pydantic-settings

Q: How do I test with different env values?

Clear the cache and pass env via monkeypatch.setenv before calling get_settings() - or use Settings(_env_file=None, api_key="test") constructor overrides.

Q: Can I load multiple env files?

Pass env_file=(".env", ".env.local") - later files do not override earlier unless real env vars win.

Q: How do nested settings work?

Define a nested BaseModel field and set env_nested_delimiter="__" so CACHE__TTL=300 maps correctly.

Q: Does this work with FastAPI Depends?

Yes - def settings_dep() -> Settings: return get_settings() and inject into routes.

Q: How do I validate mutually exclusive flags?

Use @model_validator(mode="after") to raise if both USE_SQLITE and DATABASE_URL are set.

Q: What about 12-factor PORT binding?

port: int = Field(default=8000, alias="PORT") - Heroku/Fly/Railway inject PORT automatically.

Q: Can I use TOML instead of env?

pydantic-settings focuses on env; for TOML-heavy apps combine with pyproject.toml section loaders or dynaconf.

Q: How do I document settings for operators?

Generate JSON schema from the model or maintain .env.example with every key and sample value.

Q: SecretStr vs plain str?

Use SecretStr for tokens and passwords so accidental logging does not print secrets.

pydantic-settings extends Pydantic models to load configuration from environment variables, .env files, and optional secrets directories - with validation, type coercion, and clear error messages at startup.

Recipe

from pydantic import Field
from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
    database_url: str = Field(alias="DATABASE_URL")
    debug: bool = False
    api_key: str
 
 
settings = Settings()  # raises ValidationError if api_key missing

When to reach for this:

FastAPI/Django/Flask apps reading os.environ
Workers and CLIs that need typed, documented config
Replacing manual os.getenv("PORT", "8000") casts
Validating config before forked Gunicorn workers start

Working Example

from __future__ import annotations
 
from functools import lru_cache
 
from pydantic import Field, PostgresDsn
from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
class Settings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_file_encoding="utf-8",
        extra="ignore",
    )
 
    app_name: str = "billing-api"
    database_url: PostgresDsn = Field(alias="DATABASE_URL")
    redis_url: str = Field(default="redis://localhost:6379/0", alias="REDIS_URL")
    log_level: str = "INFO"
 
 
@lru_cache
def get_settings() -> Settings:
    return Settings()
 
 
def main() -> None:
    s = get_settings()
    print(s.app_name, s.database_url, s.log_level)
 
 
if __name__ == "__main__":
    main()

What this demonstrates:

SettingsConfigDict centralizes env file and ignore-unknown-keys policy
Field(alias=...) maps screaming-snake env names to snake_case attrs
PostgresDsn validates connection string shape early
@lru_cache gives FastAPI-style singleton settings without globals

Deep Dive

How It Works

Source priority - Defaults < env file < real environment (env wins over file).
Type coercion - "true" becomes True for bool fields; comma-separated strings can become lists with custom parsers.
Nested settings - env_nested_delimiter="__" maps DB__HOST to db.host.
Secrets - secrets_dir loads Docker/Kubernetes mounted secret files as field values.

Common Fields

Field Type	Env Example	Notes
`str`	`API_KEY=abc`	Required if no default
`bool`	`DEBUG=true`	Accepts true/false/1/0
`int`	`PORT=8000`	Invalid values raise at startup
`list[str]`	`ORIGINS=a,b`	Use `NoDecode` + custom validator for CSV
`SecretStr`	`TOKEN=...`	Masks value in repr/logs

Python Notes

from pydantic import SecretStr
 
class Settings(BaseSettings):
    token: SecretStr
 
# SecretStr hides in logs
print(settings.token.get_secret_value())  # explicit reveal

Gotchas

Import-time side effects - settings = Settings() at module level fails in tests. Fix: factory + lru_cache or dependency injection.
extra="allow" in production - Typos in env names silently create attributes. Fix: extra="ignore" or "forbid".
Committing .env with secrets - Git history leaks credentials. Fix: .env.example only; load real values from platform secrets.
One Settings for monorepo - Couples unrelated services. Fix: ApiSettings, WorkerSettings per deployable.
Missing env in Docker - Empty string may coerce unexpectedly for optional ints. Fix: use Optional[int] = None and validate.
Case sensitivity - Env keys are case-sensitive on Linux. Fix: document exact names in .env.example.

