LambdaCallback
classkeras.callbacks.LambdaCallback(
on_epoch_begin=None,
on_epoch_end=None,
on_train_begin=None,
on_train_end=None,
on_train_batch_begin=None,
on_train_batch_end=None,
**kwargs
)
Callback for creating simple, custom callbacks on-the-fly.
This callback is constructed with anonymous functions that will be called
at the appropriate time (during `Model.{fit | evaluate | predict}`).
Note that the callbacks expects positional arguments, as:
- `on_epoch_begin` and `on_epoch_end` expect two positional arguments:
`epoch`, `logs`
- `on_train_begin` and `on_train_end` expect one positional argument:
`logs`
- `on_train_batch_begin` and `on_train_batch_end` expect two positional
arguments: `batch`, `logs`
- See `Callback` class definition for the full list of functions and their
expected arguments.
# Arguments
on_epoch_begin: called at the beginning of every epoch.
on_epoch_end: called at the end of every epoch.
on_train_begin: called at the beginning of model training.
on_train_end: called at the end of model training.
on_train_batch_begin: called at the beginning of every train batch.
on_train_batch_end: called at the end of every train batch.
kwargs: Any function in `Callback` that you want to override by
passing `function_name=function`. For example,
`LambdaCallback(.., on_train_end=train_end_fn)`. The custom function
needs to have same arguments as the ones defined in `Callback`.
# Example
```python
# Print the batch number at the beginning of every batch.
batch_print_callback = LambdaCallback(
on_train_batch_begin=lambda batch,logs: print(batch))
# Stream the epoch loss to a file in JSON format. The file content
# is not well-formed JSON but rather has a JSON object per line.
import json
json_log = open('loss_log.json', mode='wt', buffering=1)
json_logging_callback = LambdaCallback(
on_epoch_end=lambda epoch, logs: json_log.write(
json.dumps({'epoch': epoch, 'loss': logs['loss']}) + '
'), on_train_end=lambda logs: json_log.close() )
# Terminate some processes after having finished model training.
processes = ...
cleanup_callback = LambdaCallback(
on_train_end=lambda logs: [
p.terminate() for p in processes if p.is_alive()])
model.fit(...,
callbacks=[batch_print_callback,
json_logging_callback,
cleanup_callback])
```