BaseValidator

class BaseValidator:
    """
    BaseValidator

    A base class for creating validators.

    Attributes:
        dataloader (DataLoader): Dataloader to use for validation.
        pbar (tqdm): Progress bar to update during validation.
        args (SimpleNamespace): Configuration for the validator.
        model (nn.Module): Model to validate.
        data (dict): Data dictionary.
        device (torch.device): Device to use for validation.
        batch_i (int): Current batch index.
        training (bool): Whether the model is in training mode.
        speed (float): Batch processing speed in seconds.
        jdict (dict): Dictionary to store validation results.
        save_dir (Path): Directory to save results.
    """

    def __init__(self, dataloader=None, save_dir=None, pbar=None, args=None, _callbacks=None):
        """
        Initializes a BaseValidator instance.

        Args:
            dataloader (torch.utils.data.DataLoader): Dataloader to be used for validation.
            save_dir (Path): Directory to save results.
            pbar (tqdm.tqdm): Progress bar for displaying progress.
            args (SimpleNamespace): Configuration for the validator.
        """
        self.dataloader = dataloader
        self.pbar = pbar
        self.args = args or get_cfg(DEFAULT_CFG)
        self.model = None
        self.data = None
        self.device = None
        self.batch_i = None
        self.training = True
        self.speed = {'preprocess': 0.0, 'inference': 0.0, 'loss': 0.0, 'postprocess': 0.0}
        self.jdict = None

        project = self.args.project or Path(SETTINGS['runs_dir']) / self.args.task
        name = self.args.name or f'{self.args.mode}'
        self.save_dir = save_dir or increment_path(Path(project) / name,
                                                   exist_ok=self.args.exist_ok if RANK in (-1, 0) else True)
        (self.save_dir / 'labels' if self.args.save_txt else self.save_dir).mkdir(parents=True, exist_ok=True)

        if self.args.conf is None:
            self.args.conf = 0.001  # default conf=0.001

        self.plots = {}
        self.callbacks = _callbacks or callbacks.get_default_callbacks()

    @smart_inference_mode()
    def __call__(self, trainer=None, model=None):
        """
        Supports validation of a pre-trained model if passed or a model being trained
        if trainer is passed (trainer gets priority).
        """
        self.training = trainer is not None
        if self.training:
            self.device = trainer.device
            self.data = trainer.data
            model = trainer.ema.ema or trainer.model
            self.args.half = self.device.type != 'cpu'  # force FP16 val during training
            model = model.half() if self.args.half else model.float()
            self.model = model
            self.loss = torch.zeros_like(trainer.loss_items, device=trainer.device)
            self.args.plots = trainer.stopper.possible_stop or (trainer.epoch == trainer.epochs - 1)
            model.eval()
        else:
            callbacks.add_integration_callbacks(self)
            self.run_callbacks('on_val_start')
            assert model is not None, 'Either trainer or model is needed for validation'
            self.device = select_device(self.args.device, self.args.batch)
            self.args.half &= self.device.type != 'cpu'
            model = AutoBackend(model, device=self.device, dnn=self.args.dnn, data=self.args.data, fp16=self.args.half)
            self.model = model
            stride, pt, jit, engine = model.stride, model.pt, model.jit, model.engine
            imgsz = check_imgsz(self.args.imgsz, stride=stride)
            if engine:
                self.args.batch = model.batch_size
            else:
                self.device = model.device
                if not pt and not jit:
                    self.args.batch = 1  # export.py models default to batch-size 1
                    LOGGER.info(f'Forcing batch=1 square inference (1,3,{imgsz},{imgsz}) for non-PyTorch models')

            if isinstance(self.args.data, str) and self.args.data.endswith('.yaml'):
                self.data = check_det_dataset(self.args.data)
            elif self.args.task == 'classify':
                self.data = check_cls_dataset(self.args.data, split=self.args.split)
            else:
                raise FileNotFoundError(emojis(f"Dataset '{self.args.data}' for task={self.args.task} not found ❌"))

            if self.device.type == 'cpu':
                self.args.workers = 0  # faster CPU val as time dominated by inference, not dataloading
            if not pt:
                self.args.rect = False
            self.dataloader = self.dataloader or self.get_dataloader(self.data.get(self.args.split), self.args.batch)

            model.eval()
            model.warmup(imgsz=(1 if pt else self.args.batch, 3, imgsz, imgsz))  # warmup

