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# LoopSequentialPipelineBlocks

[`~modular_pipelines.LoopSequentialPipelineBlocks`] are a multi-block type that composes other [`~modular_pipelines.ModularPipelineBlocks`] together in a loop. Data flows circularly, using `inputs` and `intermediate_outputs`, and each block is run iteratively. This is typically used to create a denoising loop which is iterative by default.

This guide shows you how to create [`~modular_pipelines.LoopSequentialPipelineBlocks`].

## Loop wrapper

[`~modular_pipelines.LoopSequentialPipelineBlocks`], is also known as the *loop wrapper* because it defines the loop structure, iteration variables, and configuration. Within the loop wrapper, you need the following variables.

- `loop_inputs` are user provided values and equivalent to [`~modular_pipelines.ModularPipelineBlocks.inputs`].
- `loop_intermediate_outputs` are new intermediate variables created by the block and added to the [`~modular_pipelines.PipelineState`]. It is equivalent to [`~modular_pipelines.ModularPipelineBlocks.intermediate_outputs`].
- `__call__` method defines the loop structure and iteration logic.

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

class LoopWrapper(LoopSequentialPipelineBlocks):
    model_name = "test"
    @property
    def description(self):
        return "I'm a loop!!"
    @property
    def loop_inputs(self):
        return [InputParam(name="num_steps")]
    @torch.no_grad()
    def __call__(self, components, state):
        block_state = self.get_block_state(state)
        # Loop structure - can be customized to your needs
        for i in range(block_state.num_steps):
            # loop_step executes all registered blocks in sequence
            components, block_state = self.loop_step(components, block_state, i=i)
        self.set_block_state(state, block_state)
        return components, state
```

The loop wrapper can pass additional arguments, like current iteration index, to the loop blocks.

## Loop blocks

A loop block is a [`~modular_pipelines.ModularPipelineBlocks`], but the `__call__` method behaves differently.

- It receives the iteration variable from the loop wrapper.
- It works directly with the [`~modular_pipelines.BlockState`] instead of the [`~modular_pipelines.PipelineState`].
- It doesn't require retrieving or updating the [`~modular_pipelines.BlockState`].

Loop blocks share the same [`~modular_pipelines.BlockState`] to allow values to accumulate and change for each iteration in the loop.

```py
class LoopBlock(ModularPipelineBlocks):
    model_name = "test"
    @property
    def inputs(self):
        return [InputParam(name="x")]
    @property
    def intermediate_outputs(self):
        # outputs produced by this block
        return [OutputParam(name="x")]
    @property
    def description(self):
        return "I'm a block used inside the `LoopWrapper` class"
    def __call__(self, components, block_state, i: int):
        block_state.x += 1
        return components, block_state
```

## LoopSequentialPipelineBlocks

Use the [`~modular_pipelines.LoopSequentialPipelineBlocks.from_blocks_dict`] method to add the loop block to the loop wrapper to create [`~modular_pipelines.LoopSequentialPipelineBlocks`].

```py
loop = LoopWrapper.from_blocks_dict({"block1": LoopBlock})
```

Add more loop blocks to run within each iteration with [`~modular_pipelines.LoopSequentialPipelineBlocks.from_blocks_dict`]. This allows you to modify the blocks without changing the loop logic itself.

```py
loop = LoopWrapper.from_blocks_dict({"block1": LoopBlock(), "block2": LoopBlock})
```
