Reference for ultralytics/engine/trainer.py

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

def build_dataset(self, img_path, mode="train", batch=None):
    """Build dataset."""
    raise NotImplementedError("build_dataset function not implemented in trainer")

def build_optimizer(self, model, name="auto", lr=0.001, momentum=0.9, decay=1e-5, iterations=1e5):
    """
    Constructs an optimizer for the given model, based on the specified optimizer name, learning rate, momentum,
    weight decay, and number of iterations.

    Args:
        model (torch.nn.Module): The model for which to build an optimizer.
        name (str, optional): The name of the optimizer to use. If 'auto', the optimizer is selected
            based on the number of iterations. Default: 'auto'.
        lr (float, optional): The learning rate for the optimizer. Default: 0.001.
        momentum (float, optional): The momentum factor for the optimizer. Default: 0.9.
        decay (float, optional): The weight decay for the optimizer. Default: 1e-5.
        iterations (float, optional): The number of iterations, which determines the optimizer if
            name is 'auto'. Default: 1e5.

    Returns:
        (torch.optim.Optimizer): The constructed optimizer.
    """

    g = [], [], []  # optimizer parameter groups
    bn = tuple(v for k, v in nn.__dict__.items() if "Norm" in k)  # normalization layers, i.e. BatchNorm2d()
    if name == "auto":
        LOGGER.info(
            f"{colorstr('optimizer:')} 'optimizer=auto' found, "
            f"ignoring 'lr0={self.args.lr0}' and 'momentum={self.args.momentum}' and "
            f"determining best 'optimizer', 'lr0' and 'momentum' automatically... "
        )
        nc = getattr(model, "nc", 10)  # number of classes
        lr_fit = round(0.002 * 5 / (4 + nc), 6)  # lr0 fit equation to 6 decimal places
        name, lr, momentum = ("SGD", 0.01, 0.9) if iterations > 10000 else ("AdamW", lr_fit, 0.9)
        self.args.warmup_bias_lr = 0.0  # no higher than 0.01 for Adam

    for module_name, module in model.named_modules():
        for param_name, param in module.named_parameters(recurse=False):
            fullname = f"{module_name}.{param_name}" if module_name else param_name
            if "bias" in fullname:  # bias (no decay)
                g[2].append(param)
            elif isinstance(module, bn):  # weight (no decay)
                g[1].append(param)
            else:  # weight (with decay)
                g[0].append(param)

    if name in {"Adam", "Adamax", "AdamW", "NAdam", "RAdam"}:
        optimizer = getattr(optim, name, optim.Adam)(g[2], lr=lr, betas=(momentum, 0.999), weight_decay=0.0)
    elif name == "RMSProp":
        optimizer = optim.RMSprop(g[2], lr=lr, momentum=momentum)
    elif name == "SGD":
        optimizer = optim.SGD(g[2], lr=lr, momentum=momentum, nesterov=True)
    else:
        raise NotImplementedError(
            f"Optimizer '{name}' not found in list of available optimizers "
            f"[Adam, AdamW, NAdam, RAdam, RMSProp, SGD, auto]."
            "To request support for addition optimizers please visit https://github.com/ultralytics/ultralytics."
        )

    optimizer.add_param_group({"params": g[0], "weight_decay": decay})  # add g0 with weight_decay
    optimizer.add_param_group({"params": g[1], "weight_decay": 0.0})  # add g1 (BatchNorm2d weights)
    LOGGER.info(
        f"{colorstr('optimizer:')} {type(optimizer).__name__}(lr={lr}, momentum={momentum}) with parameter groups "
        f'{len(g[1])} weight(decay=0.0), {len(g[0])} weight(decay={decay}), {len(g[2])} bias(decay=0.0)'
    )
    return optimizer

def build_targets(self, preds, targets):
    """Builds target tensors for training YOLO model."""
    pass

def check_resume(self, overrides):
    """Check if resume checkpoint exists and update arguments accordingly."""
    resume = self.args.resume
    if resume:
        try:
            exists = isinstance(resume, (str, Path)) and Path(resume).exists()
            last = Path(check_file(resume) if exists else get_latest_run())

