FastAPI is a modern web framework in Python that excels in building APIs with features like built-in support for OpenAPI specs and dependency injection, which is particularly useful for managing authentication and authorization. Dependency injection in FastAPI allows for clean and reusable code, making it easier to implement token validation, user permissions, and multi-tenancy, without the verbosity and potential errors of direct parameter handling. While middleware is commonly used for authentication checks in other frameworks, FastAPI's dependency injection offers a more flexible and integrated approach, allowing user data to be directly injected into route functions. This makes complex authorization scenarios, such as multi-tenancy and role-based access control, more straightforward by using nested dependencies. PropelAuth complements FastAPI by providing a library that simplifies the setup of advanced authentication and authorization capabilities, including features like SSO and SCIM Provisioning, leveraging FastAPI's dependency injection system.