        dt = Profile(), Profile(), Profile(), Profile()
        n_batches = len(self.dataloader)
        desc = self.get_desc()
        # NOTE: keeping `not self.training` in tqdm will eliminate pbar after segmentation evaluation during training,
        # which may affect classification task since this arg is in yolov5/classify/val.py.
        # bar = tqdm(self.dataloader, desc, n_batches, not self.training, bar_format=TQDM_BAR_FORMAT)
        bar = tqdm(self.dataloader, desc, n_batches, bar_format=TQDM_BAR_FORMAT)
        self.init_metrics(de_parallel(model))
        self.jdict = []  # empty before each val
        for batch_i, batch in enumerate(bar):
            self.run_callbacks('on_val_batch_start')
            self.batch_i = batch_i
            # Preprocess
            with dt[0]:
                batch = self.preprocess(batch)

            # Inference
            with dt[1]:
                preds = model(batch['img'], augment=self.args.augment)

            # Loss
            with dt[2]:
                if self.training:
                    self.loss += model.loss(batch, preds)[1]

            # Postprocess
            with dt[3]:
                preds = self.postprocess(preds)

            self.update_metrics(preds, batch)
            if self.args.plots and batch_i < 3:
                self.plot_val_samples(batch, batch_i)
                self.plot_predictions(batch, preds, batch_i)

            self.run_callbacks('on_val_batch_end')
        stats = self.get_stats()
        self.check_stats(stats)
        self.speed = dict(zip(self.speed.keys(), (x.t / len(self.dataloader.dataset) * 1E3 for x in dt)))
        self.finalize_metrics()
        self.print_results()
        self.run_callbacks('on_val_end')
        if self.training:
            model.float()
            results = {**stats, **trainer.label_loss_items(self.loss.cpu() / len(self.dataloader), prefix='val')}
            return {k: round(float(v), 5) for k, v in results.items()}  # return results as 5 decimal place floats
        else:
            LOGGER.info('Speed: %.1fms preprocess, %.1fms inference, %.1fms loss, %.1fms postprocess per image' %
                        tuple(self.speed.values()))
            if self.args.save_json and self.jdict:
                with open(str(self.save_dir / 'predictions.json'), 'w') as f:
                    LOGGER.info(f'Saving {f.name}...')
                    json.dump(self.jdict, f)  # flatten and save
                stats = self.eval_json(stats)  # update stats
            if self.args.plots or self.args.save_json:
                LOGGER.info(f"Results saved to {colorstr('bold', self.save_dir)}")
            return stats

    def add_callback(self, event: str, callback):
        """Appends the given callback."""
        self.callbacks[event].append(callback)

    def run_callbacks(self, event: str):
        """Runs all callbacks associated with a specified event."""
        for callback in self.callbacks.get(event, []):
            callback(self)

    def get_dataloader(self, dataset_path, batch_size):
        """Get data loader from dataset path and batch size."""
        raise NotImplementedError('get_dataloader function not implemented for this validator')

    def build_dataset(self, img_path):
        """Build dataset"""
        raise NotImplementedError('build_dataset function not implemented in validator')

    def preprocess(self, batch):
        """Preprocesses an input batch."""
        return batch

    def postprocess(self, preds):
        """Describes and summarizes the purpose of 'postprocess()' but no details mentioned."""
        return preds

    def init_metrics(self, model):
        """Initialize performance metrics for the YOLO model."""
        pass

    def update_metrics(self, preds, batch):
        """Updates metrics based on predictions and batch."""
        pass

    def finalize_metrics(self, *args, **kwargs):
        """Finalizes and returns all metrics."""
        pass

    def get_stats(self):
        """Returns statistics about the model's performance."""
        return {}

    def check_stats(self, stats):
        """Checks statistics."""
        pass

    def print_results(self):
        """Prints the results of the model's predictions."""
        pass

    def get_desc(self):
        """Get description of the YOLO model."""
        pass

    @property
    def metric_keys(self):
        """Returns the metric keys used in YOLO training/validation."""
        return []

    def on_plot(self, name, data=None):
        """Registers plots (e.g. to be consumed in callbacks)"""
        self.plots[name] = {'data': data, 'timestamp': time.time()}

    # TODO: may need to put these following functions into callback
    def plot_val_samples(self, batch, ni):
        """Plots validation samples during training."""
        pass

    def plot_predictions(self, batch, preds, ni):
        """Plots YOLO model predictions on batch images."""
        pass

    def pred_to_json(self, preds, batch):
        """Convert predictions to JSON format."""
        pass

    def eval_json(self, stats):
        """Evaluate and return JSON format of prediction statistics."""
        pass