One of pydantic's most useful applications is settings management.

If you create a model that inherits from `BaseSettings`, the model initialiser will attempt to determine
the values of any fields not passed as keyword arguments by reading from the environment. (Default values
will still be used if the matching environment variable is not set.)

This makes it easy to:

* Create a clearly-defined, type-hinted application configuration class
* Automatically read modifications to the configuration from environment variables
* Manually override specific settings in the initialiser where desired (e.g. in unit tests)

For example:

```py
{!.tmp_examples/settings_main.py!}
```
_(This script is complete, it should run "as is")_

## Environment variable names

The following rules are used to determine which environment variable(s) are read for a given field:

* By default, the environment variable name is built by concatenating the prefix and field name.
  * For example, to override `special_function` above, you could use:

          export my_prefix_special_function='foo.bar'

  * Note 1: The default prefix is an empty string.
  * Note 2: Field aliases are ignored when building the environment variable name.

* Custom environment variable names can be set in two ways:
  * `Config.fields['field_name']['env']` (see `auth_key` and `redis_dsn` above)
  * `Field(..., env=...)` (see `api_key` above)
* When specifying custom environment variable names, either a string or a list of strings may be provided.
  * When specifying a list of strings, order matters: the first detected value is used.
  * For example, for `redis_dsn` above, `service_redis_dsn` would take precedence over `redis_url`.

!!! warning
    Since **v1.0** *pydantic* does not consider field aliases when finding environment variables to populate settings
    models, use `env` instead as described above.

    To aid the transition from aliases to `env`, a warning will be raised when aliases are used on settings models
    without a custom env var name. If you really mean to use aliases, either ignore the warning or set `env` to
    suppress it.

Case-sensitivity can be turned on through the `Config`:

```py
{!.tmp_examples/settings_case_sensitive.py!}
```

When `case_sensitive` is `True`, the environment variable names must match field names (optionally with a prefix),
so in this example
`redis_host` could only be modified via `export redis_host`. If you want to name environment variables
all upper-case, you should name attribute all upper-case too. You can still name environment variables anything
you like through `Field(..., env=...)`.

In Pydantic **v1** `case_sensitive` is `False` by default and all variable names are converted to lower-case internally.
If you want to define upper-case variable names on nested models like `SubModel` you have to
set `case_sensitive=True` to disable this behaviour.

!!! note
    On Windows, Python's `os` module always treats environment variables as case-insensitive, so the
    `case_sensitive` config setting will have no effect - settings will always be updated ignoring case.

## Parsing environment variable values

For most simple field types (such as `int`, `float`, `str`, etc.),
the environment variable value is parsed the same way it would
be if passed directly to the initialiser (as a string).

Complex types like `list`, `set`, `dict`, and sub-models are populated from the environment
by treating the environment variable's value as a JSON-encoded string.

Another way to populate nested complex variables is to configure your model with the `env_nested_delimiter`
config setting, then use an env variable with a name pointing to the nested module fields.
What it does is simply explodes your variable into nested models or dicts.
So if you define a variable `FOO__BAR__BAZ=123` it will convert it into `FOO={'BAR': {'BAZ': 123}}`
If you have multiple variables with the same structure they will be merged.

With the following environment variables:
```bash
# your environment
export V0=0
export SUB_MODEL='{"v1": "json-1", "v2": "json-2"}'
export SUB_MODEL__V2=nested-2
export SUB_MODEL__V3=3
export SUB_MODEL__DEEP__V4=v4
```

You could load a settings module thus:
```py
{!.tmp_examples/settings_nested_env.py!}
```

`env_nested_delimiter` can be configured via the `Config` class as shown above, or via the 
`_env_nested_delimiter` keyword argument on instantiation.

JSON is only parsed in top-level fields, if you need to parse JSON in sub-models, you will need to implement
validators on those models.

Nested environment variables take precedence over the top-level environment variable JSON
(e.g. in the example above, `SUB_MODEL__V2` trumps `SUB_MODEL`).

## Dotenv (.env) support

!!! note
    dotenv file parsing requires [python-dotenv](https://pypi.org/project/python-dotenv/) to be installed.
    This can be done with either `pip install python-dotenv` or `pip install pydantic[dotenv]`.

Dotenv files (generally named `.env`) are a common pattern that make it easy to use environment variables in a
platform-independent manner.

A dotenv file follows the same general principles of all environment variables,
and looks something like:

```bash
# ignore comment
ENVIRONMENT="production"
REDIS_ADDRESS=localhost:6379
MEANING_OF_LIFE=42
MY_VAR='Hello world'
```

Once you have your `.env` file filled with variables, *pydantic* supports loading it in two ways:

**1.** setting `env_file` (and `env_file_encoding` if you don't want the default encoding of your OS) on `Config`
in a `BaseSettings` class:

```py
class Settings(BaseSettings):
    ...

    class Config:
        env_file = '.env'
        env_file_encoding = 'utf-8'
```

**2.** instantiating a `BaseSettings` derived class with the `_env_file` keyword argument
(and the `_env_file_encoding` if needed):

```py
settings = Settings(_env_file='prod.env', _env_file_encoding='utf-8')
```

In either case, the value of the passed argument can be any valid path or filename, either absolute or relative to the
current working directory. From there, *pydantic* will handle everything for you by loading in your variables and
validating them.

