# Bring Your Own Policies

This tutorial explains how to integrate your own custom policy implementations into the LeRobot ecosystem, allowing you to leverage all LeRobot tools for training, evaluation, and deployment while using your own algorithms.

## Step 1: Create a Policy Package

Your custom policy should be organized as an installable Python package following LeRobot's plugin conventions.

### Package Structure

Create a package with the prefix `lerobot_policy_` (IMPORTANT!) followed by your policy name:

```bash
lerobot_policy_my_custom_policy/
├── pyproject.toml
└── src/
    └── lerobot_policy_my_custom_policy/
        ├── __init__.py
        ├── configuration_my_custom_policy.py
        ├── modeling_my_custom_policy.py
        └── processor_my_custom_policy.py
```

### Package Configuration

Set up your `pyproject.toml`:

```toml
[project]
name = "lerobot_policy_my_custom_policy"
version = "0.1.0"
dependencies = [
    # your policy-specific dependencies
]
requires-python = ">= 3.12"

[build-system]
build-backend = # your-build-backend
requires = # your-build-system
```

## Step 2: Define the Policy Configuration

Create a configuration class that inherits from [`PreTrainedConfig`](https://github.com/huggingface/lerobot/blob/main/src/lerobot/configs/policies.py) and registers your policy type:
Here is a template to get you started, customize the parameters and methods as needed for your policy's architecture and training requirements.

```python
# configuration_my_custom_policy.py
from dataclasses import dataclass, field
from lerobot.configs.policies import PreTrainedConfig
from lerobot.optim.optimizers import AdamWConfig
from lerobot.optim.schedulers import CosineDecayWithWarmupSchedulerConfig

@PreTrainedConfig.register_subclass("my_custom_policy")
@dataclass
class MyCustomPolicyConfig(PreTrainedConfig):
    """Configuration class for MyCustomPolicy.

    Args:
        n_obs_steps: Number of observation steps to use as input
        horizon: Action prediction horizon
        n_action_steps: Number of action steps to execute
        hidden_dim: Hidden dimension for the policy network
        # Add your policy-specific parameters here
    """

    horizon: int = 50
    n_action_steps: int = 50
    hidden_dim: int = 256

    optimizer_lr: float = 1e-4
    optimizer_weight_decay: float = 1e-4

    def __post_init__(self):
        super().__post_init__()
        if self.n_action_steps > self.horizon:
            raise ValueError("n_action_steps cannot exceed horizon")

    def validate_features(self) -> None:
        """Validate input/output feature compatibility."""
        if not self.image_features:
            raise ValueError("MyCustomPolicy requires at least one image feature.")
        if self.action_feature is None:
            raise ValueError("MyCustomPolicy requires 'action' in output_features.")

    def get_optimizer_preset(self) -> AdamWConfig:
        return AdamWConfig(lr=self.optimizer_lr, weight_decay=self.optimizer_weight_decay)

    def get_scheduler_preset(self):
        return None

    @property
    def observation_delta_indices(self) -> list[int] | None:
        """Relative timestep offsets the dataset loader provides per observation.

        Return `None` for single-frame policies. For temporal policies that consume
        multiple past or future frames, return a list of offsets, e.g. `[-20, -10, 0, 10]` for
        3 past frames at stride 10 and 1 future frame at stride 10.
        """
        return None

    @property
    def action_delta_indices(self) -> list[int]:
        """Relative timestep offsets for the action chunk the dataset loader returns.
        """
        return list(range(self.horizon))

    @property
    def reward_delta_indices(self) -> None:
        return None
```

