cb_1.py

import pytorch_lightning as pl

class MyPrintingCallback(pl.Callback):

    def on_init_start(self, trainer):
        print('Starting to init trainer!')

    def on_init_end(self, trainer):
        print('trainer is init now')

    def on_train_end(self, trainer, pl_module):
        print('do something when training ends')

# pass to trainer
trainer = pl.Trainer(callbacks=[MyPrintingCallback()])