            # Check that resume data YAML exists, otherwise strip to force re-download of dataset
            ckpt_args = attempt_load_weights(last).args
            if not Path(ckpt_args["data"]).exists():
                ckpt_args["data"] = self.args.data

            resume = True
            self.args = get_cfg(ckpt_args)
            self.args.model = self.args.resume = str(last)  # reinstate model
            for k in "imgsz", "batch", "device":  # allow arg updates to reduce memory or update device on resume
                if k in overrides:
                    setattr(self.args, k, overrides[k])

        except Exception as e:
            raise FileNotFoundError(
                "Resume checkpoint not found. Please pass a valid checkpoint to resume from, "
                "i.e. 'yolo train resume model=path/to/last.pt'"
            ) from e
    self.resume = resume

def final_eval(self):
    """Performs final evaluation and validation for object detection YOLO model."""
    for f in self.last, self.best:
        if f.exists():
            strip_optimizer(f)  # strip optimizers
            if f is self.best:
                LOGGER.info(f"\nValidating {f}...")
                self.validator.args.plots = self.args.plots
                self.metrics = self.validator(model=f)
                self.metrics.pop("fitness", None)
                self.run_callbacks("on_fit_epoch_end")

def get_dataloader(self, dataset_path, batch_size=16, rank=0, mode="train"):
    """Returns dataloader derived from torch.data.Dataloader."""
    raise NotImplementedError("get_dataloader function not implemented in trainer")

def get_dataset(self):
    """
    Get train, val path from data dict if it exists.

    Returns None if data format is not recognized.
    """
    try:
        if self.args.task == "classify":
            data = check_cls_dataset(self.args.data)
        elif self.args.data.split(".")[-1] in {"yaml", "yml"} or self.args.task in {
            "detect",
            "segment",
            "pose",
            "obb",
        }:
            data = check_det_dataset(self.args.data)
            if "yaml_file" in data:
                self.args.data = data["yaml_file"]  # for validating 'yolo train data=url.zip' usage
    except Exception as e:
        raise RuntimeError(emojis(f"Dataset '{clean_url(self.args.data)}' error ❌ {e}")) from e
    self.data = data
    return data["train"], data.get("val") or data.get("test")

def get_model(self, cfg=None, weights=None, verbose=True):
    """Get model and raise NotImplementedError for loading cfg files."""
    raise NotImplementedError("This task trainer doesn't support loading cfg files")

def get_validator(self):
    """Returns a NotImplementedError when the get_validator function is called."""
    raise NotImplementedError("get_validator function not implemented in trainer")

def label_loss_items(self, loss_items=None, prefix="train"):
    """
    Returns a loss dict with labelled training loss items tensor.

    Note:
        This is not needed for classification but necessary for segmentation & detection
    """
    return {"loss": loss_items} if loss_items is not None else ["loss"]

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

def optimizer_step(self):
    """Perform a single step of the training optimizer with gradient clipping and EMA update."""
    self.scaler.unscale_(self.optimizer)  # unscale gradients
    torch.nn.utils.clip_grad_norm_(self.model.parameters(), max_norm=10.0)  # clip gradients
    self.scaler.step(self.optimizer)
    self.scaler.update()
    self.optimizer.zero_grad()
    if self.ema:
        self.ema.update(self.model)

def plot_metrics(self):
    """Plot and display metrics visually."""
    pass

def plot_training_labels(self):
    """Plots training labels for YOLO model."""
    pass

def plot_training_samples(self, batch, ni):
    """Plots training samples during YOLO training."""
    pass

def preprocess_batch(self, batch):
    """Allows custom preprocessing model inputs and ground truths depending on task type."""
    return batch

def progress_string(self):
    """Returns a string describing training progress."""
    return ""

def resume_training(self, ckpt):
    """Resume YOLO training from given epoch and best fitness."""
    if ckpt is None or not self.resume:
        return
    best_fitness = 0.0
    start_epoch = ckpt.get("epoch", -1) + 1
    if ckpt.get("optimizer", None) is not None:
        self.optimizer.load_state_dict(ckpt["optimizer"])  # optimizer
        best_fitness = ckpt["best_fitness"]
    if self.ema and ckpt.get("ema"):
        self.ema.ema.load_state_dict(ckpt["ema"].float().state_dict())  # EMA
        self.ema.updates = ckpt["updates"]
    assert start_epoch > 0, (
        f"{self.args.model} training to {self.epochs} epochs is finished, nothing to resume.\n"
        f"Start a new training without resuming, i.e. 'yolo train model={self.args.model}'"
    )
    LOGGER.info(f"Resuming training {self.args.model} from epoch {start_epoch + 1} to {self.epochs} total epochs")
    if self.epochs < start_epoch:
        LOGGER.info(
            f"{self.model} has been trained for {ckpt['epoch']} epochs. Fine-tuning for {self.epochs} more epochs."
        )
        self.epochs += ckpt["epoch"]  # finetune additional epochs
    self.best_fitness = best_fitness
    self.start_epoch = start_epoch
    if start_epoch > (self.epochs - self.args.close_mosaic):
        self._close_dataloader_mosaic()

