Optional. Any code that needs to be run after training
        # finishes goes here
```

If you write a logger than may be useful to others, please send
a pull request to add it to Lighting!

---
### Using loggers

#### Display metrics in progress bar 
``` {.python}
# DEFAULT
trainer = Trainer(show_progress_bar=True)
```

---
#### Log metric row every k batches 
Every k batches lightning will make an entry in the metrics log
``` {.python}
# DEFAULT (ie: save a .csv log file every 10 batches)
trainer = Trainer(row_log_interval=10)
```   

---
#### Log GPU memory
Logs GPU memory when metrics are logged.   
``` {.python}
# DEFAULT
trainer = Trainer(log_gpu_memory=False)
```

---
#### Process position
When running multiple models on the same machine we want to decide which progress bar to use.
Lightning will stack progress bars according to this value. 
``` {.python}
# DEFAULT
trainer = Trainer(process_position=0)

# if this is the second model on the node, show the second progress bar below
trainer = Trainer(process_position=1)
```

---
#### Save a snapshot of all hyperparameters 
Log hyperparameters using the logger
``` {.python}
logger = TestTubeLogger(...)
logger.log_hyperparams(args)

Trainer(logger=logger)
```

---
#### Write logs file to csv every k batches 
Every k batches, lightning will write the new logs to disk
``` {.python}
# DEFAULT (ie: save a .csv log file every 100 batches)
trainer = Trainer(log_save_interval=100)
```