Python Configuration

Python Configuration Management

Externalize configuration from code using environment variables and typed settings. Well-managed configuration enables the same code to run in any environment without modification.

When to Use This Skill

• Setting up a new project's configuration system
• Migrating from hardcoded values to environment variables
• Implementing pydantic-settings for typed configuration
• Managing secrets and sensitive values
• Creating environment-specific settings (dev/staging/prod)
• Validating configuration at application startup

Core Concepts

1. Externalized Configuration

All environment-specific values (URLs, secrets, feature flags) come from environment variables, not code.

2. Typed Settings

Parse and validate configuration into typed objects at startup, not scattered throughout code.

3. Fail Fast

Validate all required configuration at application boot. Missing config should crash immediately with a clear message.

4. Sensible Defaults

Provide reasonable defaults for local development while requiring explicit values for sensitive settings.

Quick Start

from pydantic_settings import BaseSettings
from pydantic import Field

class Settings(BaseSettings):
    database_url: str = Field(alias="DATABASE_URL")
    api_key: str = Field(alias="API_KEY")
    debug: bool = Field(default=False, alias="DEBUG")

settings = Settings()  # Loads from environment

Fundamental Patterns

Pattern 1: Typed Settings with Pydantic

Create a central settings class that loads and validates all configuration.

from pydantic_settings import BaseSettings
from pydantic import Field, PostgresDsn, ValidationError
import sys

class Settings(BaseSettings):
    """Application configuration loaded from environment variables."""

    # Database
    db_host: str = Field(alias="DB_HOST")
    db_port: int = Field(default=5432, alias="DB_PORT")
    db_name: str = Field(alias="DB_NAME")
    db_user: str = Field(alias="DB_USER")
    db_password: str = Field(alias="DB_PASSWORD")

    # Redis
    redis_url: str = Field(default="redis://localhost:6379", alias="REDIS_URL")

    # API Keys
    api_secret_key: str = Field(alias="API_SECRET_KEY")

    # Feature flags
    enable_new_feature: bool = Field(default=False, alias="ENABLE_NEW_FEATURE")

    model_config = {
        "env_file": ".env",
        "env_file_encoding": "utf-8",
    }

# Create singleton instance at module load
try:
    settings = Settings()
except ValidationError as e:
    print(f"Configuration error:\n{e}")
    sys.exit(1)

Import settings throughout your application:

from myapp.config import settings

def get_database_connection():
    return connect(
        host=settings.db_host,
        port=settings.db_port,
        database=settings.db_name,
    )

Pattern 2: Fail Fast on Missing Configuration

Required settings should crash the application immediately with a clear error.

from pydantic_settings import BaseSettings
from pydantic import Field, ValidationError
import sys

class Settings(BaseSettings):
    # Required - no default means it must be set
    api_key: str = Field(alias="API_KEY")
    database_url: str = Field(alias="DATABASE_URL")

    # Optional with defaults
    log_level: str = Field(default="INFO", alias="LOG_LEVEL")

try:
    settings = Settings()
except ValidationError as e:
    print("=" * 60)
    print("CONFIGURATION ERROR")
    print("=" * 60)
    for error in e.errors():
        field = error["loc"][0]
        print(f"  - {field}: {error['msg']}")
    print("\nPlease set the required environment variables.")
    sys.exit(1)

A clear error at startup is better than a cryptic None failure mid-request.

Pattern 3: Local Development Defaults

Provide sensible defaults for local development while requiring explicit values for secrets.

class Settings(BaseSettings):
    # Has local default, but prod will override
    db_host: str = Field(default="localhost", alias="DB_HOST")
    db_port: int = Field(default=5432, alias="DB_PORT")

    # Always required - no default for secrets
    db_password: str = Field(alias="DB_PASSWORD")
    api_secret_key: str = Field(alias="API_SECRET_KEY")

    # Development convenience
    debug: bool = Field(default=False, alias="DEBUG")

    model_config = {"env_file": ".env"}

Create a .env file for local development (never commit this):

# .env (add to .gitignore)
DB_PASSWORD=local_dev_password
API_SECRET_KEY=dev-secret-key
DEBUG=true

Pattern 4: Namespaced Environment Variables

Prefix related variables for clarity and easy debugging.

# Database configuration
DB_HOST=localhost
DB_PORT=5432
DB_NAME=myapp
DB_USER=admin
DB_PASSWORD=secret

# Redis configuration
REDIS_URL=redis://localhost:6379
REDIS_MAX_CONNECTIONS=10

# Authentication
AUTH_SECRET_KEY=your-secret-key
AUTH_TOKEN_EXPIRY_SECONDS=3600
AUTH_ALGORITHM=HS256

# Feature flags
FEATURE_NEW_CHECKOUT=true
FEATURE_BETA_UI=false

Makes env | grep DB_ useful for debugging.