## Step 3: Implement the Policy Class

Create your policy implementation by inheriting from [`PreTrainedPolicy`](https://github.com/huggingface/lerobot/blob/main/src/lerobot/policies/pretrained.py):

```python
# modeling_my_custom_policy.py
import torch
import torch.nn as nn
from typing import Any

from lerobot.policies.pretrained import PreTrainedPolicy
from lerobot.utils.constants import ACTION
from .configuration_my_custom_policy import MyCustomPolicyConfig

class MyCustomPolicy(PreTrainedPolicy):
    config_class = MyCustomPolicyConfig  # must match the string in @register_subclass
    name = "my_custom_policy"

    def __init__(self, config: MyCustomPolicyConfig, dataset_stats: dict[str, Any] = None):
        super().__init__(config, dataset_stats)
        config.validate_features()  # not called automatically by the base class
        self.config = config
        self.model = ...  # your nn.Module here

    def reset(self):
        """Reset episode state."""
        ...

    def get_optim_params(self) -> dict:
        """Return parameters to pass to the optimizer (e.g. with per-group lr/wd)."""
        return {"params": self.parameters()}

    def predict_action_chunk(self, batch: dict[str, torch.Tensor], **kwargs) -> torch.Tensor:
        """Return the full action chunk (B, chunk_size, action_dim) for the current observation."""
        ...

    def select_action(self, batch: dict[str, torch.Tensor], **kwargs) -> torch.Tensor:
        """Return a single action for the current timestep (called at inference)."""
        ...

    def forward(self, batch: dict[str, torch.Tensor]) -> dict[str, torch.Tensor]:
        """Compute the training loss.

        `batch["action_is_pad"]` is a bool mask of shape (B, horizon) that marks
        timesteps padded because the episode ended before `horizon` steps, you
        can exclude those from your loss.
        """
        actions = batch[ACTION]
        action_is_pad = batch.get("action_is_pad")
        ...
        return {"loss": ...}
```

## Step 4: Add Data Processors

Create processor functions. For a concrete reference, see [processor_act.py](https://github.com/huggingface/lerobot/blob/main/src/lerobot/policies/act/processor_act.py) or [processor_diffusion.py](https://github.com/huggingface/lerobot/blob/main/src/lerobot/policies/diffusion/processor_diffusion.py).

```python
# processor_my_custom_policy.py
from typing import Any
import torch

from lerobot.processor import PolicyAction, PolicyProcessorPipeline


def make_my_custom_policy_pre_post_processors(
    config,
    dataset_stats: dict[str, dict[str, torch.Tensor]] | None = None,
) -> tuple[
    PolicyProcessorPipeline[dict[str, Any], dict[str, Any]],
    PolicyProcessorPipeline[PolicyAction, PolicyAction],
]:
    preprocessor = ...   # build your PolicyProcessorPipeline for inputs
    postprocessor = ...  # build your PolicyProcessorPipeline for outputs
    return preprocessor, postprocessor
```

**Important - function naming:** LeRobot discovers your processor by name. The function **must** be called `make_{policy_name}_pre_post_processors` (matching the string you passed to `@PreTrainedConfig.register_subclass`).

## Step 5: Package Initialization

Expose your classes in the package's `__init__.py`:

```python
# __init__.py
"""Custom policy package for LeRobot."""

try:
    import lerobot  # noqa: F401
except ImportError:
    raise ImportError(
        "lerobot is not installed. Please install lerobot to use this policy package."
    )

from .configuration_my_custom_policy import MyCustomPolicyConfig
from .modeling_my_custom_policy import MyCustomPolicy
from .processor_my_custom_policy import make_my_custom_policy_pre_post_processors

__all__ = [
    "MyCustomPolicyConfig",
    "MyCustomPolicy",
    "make_my_custom_policy_pre_post_processors",
]
```

## Step 6: Installation and Usage

### Install Your Policy Package

```bash
cd lerobot_policy_my_custom_policy
pip install -e .

# Or install from PyPI if published
pip install lerobot_policy_my_custom_policy
```

### Use Your Policy

Once installed, your policy automatically integrates with LeRobot's training and evaluation tools:

```bash
lerobot-train \
    --policy.type my_custom_policy \
    --env.type pusht \
    --steps 200000
```

## Examples and Community Contributions

Check out these example policy implementations:

- [DiTFlow Policy](https://github.com/danielsanjosepro/lerobot_policy_ditflow) - Diffusion Transformer policy with flow-matching objective. Try it out in this example: [DiTFlow Example](https://github.com/danielsanjosepro/test_lerobot_policy_ditflow)

Share your policy implementations with the community! 🤗
