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Version: 0.1

Prepare Custom Model

In this tutorial, we will learn how to prepare a custom AI model so that it can be integrated into a PyTorch Live demo.

PyTorch Live works with high-level data types such as images and strings. To run inference, these high-level data types need to be transformed into tensors and the model output needs to be transformed into high-level data types. Transforming a high-level data type into a tensor is called pack and transforming a tensor into a high-level data type is called unpack.

Each model will have a pack and an unpack. Both are specified in the PyTorch Live Spec (or Live Spec for short). The Live Spec has to be bundled with the model ptl file as extra_files.

This section will guide you step-by-step for how to bundle the Live Spec with pack and unpack for an image classification model, more specifically the MobileNet V3 (small) model.

We can do this bundling step from anywhere in our filesystem that has access to python. If you already have a PyTorch Live project, follow these steps from a subdirectory in the project directory, for example, <PYTORCH_LIVE_PROJECT>/models, so that the bundled model is in place and ready to go when we're done.

Create Live Spec

Create a live.spec.json file and insert the following definition.

"pack": {
"type": "tensor_from_image",
"image": "image",
"transforms": [
"type": "image_to_image",
"name": "center_crop"
"type": "image_to_image",
"name": "scale",
"width": 224,
"height": 224
"type": "image_to_tensor",
"name": "rgb_norm",
"mean": [0.485, 0.456, 0.406],
"std": [0.229, 0.224, 0.225]
"unpack": {
"type": "argmax",
"dtype": "float",
"key": "maxIdx",
"valueKey": "confidence"

Bundle Live Spec with Model

Next is to download and bundle the Live Spec with the MobileNet V3 model using a Python script.

Set up Python virtual environment

It is recommended to run the Pyhton script in a virtual environment. Python offers a command to create a virtual environment with the following command.

python3 -m venv venv
source venv/bin/activate

Install torch and torchvision dependencies

The Python script requires torch and torchvision. Use the Python package manager (pip3 or simply pip in a virtual environment) to install both dependencies.

pip3 install torch torchvision

Build the Model

The following script will download the MobileNet V3 model from the PyTorch Hub, optimize it for mobile use, and bundle the Live Spec as extra file with the ptl file.
from pathlib import Path

import torch
import torchvision
from torch.utils.mobile_optimizer import optimize_for_mobile

model = torchvision.models.mobilenet_v3_small(pretrained=True)

scripted_model = torch.jit.script(model)
optimized_model = optimize_for_mobile(scripted_model)
spec = Path("live.spec.json").read_text()
extra_files = {}
extra_files["model/live.spec.json"] = spec
optimized_model._save_for_lite_interpreter("mobilenet_v3_small.ptl", _extra_files=extra_files)

print("model successfully exported")

Create the file, add the Python script above, and then run Python script to create mobilenet_v3_small.ptl.


If you already have a PyTorch Live project, and you haven't run the commands in the <PYTORCH_LIVE_PROJECT>/models directory, copy mobilenet_v3_small.ptl to the <PYTORCH_LIVE_PROJECT>/models directory. It will now be accessible to load and run in your PyTorch Live project using the path models/mobilenet_v3_small.ptl.

Learn More About Live Spec

For additional details about the Live Spec JSON format and its transforms, read the model specification page in the API documentation.

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