pydantic/docs/usage/settings.md

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:

{!.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:

{!.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=...).

!!! 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.

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. The default field values for the Settings model.