| --- |
| comments: true |
| description: Learn how to utilize callbacks in the Ultralytics framework during train, val, export, and predict modes for enhanced functionality. |
| keywords: Ultralytics, YOLO, callbacks guide, training callback, validation callback, export callback, prediction callback |
| --- |
| |
| ## Callbacks |
|
|
| Ultralytics framework supports callbacks as entry points in strategic stages of train, val, export, and predict modes. |
| Each callback accepts a `Trainer`, `Validator`, or `Predictor` object depending on the operation type. All properties of |
| these objects can be found in Reference section of the docs. |
|
|
| ## Examples |
|
|
| ### Returning additional information with Prediction |
|
|
| In this example, we want to return the original frame with each result object. Here's how we can do that |
|
|
| ```python |
| def on_predict_batch_end(predictor): |
| # Retrieve the batch data |
| _, im0s, _, _ = predictor.batch |
| |
| # Ensure that im0s is a list |
| im0s = im0s if isinstance(im0s, list) else [im0s] |
| |
| # Combine the prediction results with the corresponding frames |
| predictor.results = zip(predictor.results, im0s) |
| |
| # Create a YOLO model instance |
| model = YOLO(f'yolov8n.pt') |
| |
| # Add the custom callback to the model |
| model.add_callback("on_predict_batch_end", on_predict_batch_end) |
| |
| # Iterate through the results and frames |
| for (result, frame) in model.track/predict(): |
| pass |
| ``` |
|
|
| ## All callbacks |
|
|
| Here are all supported callbacks. See callbacks [source code](https://github.com/ultralytics/ultralytics/blob/main/ultralytics/utils/callbacks/base.py) for additional details. |
|
|
| ### Trainer Callbacks |
|
|
| | Callback | Description | |
| |-----------------------------|---------------------------------------------------------| |
| | `on_pretrain_routine_start` | Triggered at the beginning of pre-training routine | |
| | `on_pretrain_routine_end` | Triggered at the end of pre-training routine | |
| | `on_train_start` | Triggered when the training starts | |
| | `on_train_epoch_start` | Triggered at the start of each training epoch | |
| | `on_train_batch_start` | Triggered at the start of each training batch | |
| | `optimizer_step` | Triggered during the optimizer step | |
| | `on_before_zero_grad` | Triggered before gradients are zeroed | |
| | `on_train_batch_end` | Triggered at the end of each training batch | |
| | `on_train_epoch_end` | Triggered at the end of each training epoch | |
| | `on_fit_epoch_end` | Triggered at the end of each fit epoch | |
| | `on_model_save` | Triggered when the model is saved | |
| | `on_train_end` | Triggered when the training process ends | |
| | `on_params_update` | Triggered when model parameters are updated | |
| | `teardown` | Triggered when the training process is being cleaned up | |
|
|
| ### Validator Callbacks |
|
|
| | Callback | Description | |
| |----------------------|-------------------------------------------------| |
| | `on_val_start` | Triggered when the validation starts | |
| | `on_val_batch_start` | Triggered at the start of each validation batch | |
| | `on_val_batch_end` | Triggered at the end of each validation batch | |
| | `on_val_end` | Triggered when the validation ends | |
|
|
| ### Predictor Callbacks |
|
|
| | Callback | Description | |
| |------------------------------|---------------------------------------------------| |
| | `on_predict_start` | Triggered when the prediction process starts | |
| | `on_predict_batch_start` | Triggered at the start of each prediction batch | |
| | `on_predict_postprocess_end` | Triggered at the end of prediction postprocessing | |
| | `on_predict_batch_end` | Triggered at the end of each prediction batch | |
| | `on_predict_end` | Triggered when the prediction process ends | |
|
|
| ### Exporter Callbacks |
|
|
| | Callback | Description | |
| |-------------------|------------------------------------------| |
| | `on_export_start` | Triggered when the export process starts | |
| | `on_export_end` | Triggered when the export process ends | |
|
|
