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# How to build a community pipeline

*Note*: this page was built from the GitHub Issue on Community Pipelines [#841](https://github.com/huggingface/diffusers/issues/841).

Let's make an example!
Say you want to define a pipeline that just does a single forward pass to a U-Net and then calls a scheduler only once (Note, this doesn't make any sense from a scientific point of view, but only represents an example of how things work under the hood).

Cool! So you open your favorite IDE and start creating your pipeline 💻.
First, what model weights and configurations do we need?
We have a U-Net and a scheduler, so our pipeline should take a U-Net and a scheduler as an argument.
Also, as stated above, you'd like to be able to load weights and the scheduler config for Hub and share your code with others, so we'll inherit from `DiffusionPipeline`:

```python
from diffusers import DiffusionPipeline
import torch


class UnetSchedulerOneForwardPipeline(DiffusionPipeline):
    def __init__(self, unet, scheduler):
        super().__init__()
```

Now, we must save the `unet` and `scheduler` in a config file so that you can save your pipeline with `save_pretrained`.
Therefore, make sure you add every component that is save-able to the `register_modules` function:

```python
from diffusers import DiffusionPipeline
import torch


class UnetSchedulerOneForwardPipeline(DiffusionPipeline):
    def __init__(self, unet, scheduler):
        super().__init__()

        self.register_modules(unet=unet, scheduler=scheduler)
```

Cool, the init is done! 🔥 Now, let's go into the forward pass, which we recommend defining as `__call__` . Here you're given all the creative freedom there is. For our amazing "one-step" pipeline, we simply create a random image and call the unet once and the scheduler once:

```python
from diffusers import DiffusionPipeline
import torch


class UnetSchedulerOneForwardPipeline(DiffusionPipeline):
    def __init__(self, unet, scheduler):
        super().__init__()

        self.register_modules(unet=unet, scheduler=scheduler)

    def __call__(self):
        image = torch.randn(
            (1, self.unet.in_channels, self.unet.sample_size, self.unet.sample_size),
        )
        timestep = 1

        model_output = self.unet(image, timestep).sample
        scheduler_output = self.scheduler.step(model_output, timestep, image).prev_sample

        return scheduler_output
```

Cool, that's it! 🚀 You can now run this pipeline by passing a `unet` and a `scheduler` to the init:

```python
from diffusers import DDPMScheduler, Unet2DModel

scheduler = DDPMScheduler()
unet = UNet2DModel()

pipeline = UnetSchedulerOneForwardPipeline(unet=unet, scheduler=scheduler)

output = pipeline()
```

But what's even better is that you can load pre-existing weights into the pipeline if they match exactly your pipeline structure. This is e.g. the case for [https://huggingface.co/google/ddpm-cifar10-32](https://huggingface.co/google/ddpm-cifar10-32) so that we can do the following:

```python
pipeline = UnetSchedulerOneForwardPipeline.from_pretrained("google/ddpm-cifar10-32")

output = pipeline()
```

We want to share this amazing pipeline with the community, so we would open a PR request to add the following code under `one_step_unet.py` to [https://github.com/huggingface/diffusers/tree/main/examples/community](https://github.com/huggingface/diffusers/tree/main/examples/community) .

```python
from diffusers import DiffusionPipeline
import torch


class UnetSchedulerOneForwardPipeline(DiffusionPipeline):
    def __init__(self, unet, scheduler):
        super().__init__()

        self.register_modules(unet=unet, scheduler=scheduler)

    def __call__(self):
        image = torch.randn(
            (1, self.unet.in_channels, self.unet.sample_size, self.unet.sample_size),
        )
        timestep = 1

        model_output = self.unet(image, timestep).sample
        scheduler_output = self.scheduler.step(model_output, timestep, image).prev_sample

        return scheduler_output
```

Our amazing pipeline got merged here: [#840](https://github.com/huggingface/diffusers/pull/840).
Now everybody that has `diffusers >= 0.4.0` installed can use our pipeline magically 🪄 as follows:

```python
from diffusers import DiffusionPipeline

pipe = DiffusionPipeline.from_pretrained("google/ddpm-cifar10-32", custom_pipeline="one_step_unet")
pipe()
```

Another way to upload your custom_pipeline, besides sending a PR, is uploading the code that contains it to the Hugging Face Hub, [as exemplified here](https://huggingface.co/docs/diffusers/using-diffusers/custom_pipeline_overview#loading-custom-pipelines-from-the-hub).

**Try it out now - it works!**

In general, you will want to create much more sophisticated pipelines, so we recommend looking at existing pipelines here: [https://github.com/huggingface/diffusers/tree/main/examples/community](https://github.com/huggingface/diffusers/tree/main/examples/community).

IMPORTANT:
You can use whatever package you want in your community pipeline file - as long as the user has it installed, everything will work fine. Make sure you have one and only one pipeline class that inherits from `DiffusionPipeline` as this will be automatically detected.

## How do community pipelines work?
A community pipeline is a class that has to inherit from ['DiffusionPipeline']:
and that has been added to `examples/community` [files](https://github.com/huggingface/diffusers/tree/main/examples/community).
The community can load the pipeline code via the custom_pipeline argument from DiffusionPipeline. See docs [here](https://huggingface.co/docs/diffusers/api/diffusion_pipeline#diffusers.DiffusionPipeline.from_pretrained.custom_pipeline):

This means:
The model weights and configs of the pipeline should be loaded from the `pretrained_model_name_or_path` [argument](https://huggingface.co/docs/diffusers/api/diffusion_pipeline#diffusers.DiffusionPipeline.from_pretrained.pretrained_model_name_or_path):
whereas the code that powers the community pipeline is defined in a file added in [`examples/community`](https://github.com/huggingface/diffusers/tree/main/examples/community).

Now, it might very well be that only some of your pipeline components weights can be downloaded from an official repo.
The other components should then be passed directly to init as is the case for the ClIP guidance notebook [here](https://colab.research.google.com/github/huggingface/notebooks/blob/main/diffusers/CLIP_Guided_Stable_diffusion_with_diffusers.ipynb#scrollTo=z9Kglma6hjki).

The magic behind all of this is that we load the code directly from GitHub. You can check it out in more detail if you follow the functionality defined here:

```python
# 2. Load the pipeline class, if using custom module then load it from the hub
# if we load from explicit class, let's use it
if custom_pipeline is not None:
    pipeline_class = get_class_from_dynamic_module(
        custom_pipeline, module_file=CUSTOM_PIPELINE_FILE_NAME, cache_dir=custom_pipeline
    )
elif cls != DiffusionPipeline:
    pipeline_class = cls
else:
    diffusers_module = importlib.import_module(cls.__module__.split(".")[0])
    pipeline_class = getattr(diffusers_module, config_dict["_class_name"])
```

This is why a community pipeline merged to GitHub will be directly available to all `diffusers` packages.