docs/source/en/modular_diffusers/pipeline_block.md

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# ModularPipelineBlocks

[`~modular_pipelines.ModularPipelineBlocks`] is the basic block for building a [`ModularPipeline`]. It defines what components, inputs/outputs, and computation a block should perform for a specific step in a pipeline. A [`~modular_pipelines.ModularPipelineBlocks`] connects with other blocks, using [state](./modular_diffusers_states), to enable the modular construction of workflows.

A [`~modular_pipelines.ModularPipelineBlocks`] on it's own can't be executed. It is a blueprint for what a step should do in a pipeline. To actually run and execute a pipeline, the [`~modular_pipelines.ModularPipelineBlocks`] needs to be converted into a [`ModularPipeline`].

This guide will show you how to create a [`~modular_pipelines.ModularPipelineBlocks`].

## Inputs and outputs

> [!TIP]
> Refer to the [States](./modular_diffusers_states) guide if you aren't familiar with how state works in Modular Diffusers.

A [`~modular_pipelines.ModularPipelineBlocks`] requires `inputs`, and `intermediate_outputs`.

- `inputs` are values provided by a user and retrieved from the [`~modular_pipelines.PipelineState`]. This is useful because some workflows resize an image, but the original image is still required. The [`~modular_pipelines.PipelineState`] maintains the original image.

    Use `InputParam` to define `inputs`.

    ```py
    from diffusers.modular_pipelines import InputParam

    user_inputs = [
        InputParam(name="image", type_hint="PIL.Image", description="raw input image to process")
    ]
    ```

- `intermediate_outputs` are new values created by a block and added to the [`~modular_pipelines.PipelineState`]. The `intermediate_outputs` are available as `inputs` for subsequent blocks or available as the final output from running the pipeline.

    Use `OutputParam` to define `intermediate_outputs`.

    ```py
    from diffusers.modular_pipelines import OutputParam

        user_intermediate_outputs = [
        OutputParam(name="image_latents", description="latents representing the image")
    ]
    ```

The intermediate inputs and outputs share data to connect blocks. They are accessible at any point, allowing you to track the workflow's progress.

## Computation logic

The computation a block performs is defined in the `__call__` method and it follows a specific structure.

1. Retrieve the [`~modular_pipelines.BlockState`] to get a local view of the `inputs`
2. Implement the computation logic on the `inputs`.
3. Update [`~modular_pipelines.PipelineState`] to push changes from the local [`~modular_pipelines.BlockState`] back to the global [`~modular_pipelines.PipelineState`].
4. Return the components and state which becomes available to the next block.

```py
def __call__(self, components, state):
    # Get a local view of the state variables this block needs
    block_state = self.get_block_state(state)

    # Your computation logic here
    # block_state contains all your inputs
    # Access them like: block_state.image, block_state.processed_image

    # Update the pipeline state with your updated block_states
    self.set_block_state(state, block_state)
    return components, state
```

### Components and configs

The components and pipeline-level configs a block needs are specified in [`ComponentSpec`] and [`~modular_pipelines.ConfigSpec`].

- [`ComponentSpec`] contains the expected components used by a block. You need the `name` of the component and ideally a `type_hint` that specifies exactly what the component is.
- [`~modular_pipelines.ConfigSpec`] contains pipeline-level settings that control behavior across all blocks.

```py
from diffusers import ComponentSpec, ConfigSpec

expected_components = [
    ComponentSpec(name="unet", type_hint=UNet2DConditionModel),
    ComponentSpec(name="scheduler", type_hint=EulerDiscreteScheduler)
]

expected_config = [
    ConfigSpec("force_zeros_for_empty_prompt", True)
]
```

When the blocks are converted into a pipeline, the components become available to the block as the first argument in `__call__`.

```py
def __call__(self, components, state):
    # Access components using dot notation
    unet = components.unet
    vae = components.vae
    scheduler = components.scheduler
```