Advanced Patterns

Pattern 5: Type Coercion

Pydantic handles common conversions automatically.

from pydantic_settings import BaseSettings
from pydantic import Field, field_validator

class Settings(BaseSettings):
    # Automatically converts "true", "1", "yes" to True
    debug: bool = False

    # Automatically converts string to int
    max_connections: int = 100

    # Parse comma-separated string to list
    allowed_hosts: list[str] = Field(default_factory=list)

    @field_validator("allowed_hosts", mode="before")
    @classmethod
    def parse_allowed_hosts(cls, v: str | list[str]) -> list[str]:
        if isinstance(v, str):
            return [host.strip() for host in v.split(",") if host.strip()]
        return v

Usage:

ALLOWED_HOSTS=example.com,api.example.com,localhost
MAX_CONNECTIONS=50
DEBUG=true

Pattern 6: Environment-Specific Configuration

Use an environment enum to switch behavior.

from enum import Enum
from pydantic_settings import BaseSettings
from pydantic import Field, computed_field

class Environment(str, Enum):
    LOCAL = "local"
    STAGING = "staging"
    PRODUCTION = "production"

class Settings(BaseSettings):
    environment: Environment = Field(
        default=Environment.LOCAL,
        alias="ENVIRONMENT",
    )

    # Settings that vary by environment
    log_level: str = Field(default="DEBUG", alias="LOG_LEVEL")

    @computed_field
    @property
    def is_production(self) -> bool:
        return self.environment == Environment.PRODUCTION

    @computed_field
    @property
    def is_local(self) -> bool:
        return self.environment == Environment.LOCAL

# Usage
if settings.is_production:
    configure_production_logging()
else:
    configure_debug_logging()

Pattern 7: Nested Configuration Groups

Organize related settings into nested models.

from pydantic import BaseModel
from pydantic_settings import BaseSettings

class DatabaseSettings(BaseModel):
    host: str = "localhost"
    port: int = 5432
    name: str
    user: str
    password: str

class RedisSettings(BaseModel):
    url: str = "redis://localhost:6379"
    max_connections: int = 10

class Settings(BaseSettings):
    database: DatabaseSettings
    redis: RedisSettings
    debug: bool = False

    model_config = {
        "env_nested_delimiter": "__",
        "env_file": ".env",
    }

Environment variables use double underscore for nesting:

DATABASE__HOST=db.example.com
DATABASE__PORT=5432
DATABASE__NAME=myapp
DATABASE__USER=admin
DATABASE__PASSWORD=secret
REDIS__URL=redis://redis.example.com:6379

Pattern 8: Secrets from Files

For container environments, read secrets from mounted files.

from pydantic_settings import BaseSettings
from pydantic import Field
from pathlib import Path

class Settings(BaseSettings):
    # Read from environment variable or file
    db_password: str = Field(alias="DB_PASSWORD")

    model_config = {
        "secrets_dir": "/run/secrets",  # Docker secrets location
    }

Pydantic will look for /run/secrets/db_password if the env var isn't set.

Pattern 9: Configuration Validation

Add custom validation for complex requirements.

from pydantic_settings import BaseSettings
from pydantic import Field, model_validator

class Settings(BaseSettings):
    db_host: str = Field(alias="DB_HOST")
    db_port: int = Field(alias="DB_PORT")
    read_replica_host: str | None = Field(default=None, alias="READ_REPLICA_HOST")
    read_replica_port: int = Field(default=5432, alias="READ_REPLICA_PORT")

    @model_validator(mode="after")
    def validate_replica_settings(self):
        if self.read_replica_host and self.read_replica_port == self.db_port:
            if self.read_replica_host == self.db_host:
                raise ValueError(
                    "Read replica cannot be the same as primary database"
                )
        return self

Best Practices Summary

1. Never hardcode config - All environment-specific values from env vars 2. Use typed settings - Pydantic-settings with validation 3. Fail fast - Crash on missing required config at startup 4. Provide dev defaults - Make local development easy 5. Never commit secrets - Use .env files (gitignored) or secret managers 6. Namespace variables - DB_HOST, REDIS_URL for clarity 7. Import settings singleton - Don't call os.getenv() throughout code 8. Document all variables - README should list required env vars 9. Validate early - Check config correctness at boot time 10. Use secrets_dir - Support mounted secrets in containers

---

Gotchas

• `pydantic-settings` reads env at class IMPORT, not at instantiation — tests that monkeypatch env after import see cached values. Reload the module or instantiate before patching.
• Nested BaseSettings models need `env_nested_delimiter` explicitly set — without it, DB_HOST and DB_PORT don't auto-bind to Settings.db.host.
• `SecretStr` redacts in `__repr__` but NOT in `__str__` — print(secret) leaks; f"{secret}" leaks; only repr(secret) redacts.
• `.env` file precedence vs process env: pydantic-settings reads .env FIRST then overlays process env. A CI secret in process env wins over a developer's .env — but stale .env values stick if process env is missing the key.
• Pydantic v1 vs v2 BaseSettings have different API surfaces — Field(env=...) vs model_config = SettingsConfigDict(...). Mixing breaks silently in some paths.