Alternatives

Alternative	Use When	Don't Use When
`os.environ` + manual casts	Tiny scripts with 2-3 vars	More than a handful of settings
`python-decouple`	Legacy Django projects already using it	Greenfield Pydantic/FastAPI stacks
`dynaconf`	Multi-environment YAML/TOML layering	You want Pydantic types end-to-end
`django-environ`	Django-only configuration	FastAPI or standalone workers

FAQs

How do I test with different env values?

Clear the cache and pass env via monkeypatch.setenv before calling get_settings() - or use Settings(_env_file=None, api_key="test") constructor overrides.

Can I load multiple env files?

Pass env_file=(".env", ".env.local") - later files do not override earlier unless real env vars win.

How do nested settings work?

Define a nested BaseModel field and set env_nested_delimiter="__" so CACHE__TTL=300 maps correctly.

Does this work with FastAPI Depends?

Yes - def settings_dep() -> Settings: return get_settings() and inject into routes.

How do I validate mutually exclusive flags?

Use @model_validator(mode="after") to raise if both USE_SQLITE and DATABASE_URL are set.

What about 12-factor PORT binding?

port: int = Field(default=8000, alias="PORT") - Heroku/Fly/Railway inject PORT automatically.

Can I use TOML instead of env?

pydantic-settings focuses on env; for TOML-heavy apps combine with pyproject.toml section loaders or dynaconf.

How do I document settings for operators?

Generate JSON schema from the model or maintain .env.example with every key and sample value.

SecretStr vs plain str?

Use SecretStr for tokens and passwords so accidental logging does not print secrets.

Does pydantic-settings work with Django?

Yes in standalone scripts and ASGI workers; Django projects often use django-environ but Pydantic settings work for Celery workers sharing the same env.

Pydantic Basics - models and validation
Configuration and Settings - architecture patterns
FastAPI Basics - dependency injection
Secrets Management - production secret handling

Python Package Index (PyPI) - Official repository of Python packages.
Read the Docs - Hosting for thousands of Python project documentations.
The Hitchhiker's Guide to Python - Best-practices handbook for installation, configuration, style, and tooling.
Fluent Python (2nd Edition, O'Reilly) - Same book on O'Reilly.
The Quick Python Book (4th Edition) - Concise Python overview (Naomi Ceder).

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Recipe

from pydantic import Field
from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
    database_url: str = Field(alias="DATABASE_URL")
    debug: bool = False
    api_key: str
 
 
settings = Settings()  # raises ValidationError if api_key missing

When to reach for this:

FastAPI/Django/Flask apps reading os.environ
Workers and CLIs that need typed, documented config
Replacing manual os.getenv("PORT", "8000") casts
Validating config before forked Gunicorn workers start

Working Example

from __future__ import annotations
 
from functools import lru_cache
 
from pydantic import Field, PostgresDsn
from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
class Settings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_file_encoding="utf-8",
        extra="ignore",
    )
 
    app_name: str = "billing-api"
    database_url: PostgresDsn = Field(alias="DATABASE_URL")
    redis_url: str = Field(default="redis://localhost:6379/0", alias="REDIS_URL")
    log_level: str = "INFO"
 
 
@lru_cache
def get_settings() -> Settings:
    return Settings()
 
 
def main() -> None:
    s = get_settings()
    print(s.app_name, s.database_url, s.log_level)
 
 
if __name__ == "__main__":
    main()

What this demonstrates:

SettingsConfigDict centralizes env file and ignore-unknown-keys policy
Field(alias=...) maps screaming-snake env names to snake_case attrs
PostgresDsn validates connection string shape early
@lru_cache gives FastAPI-style singleton settings without globals

Deep Dive

How It Works

Source priority - Defaults < env file < real environment (env wins over file).
Type coercion - "true" becomes True for bool fields; comma-separated strings can become lists with custom parsers.
Nested settings - env_nested_delimiter="__" maps DB__HOST to db.host.
Secrets - secrets_dir loads Docker/Kubernetes mounted secret files as field values.