!!! note
    If a filename is specified for `env_file`, Pydantic will only check the current working directory and
    won't check any parent directories for the `.env` file.

Even when using a dotenv file, *pydantic* will still read environment variables as well as the dotenv file,
**environment variables will always take priority over values loaded from a dotenv file**.

Passing a file path via the `_env_file` keyword argument on instantiation (method 2) will override
the value (if any) set on the `Config` class. If the above snippets were used in conjunction, `prod.env` would be loaded
while `.env` would be ignored.

You can also use the keyword argument override to tell Pydantic not to load any file at all (even if one is set in
the `Config` class) by passing `None` as the instantiation keyword argument, e.g. `settings = Settings(_env_file=None)`.

Because python-dotenv is used to parse the file, bash-like semantics such as `export` can be used which
(depending on your OS and environment) may allow your dotenv file to also be used with `source`,
see [python-dotenv's documentation](https://saurabh-kumar.com/python-dotenv/#usages) for more details.

## Secret Support

Placing secret values in files is a common pattern to provide sensitive configuration to an application.

A secret file follows the same principal as a dotenv file except it only contains a single value and the file name 
is used as the key. A secret file will look like the following:

`/var/run/database_password`:
```
super_secret_database_password
```

Once you have your secret files, *pydantic* supports loading it in two ways:

**1.** setting `secrets_dir` on `Config` in a `BaseSettings` class to the directory where your secret files are stored:

```py
class Settings(BaseSettings):
    ...
    database_password: str

    class Config:
        secrets_dir = '/var/run'
```

**2.** instantiating a `BaseSettings` derived class with the `_secrets_dir` keyword argument:

```py
settings = Settings(_secrets_dir='/var/run')
```

In either case, the value of the passed argument can be any valid directory, either absolute or relative to the
current working directory. **Note that a non existent directory will only generate a warning**.
From there, *pydantic* will handle everything for you by loading in your variables and validating them.

Even when using a secrets directory, *pydantic* will still read environment variables from a dotenv file or the environment,
**a dotenv file and environment variables will always take priority over values loaded from the secrets directory**.

Passing a file path via the `_secrets_dir` keyword argument on instantiation (method 2) will override
the value (if any) set on the `Config` class.

### Use Case: Docker Secrets

Docker Secrets can be used to provide sensitive configuration to an application running in a Docker container.
To use these secrets in a *pydantic* application the process is simple. More information regarding creating, managing
and using secrets in Docker see the official
[Docker documentation](https://docs.docker.com/engine/reference/commandline/secret/).

First, define your Settings
```py
class Settings(BaseSettings):
    my_secret_data: str

    class Config:
        secrets_dir = '/run/secrets'
```
!!! note
    By default Docker uses `/run/secrets` as the target mount point. If you want to use a different location, change 
    `Config.secrets_dir` accordingly.

Then, create your secret via the Docker CLI
```bash
printf "This is a secret" | docker secret create my_secret_data -
```

Last, run your application inside a Docker container and supply your newly created secret
```bash
docker service create --name pydantic-with-secrets --secret my_secret_data pydantic-app:latest
```

## Field value priority

In the case where a value is specified for the same `Settings` field in multiple ways,
the selected value is determined as follows (in descending order of priority):

1. Arguments passed to the `Settings` class initialiser.
2. Environment variables, e.g. `my_prefix_special_function` as described above.
3. Variables loaded from a dotenv (`.env`) file.
4. Variables loaded from the secrets directory.
5. The default field values for the `Settings` model.

## Customise settings sources

If the default order of priority doesn't match your needs, it's possible to change it by overriding
the `customise_sources` method on the `Config` class of your `Settings` .

`customise_sources` takes three callables as arguments and returns any number of callables as a tuple. In turn these
callables are called to build the inputs to the fields of the settings class.

Each callable should take an instance of the settings class as its sole argument and return a `dict`.

### Changing Priority

The order of the returned callables decides the priority of inputs; first item is the highest priority.

```py
{!.tmp_examples/settings_env_priority.py!}
```
_(This script is complete, it should run "as is")_

By flipping `env_settings` and `init_settings`, environment variables now have precedence over `__init__` kwargs.

### Adding sources

As explained earlier, *pydantic* ships with multiples built-in settings sources. However, you may occasionally
need to add your own custom sources, `customise_sources` makes this very easy:

```py
{!.tmp_examples/settings_add_custom_source.py!}
```
_(This script is complete, it should run "as is")_

### Removing sources

You might also want to disable a source:

```py
{!.tmp_examples/settings_disable_source.py!}
```
_(This script is complete, it should run "as is", here you might need to set the `my_api_key` environment variable)_