In modern software development, maintaining flexibility and control over feature releases is paramount. Feature flags, also known as feature toggles or switches, offer a powerful mechanism to enable or disable features at runtime. Integrating feature flags with a framework like FastAPI allows for dynamic route handling, empowering developers to control endpoints based on various conditions.

Understanding Feature Flags

Before diving into the implementation details, let’s briefly recap what feature flags are and why they’re valuable.

Feature flags are conditional statements that determine whether a particular feature or functionality is enabled or disabled. By controlling these flags, developers can toggle features on or off without redeploying code, allowing for gradual rollouts, A/B testing, and targeted releases.

Implementation in FastAPI

We’ll use FastAPI, a modern web framework for building APIs with Python, to demonstrate how feature flags can be seamlessly integrated into route handling.

Feature Flag Routing

Our implementation involves two main components: the route definitions and the feature flag logic.

from fastapi import APIRouter, HTTPException
from fastapi.responses import JSONResponse
from utils.feature_flag_routing import validate_flag_name, retrieve_feature_flag
router = APIRouter()

@validate_flag_name(“my_platform_1”, retrieve_feature_flag)
def get_feature():
return JSONResponse(status_code=200, content={“message”: “hello”})

@validate_flag_name(“my_platform_1”, retrieve_feature_flag)
async def get_feature_async():
return JSONResponse(status_code=200, content={“message”: “hello”})

In the above code snippet, we define two routes `/feature` and `/feature_async` using FastAPI’s `APIRouter`. Each route is decorated with `validate_flag_name`, which checks whether a specific feature flag is enabled before executing the route logic.

Feature Flag Logic

The feature flag logic resides in a separate module (`utils.feature_flag_routing`). Here, we define the `retrieve_feature_flag` function, responsible for fetching the state of a given feature flag.

def retrieve_feature_flag(platform):
return FEATURE_FLAGS_COMPLEX.get(“my_feature_flag”).get(platform, False)

Additionally, we implement a decorator `validate_flag_name` that wraps our route functions. This decorator ensures that the feature flag is enabled before executing the route logic.

Dynamic Route Handling

With this setup, we achieve dynamic route handling based on feature flags. When a request is made to a route, the associated feature flag is checked. If the flag is enabled, the route logic proceeds as usual. Otherwise, a suitable HTTP exception is raised, indicating that the feature is not enabled.


In conclusion, integrating feature flags with FastAPI offers a robust mechanism for controlling route enablement based on runtime conditions. By leveraging feature flags, developers gain fine-grained control over feature releases, enabling gradual rollouts, experimentation, and improved stability.

Implementing feature flag-based route handling in FastAPI enhances flexibility and empowers teams to deliver features with confidence, ensuring a seamless and controlled user experience.

To explore the complete implementation and further customize it to your needs, you can find the code on [GitHub]( Feel free to fork, contribute, or use it as a reference for your projects. Happy coding!

Leveraging Feature Flags for Dynamic Route Handling in FastAPI was originally published in Level Up Coding on Medium, where people are continuing the conversation by highlighting and responding to this story.

​ Level Up Coding – Medium

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