Configuration management in FastAPI - Step-by-Step Execution

Concept Flow - Configuration management

Start Application

↓

Load Config File

↓

Parse Config Values

↓

Set Environment Variables

↓

Create Config Object

↓

Inject Config into App

↓

Run App with Config

This flow shows how FastAPI loads and uses configuration settings step-by-step when starting an app.

Execution Sample

FastAPI

from pydantic import BaseSettings

class Settings(BaseSettings):
    app_name: str = "My FastAPI App"

settings = Settings()
print(settings.app_name)

This code loads configuration using Pydantic BaseSettings and prints the app name.

Execution Table

Step	Action	Evaluation	Result
1	Create Settings instance	Settings() called	Defaults loaded: app_name='My FastAPI App'
2	Check environment variables	No override found	app_name remains 'My FastAPI App'
3	Print app_name	settings.app_name	'My FastAPI App' printed
4	Exit	No more code	Program ends

💡 All config loaded and printed, program ends normally

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
settings.app_name	undefined	My FastAPI App	My FastAPI App	My FastAPI App

Key Moments - 2 Insights

Why does settings.app_name keep the default value even if environment variables exist?

Because in the execution_table step 2, no environment variable overrides were found, so the default value remains.

How does BaseSettings know which environment variables to use?

BaseSettings automatically looks for environment variables matching the field names, but if none are set, it uses defaults as shown in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of settings.app_name after step 1?

A undefined

B "My FastAPI App"

C "Default App"

D Empty string

💡 Hint

Check the 'Result' column in row for step 1 in execution_table

At which step does the program print the app_name?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the action 'Print app_name' in execution_table

If an environment variable APP_NAME="Custom App" was set, how would the value of settings.app_name change after step 2?

A "Custom App"

B "My FastAPI App"

C undefined

D Error

💡 Hint

BaseSettings overrides defaults with environment variables as explained in key_moments and step 2

Concept Snapshot

FastAPI uses Pydantic BaseSettings for config management.
Define a class with config fields and defaults.
It loads environment variables automatically.
Create an instance to access config values.
Use this instance throughout your app.

Full Transcript

Configuration management in FastAPI uses Pydantic's BaseSettings class. When the app starts, it creates a Settings instance that loads default values and overrides them with environment variables if present. This process ensures your app can be configured easily without changing code. The example code defines a Settings class with an app_name field defaulting to 'My FastAPI App'. When running, it prints this value unless an environment variable overrides it. This approach helps keep configuration clean and flexible.