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

def save_metrics(self, metrics):
    """Saves training metrics to a CSV file."""
    keys, vals = list(metrics.keys()), list(metrics.values())
    n = len(metrics) + 1  # number of cols
    s = "" if self.csv.exists() else (("%23s," * n % tuple(["epoch"] + keys)).rstrip(",") + "\n")  # header
    with open(self.csv, "a") as f:
        f.write(s + ("%23.5g," * n % tuple([self.epoch + 1] + vals)).rstrip(",") + "\n")

def save_model(self):
    """Save model training checkpoints with additional metadata."""
    import io

    import pandas as pd  # scope for faster 'import ultralytics'

    # Serialize ckpt to a byte buffer once (faster than repeated torch.save() calls)
    buffer = io.BytesIO()
    torch.save(
        {
            "epoch": self.epoch,
            "best_fitness": self.best_fitness,
            "model": None,  # resume and final checkpoints derive from EMA
            "ema": deepcopy(self.ema.ema).half(),
            "updates": self.ema.updates,
            "optimizer": convert_optimizer_state_dict_to_fp16(deepcopy(self.optimizer.state_dict())),
            "train_args": vars(self.args),  # save as dict
            "train_metrics": {**self.metrics, **{"fitness": self.fitness}},
            "train_results": {k.strip(): v for k, v in pd.read_csv(self.csv).to_dict(orient="list").items()},
            "date": datetime.now().isoformat(),
            "version": __version__,
            "license": "AGPL-3.0 (https://ultralytics.com/license)",
            "docs": "https://docs.ultralytics.com",
        },
        buffer,
    )
    serialized_ckpt = buffer.getvalue()  # get the serialized content to save

    # Save checkpoints
    self.last.write_bytes(serialized_ckpt)  # save last.pt
    if self.best_fitness == self.fitness:
        self.best.write_bytes(serialized_ckpt)  # save best.pt
    if (self.save_period > 0) and (self.epoch % self.save_period == 0):
        (self.wdir / f"epoch{self.epoch}.pt").write_bytes(serialized_ckpt)  # save epoch, i.e. 'epoch3.pt'

def set_callback(self, event: str, callback):
    """Overrides the existing callbacks with the given callback."""
    self.callbacks[event] = [callback]

def set_model_attributes(self):
    """To set or update model parameters before training."""
    self.model.names = self.data["names"]

def setup_model(self):
    """Load/create/download model for any task."""
    if isinstance(self.model, torch.nn.Module):  # if model is loaded beforehand. No setup needed
        return

    cfg, weights = self.model, None
    ckpt = None
    if str(self.model).endswith(".pt"):
        weights, ckpt = attempt_load_one_weight(self.model)
        cfg = weights.yaml
    elif isinstance(self.args.pretrained, (str, Path)):
        weights, _ = attempt_load_one_weight(self.args.pretrained)
    self.model = self.get_model(cfg=cfg, weights=weights, verbose=RANK == -1)  # calls Model(cfg, weights)
    return ckpt

def train(self):
    """Allow device='', device=None on Multi-GPU systems to default to device=0."""
    if isinstance(self.args.device, str) and len(self.args.device):  # i.e. device='0' or device='0,1,2,3'
        world_size = len(self.args.device.split(","))
    elif isinstance(self.args.device, (tuple, list)):  # i.e. device=[0, 1, 2, 3] (multi-GPU from CLI is list)
        world_size = len(self.args.device)
    elif torch.cuda.is_available():  # i.e. device=None or device='' or device=number
        world_size = 1  # default to device 0
    else:  # i.e. device='cpu' or 'mps'
        world_size = 0

    # Run subprocess if DDP training, else train normally
    if world_size > 1 and "LOCAL_RANK" not in os.environ:
        # Argument checks
        if self.args.rect:
            LOGGER.warning("WARNING ⚠️ 'rect=True' is incompatible with Multi-GPU training, setting 'rect=False'")
            self.args.rect = False
        if self.args.batch < 1.0:
            LOGGER.warning(
                "WARNING ⚠️ 'batch<1' for AutoBatch is incompatible with Multi-GPU training, setting "
                "default 'batch=16'"
            )
            self.args.batch = 16

        # Command
        cmd, file = generate_ddp_command(world_size, self)
        try:
            LOGGER.info(f'{colorstr("DDP:")} debug command {" ".join(cmd)}')
            subprocess.run(cmd, check=True)
        except Exception as e:
            raise e
        finally:
            ddp_cleanup(self, str(file))

    else:
        self._do_train(world_size)

def validate(self):
    """
    Runs validation on test set using self.validator.

    The returned dict is expected to contain "fitness" key.
    """
    metrics = self.validator(self)
    fitness = metrics.pop("fitness", -self.loss.detach().cpu().numpy())  # use loss as fitness measure if not found
    if not self.best_fitness or self.best_fitness < fitness:
        self.best_fitness = fitness
    return metrics, fitness