Common Fields

Field Type	Env Example	Notes
`str`	`API_KEY=abc`	Required if no default
`bool`	`DEBUG=true`	Accepts true/false/1/0
`int`	`PORT=8000`	Invalid values raise at startup
`list[str]`	`ORIGINS=a,b`	Use `NoDecode` + custom validator for CSV
`SecretStr`	`TOKEN=...`	Masks value in repr/logs

Python Notes

from pydantic import SecretStr
 
class Settings(BaseSettings):
    token: SecretStr
 
# SecretStr hides in logs
print(settings.token.get_secret_value())  # explicit reveal

Gotchas

Import-time side effects - settings = Settings() at module level fails in tests. Fix: factory + lru_cache or dependency injection.
extra="allow" in production - Typos in env names silently create attributes. Fix: extra="ignore" or "forbid".
Committing .env with secrets - Git history leaks credentials. Fix: .env.example only; load real values from platform secrets.
One Settings for monorepo - Couples unrelated services. Fix: ApiSettings, WorkerSettings per deployable.
Missing env in Docker - Empty string may coerce unexpectedly for optional ints. Fix: use Optional[int] = None and validate.
Case sensitivity - Env keys are case-sensitive on Linux. Fix: document exact names in .env.example.

Alternatives

Alternative	Use When	Don't Use When
`os.environ` + manual casts	Tiny scripts with 2-3 vars	More than a handful of settings
`python-decouple`	Legacy Django projects already using it	Greenfield Pydantic/FastAPI stacks
`dynaconf`	Multi-environment YAML/TOML layering	You want Pydantic types end-to-end
`django-environ`	Django-only configuration	FastAPI or standalone workers

FAQs

How do I test with different env values?

Clear the cache and pass env via monkeypatch.setenv before calling get_settings() - or use Settings(_env_file=None, api_key="test") constructor overrides.

Can I load multiple env files?

Pass env_file=(".env", ".env.local") - later files do not override earlier unless real env vars win.

How do nested settings work?

Define a nested BaseModel field and set env_nested_delimiter="__" so CACHE__TTL=300 maps correctly.

Does this work with FastAPI Depends?

Yes - def settings_dep() -> Settings: return get_settings() and inject into routes.

How do I validate mutually exclusive flags?

Use @model_validator(mode="after") to raise if both USE_SQLITE and DATABASE_URL are set.

What about 12-factor PORT binding?

port: int = Field(default=8000, alias="PORT") - Heroku/Fly/Railway inject PORT automatically.

Can I use TOML instead of env?

pydantic-settings focuses on env; for TOML-heavy apps combine with pyproject.toml section loaders or dynaconf.

How do I document settings for operators?

Generate JSON schema from the model or maintain .env.example with every key and sample value.

SecretStr vs plain str?

Use SecretStr for tokens and passwords so accidental logging does not print secrets.

Does pydantic-settings work with Django?

Yes in standalone scripts and ASGI workers; Django projects often use django-environ but Pydantic settings work for Celery workers sharing the same env.

Pydantic Basics - models and validation
Configuration and Settings - architecture patterns
FastAPI Basics - dependency injection
Secrets Management - production secret handling

Python Package Index (PyPI) - Official repository of Python packages.
Read the Docs - Hosting for thousands of Python project documentations.
The Hitchhiker's Guide to Python - Best-practices handbook for installation, configuration, style, and tooling.
Fluent Python (2nd Edition, O'Reilly) - Same book on O'Reilly.
The Quick Python Book (4th Edition) - Concise Python overview (Naomi Ceder).

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

pydantic-settings

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