docs/source/en/modular_diffusers/sequential_pipeline_blocks.md

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# SequentialPipelineBlocks

[`~modular_pipelines.SequentialPipelineBlocks`] are a multi-block type that composes other [`~modular_pipelines.ModularPipelineBlocks`] together in a sequence. Data flows linearly from one block to the next using `inputs` and `intermediate_outputs`. Each block in [`~modular_pipelines.SequentialPipelineBlocks`] usually represents a step in the pipeline, and by combining them, you gradually build a pipeline.

This guide shows you how to connect two blocks into a [`~modular_pipelines.SequentialPipelineBlocks`].

Create two [`~modular_pipelines.ModularPipelineBlocks`]. The first block, `InputBlock`, outputs a `batch_size` value and the second block, `ImageEncoderBlock` uses `batch_size` as `inputs`.

<hfoptions id="sequential">
<hfoption id="InputBlock">

```py
from diffusers.modular_pipelines import ModularPipelineBlocks, InputParam, OutputParam

class InputBlock(ModularPipelineBlocks):

    @property
    def inputs(self):
        return [
            InputParam(name="prompt", type_hint=list, description="list of text prompts"),
            InputParam(name="num_images_per_prompt", type_hint=int, description="number of images per prompt"),
        ]

    @property
    def intermediate_outputs(self):
        return [
            OutputParam(name="batch_size", description="calculated batch size"),
        ]

    @property
    def description(self):
        return "A block that determines batch_size based on the number of prompts and num_images_per_prompt argument."

    def __call__(self, components, state):
        block_state = self.get_block_state(state)
        batch_size = len(block_state.prompt)
        block_state.batch_size = batch_size * block_state.num_images_per_prompt
        self.set_block_state(state, block_state)
        return components, state
```

</hfoption>
<hfoption id="ImageEncoderBlock">

```py
import torch
from diffusers.modular_pipelines import ModularPipelineBlocks, InputParam, OutputParam

class ImageEncoderBlock(ModularPipelineBlocks):

    @property
    def inputs(self):
        return [
            InputParam(name="image", type_hint="PIL.Image", description="raw input image to process"),
            InputParam(name="batch_size", type_hint=int),
        ]

    @property
    def intermediate_outputs(self):
        return [
            OutputParam(name="image_latents", description="latents representing the image"),
        ]

    @property
    def description(self):
        return "Encode raw image into its latent presentation"

    def __call__(self, components, state):
        block_state = self.get_block_state(state)
        # Simulate processing the image
        # This will change the state of the image from a PIL image to a tensor for all blocks
        block_state.image = torch.randn(1, 3, 512, 512)
        block_state.batch_size = block_state.batch_size * 2
        block_state.image_latents = torch.randn(1, 4, 64, 64)
        self.set_block_state(state, block_state)
        return components, state
```

</hfoption>
</hfoptions>

Connect the two blocks by defining an [`InsertableDict`] to map the block names to the block instances. Blocks are executed in the order they're registered in `blocks_dict`.

Use [`~modular_pipelines.SequentialPipelineBlocks.from_blocks_dict`] to create a [`~modular_pipelines.SequentialPipelineBlocks`].

```py
from diffusers.modular_pipelines import SequentialPipelineBlocks, InsertableDict

blocks_dict = InsertableDict()
blocks_dict["input"] = input_block
blocks_dict["image_encoder"] = image_encoder_block

blocks = SequentialPipelineBlocks.from_blocks_dict(blocks_dict)
```

Inspect the sub-blocks in [`~modular_pipelines.SequentialPipelineBlocks`] by calling `blocks`, and for more details about the inputs and outputs, access the `docs` attribute.

```py
print(blocks)
print(blocks.doc)
```