---
title: __init__ API Reference
description: Learn how Ultralytics uses lazy imports with __getattr__ to speed up package startup and load model classes like YOLO, NAS, RTDETR, and SAM only when needed.
keywords: Ultralytics, YOLO, lazy import, __getattr__, NAS, RTDETR, SAM, FastSAM, YOLOWorld, performance, startup time, optimization
---

# Reference for `ultralytics/__init__.py`

!!! success "Improvements"

    This page is sourced from [https://github.com/ultralytics/ultralytics/blob/main/ultralytics/\_\_init\_\_.py](https://github.com/ultralytics/ultralytics/blob/main/ultralytics/__init__.py). Have an improvement or example to add? Open a [Pull Request](https://docs.ultralytics.com/help/contributing) — thank you! 🙏

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## ::: ultralytics.__init__.__getattr__

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## ::: ultralytics.__init__.__dir__

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