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FastAPI Backend Development Bootcamp · Lesson

Custom Data Types & Settings

Define custom Pydantic data types and manage application settings using Pydantic's `BaseSettings`.

Custom Data Types & Settings is a free FastAPI Backend Development Bootcamp lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the FastAPI Backend Development Bootcamp learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome to Custom Types!

Pydantic is great for validating data, but sometimes you need validation beyond its built-in types.

  • Custom Data Types let you define your own rules for data.
  • This ensures your data adheres to specific formats or business logic.
  • Think of it as extending Pydantic's power for unique needs.

`Annotated` for Custom Validation

Pydantic v2 uses Python's typing.Annotated alongside validator functions to create custom types.

  • Annotated: Adds metadata to a type hint.
  • BeforeValidator: Runs a function before Pydantic's standard validation.
  • This allows you to transform or validate input data before it's assigned.

Crafting a `CapitalizedString`

Let's create a custom type called CapitalizedString that ensures the first letter of a string is always uppercase.

Our validator function will check this rule. If the string isn't capitalized, it will raise an error.

Using Your Custom Type

Here's how to define and use our new CapitalizedString type in a Pydantic model. Try changing the name to start with a lowercase letter to see the validation error!

from typing import Annotated
from pydantic import BaseModel, BeforeValidator, ValidationError

def validate_capitalized(v: str) -> str:
    if not isinstance(v, str):
        raise TypeError("String required")
    if v and not v[0].isupper():
        raise ValueError("Must start with uppercase")
    return v

CapitalizedString = Annotated[str, BeforeValidator(validate_capitalized)]

class Product(BaseModel):
    name: CapitalizedString
    price: float

if __name__ == "__main__":
    try:
        product1 = Product(name="Laptop", price=1200.50)
        print(f"Product: {product1.name}")

        # This will raise a ValidationError
        # product2 = Product(name="keyboard", price=75.00)
    except ValidationError as e:
        print(f"Validation Error: {e}")

Manage Settings with `BaseSettings`

Application settings (like database URLs, API keys) often change between development and production environments.

BaseSettings, from pydantic-settings, is designed to manage these configurations easily. It automatically loads settings from:

  • Environment variables
  • .env files
  • Default values

Default Settings in Action

Define your settings as attributes in a class inheriting from BaseSettings. Pydantic handles the rest, providing default values if nothing else is specified.

from pydantic_settings import BaseSettings

class AppConfig(BaseSettings):
    app_name: str = "My FastAPI App"
    debug_mode: bool = False
    version: str = "1.0.0"

if __name__ == "__main__":
    settings = AppConfig()
    print(f"App Name: {settings.app_name}")
    print(f"Debug Mode: {settings.debug_mode}")
    print(f"Version: {settings.version}")

Loading from Environment Variables

BaseSettings automatically looks for environment variables that match your setting names (case-insensitive).

In this example, we temporarily set an environment variable to demonstrate how Pydantic picks it up, overriding the default.

import os
from pydantic_settings import BaseSettings

class AppConfig(BaseSettings):
    app_name: str = "Default App"
    database_url: str = "sqlite:///./test.db"

if __name__ == "__main__":
    print("--- Without env var ---")
    settings_default = AppConfig()
    print(f"App Name: {settings_default.app_name}")

    # Simulate setting an environment variable
    os.environ["APP_NAME"] = "Production App"
    os.environ["DATABASE_URL"] = "postgresql://user:pass@host:5432/db"

    print("\n--- With env var ---")
    settings_env = AppConfig()
    print(f"App Name: {settings_env.app_name}")
    print(f"DB URL: {settings_env.database_url}")

    # Clean up the environment variable for subsequent runs
    del os.environ["APP_NAME"]
    del os.environ["DATABASE_URL"]

Leveraging `.env` Files

For local development, it's common to store settings in a .env file (e.g., .env) in your project root.

You can configure BaseSettings to load from this file using SettingsConfigDict(env_file='.env') in your settings class.

Example .env content:
APP_NAME="Dev App"
API_KEY="your_dev_api_key"

from pydantic_settings import BaseSettings, SettingsConfigDict

class ProjectSettings(BaseSettings):
    model_config = SettingsConfigDict(env_file='.env', extra='ignore')

    app_name: str = "Default Project"
    api_key: str = "default_key"

# To make this runnable, you would need python-dotenv installed
# and an actual .env file in the same directory as the script.
# For this lesson, we show the setup.

# Example usage (if .env existed and python-dotenv was active):
# if __name__ == "__main__":
#     settings = ProjectSettings()
#     print(f"App Name: {settings.app_name}")
#     print(f"API Key: {settings.api_key}")

Understanding Settings Priority

When BaseSettings looks for a value, it follows a specific order of precedence:

  1. Environment variables (highest priority)
  2. .env file variables
  3. Default values defined in the BaseSettings class (lowest priority)

This ensures that environment variables can always override local .env files and class defaults, which is crucial for deployment.

Test Your Knowledge!

Which of the following are true about Pydantic's BaseSettings and custom types?

Recap: Custom Types & Settings

Great job! You've learned how to create powerful custom data types with Annotated and BeforeValidator, extending Pydantic's validation.

You also mastered BaseSettings for robust application configuration, understanding how it loads values from defaults, .env files, and environment variables with clear priority rules.

These tools are essential for building flexible and maintainable FastAPI applications!

Frequently asked questions

Is the “Custom Data Types & Settings” lesson free?

Yes — the full text of “Custom Data Types & Settings” is free to read here on the web, and the FastAPI Backend Development Bootcamp course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the FastAPI Backend Development Bootcamp course, upgrade to CoddyKit PRO.

What will I learn in “Custom Data Types & Settings”?

Define custom Pydantic data types and manage application settings using Pydantic's `BaseSettings`. You practise FastAPI Backend Development Bootcamp with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start FastAPI Backend Development Bootcamp?

No prior experience is required. FastAPI Backend Development Bootcamp on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Custom Data Types & Settings” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this FastAPI Backend Development Bootcamp lesson?

Yes. Every FastAPI Backend Development Bootcamp lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Pydantic Field Validation & Validators
  2. Custom Data Types & Settings
  3. Nested Models & Recursive Structures
  4. Serialization with model_dump and Aliases
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