How to Use Config Class in FastAPI for Settings Management

In FastAPI, you can use a config class by creating a Pydantic model to hold your settings and then loading it once for your app. This class centralizes configuration like database URLs or API keys, making your code cleaner and easier to manage.

Syntax

Define a config class by subclassing pydantic.BaseSettings. Each attribute represents a setting with an optional default value. FastAPI can then use this class to load environment variables automatically.

Example parts:

• class Config(BaseSettings): - declares the config class.
• Attributes like database_url: str - define settings.
• Optional Config inner class to customize behavior.

python

from pydantic import BaseSettings

class Config(BaseSettings):
    database_url: str
    api_key: str = "default_key"

    class Config:
        env_file = ".env"  # Optional: load from .env file

Example

This example shows how to create a config class, load settings from environment variables or a .env file, and use it in a FastAPI app.

python

from fastapi import FastAPI
from pydantic import BaseSettings

class Config(BaseSettings):
    database_url: str
    api_key: str = "default_key"

    class Config:
        env_file = ".env"

app = FastAPI()
settings = Config()

@app.get("/settings")
def read_settings():
    return {"database_url": settings.database_url, "api_key": settings.api_key}

Output

{"database_url": "postgresql://user:pass@localhost/db", "api_key": "default_key"}

Common Pitfalls

Common mistakes when using config classes in FastAPI include:

• Not subclassing BaseSettings, so environment variables won't load automatically.
• Forgetting to create a single instance of the config class and instead recreating it multiple times.
• Not specifying env_file if you rely on a .env file for local development.
• Using mutable default values in the config class attributes.

python

from pydantic import BaseSettings

# Wrong: Not subclassing BaseSettings
class WrongConfig:
    database_url = "sqlite:///test.db"

# Right: Subclass BaseSettings and use env_file
class RightConfig(BaseSettings):
    database_url: str

    class Config:
        env_file = ".env"

Quick Reference

• Define config: subclass BaseSettings.
• Load env vars: use env_file in inner Config class.
• Use single instance: create one config object and reuse it.
• Access settings: use attributes like settings.database_url.

Key Takeaways

Use a Pydantic class subclassing BaseSettings to manage FastAPI config cleanly.

Load environment variables automatically by specifying an env_file in the config class.

Create one config instance and reuse it throughout your FastAPI app.

Avoid mutable defaults and always subclass BaseSettings for automatic env loading.

Access config values via attributes on the config instance for clarity and safety.